Glycemic index, glycemic load and glycemic response: An International Scientific Consensus Summit from the International Carbohydrate Quality Consortium (ICQC)

BACKGROUND AND AIMS

The positive and negative health effects of dietary carbohydrates are of interest to both researchers and consumers.

METHODS

International experts on carbohydrate research held a scientific summit in Stresa, Italy, in June 2013 to discuss controversies surrounding the utility of the glycemic index (GI), glycemic load (GL) and glycemic response (GR).

RESULTS

The outcome was a scientific consensus statement which recognized the importance of postprandial glycemia in overall health, and the GI as a valid and reproducible method of classifying carbohydrate foods for this purpose. There was consensus that diets low in GI and GL were relevant to the prevention and management of diabetes and coronary heart disease, and probably obesity. Moderate to weak associations were observed for selected cancers. The group affirmed that diets low in GI and GL should always be considered in the context of diets otherwise understood as healthy, complementing additional ways of characterizing carbohydrate foods, such as fiber and whole grain content. Diets of low GI and GL were considered particularly important in individuals with insulin resistance.

CONCLUSIONS

Given the high prevalence of diabetes and pre-diabetes worldwide and the consistency of the scientific evidence reviewed, the expert panel confirmed an urgent need to communicate information on GI and GL to the general public and health professionals, through channels such as national dietary guidelines, food composition tables and food labels.

Thermogenesis associated with fermentable carbohydrate in humans, validity of indirect calorimetry, and implications of dietary thermogenesis for energy requirements, food energy and body weight

BACKGROUND

Animal studies and theory show fermentable carbohydrate (FC) intake causes appreciably thermogenesis, but a similar occurrence in humans is controversial.

HYPOTHESES

(a) That indirect calorimetry (IDC) is a valid method to assess thermogenesis during fermentation. (b) That a consistent and rigorous approach to the analysis of published IDC data from human studies will establish a representative thermogenic response to FC. (c) That conventional estimates of food energy and energy requirements can mismatch appreciably, more especially when thermogenesis is ignored.

PURPOSE

To derive information and understanding of IDC, thermogenesis and energy balance in relation to food energy and energy requirement estimates.

METHODS

(a) The validities of IDC equations that estimate the heat of reaction and carbohydrate utilization were assessed for various types of FCs under various circumstances. (b) Pooled analysis of eight published randomized cross-over studies in humans with elevation of FC intake. Studies were analysed for the first time or reanalysed according to a consistent approach with appropriate corrections for confounders. (c) Some 1500 regular and 'special' diet compositions were examined to assess the extent to which Atwater general food energy factors and updated estimates of energy requirements mismatch due to variation in substrate-associated thermogenesis and substrate-associated faecal+urinary energy losses. Impact of such mismatches on BMI was assessed under conditions of all else being equal.

RESULTS

(a) Indirect calorimetry was valid, providing robust estimates of heat production during various types of fermentation; only small correction factors were necessary. By contrast, IDC equations for carbohydrate utilization sometimes applied poorly to FC. (b) A best estimate of thermogenesis in humans due to fermentation, above that due to oral glucose as a reference standard, was 0.39 (s.e.m. 0.14) kJ per kJ net metabolizable energy (NME; P<0.05, n=8 studies, total 72 humans) compared with 0.34 kJ/kJ from theory. Six sources of bias were identified; all had potential to underestimate FC thermogenesis. (c) Mismatches in energy availability and requirement estimates were often marked and translated into long-term differences in body mass index from approximately 20 to 33 kg/m(2) in average-height middle-aged initially obese women, and from approximately 22 to a non-survivable 13 kg/m(2) in initially slim women.

CONCLUSIONS

(a) Indirect calorimetry is valid for the present purpose. (b) Thermogenesis in response to FC is real in humans and is comparable to that in animals and in theory. (c) Mismatches between estimates of energy requirements and dietary energy as metabolizable energy means the two expressions are not directly comparable, which has implications for the expression of food energy, energy requirements and the conduct and interpretation of research related to body weight.

A perspective on food energy standards for nutrition labelling

Food energy values used for nutrition labelling and other purposes are traditionally based on the metabolisable energy (ME) standard, which has recent support from. By reference to current practices and published data, the present review critically examines the ME standard and support for it. Theoretical and experimental evidence on the validity of ME and alternatives are considered. ME and alternatives are applied to 1189 foods to assess outcomes. The potential impact of implementing a better standard in food labelling, documentation of energy requirements and food tables, and its impact on users including consumers, trade and professionals, are also examined. Since 1987 twenty-two expert reviews, reports and regulatory documents have fully or partly dropped the ME standard. The principal reason given is that ME only approximates energy supply by nutrients, particularly fermentable carbohydrates. ME has been replaced by net metabolisable energy (NME), which accounts for the efficiency of fuel utilisation in metabolism. Data collated from modern indirect calorimetry studies in human subjects show NME to be valid and applicable to each source of food energy, not just carbohydrates. NME is robust; two independent approaches give almost identical results (human calorimetry and calculation of free energy or net ATP yield) and these approaches are well supported by studies in animals. By contrast, the theoretical basis of ME is totally flawed. ME incompletely represents the energy balance equation, with substantial energy losses in a missing term. In using NME factors an account is made of frequent over-approximations by the ME system, up to 25 % of the NME for individual foods among 1189 foods in British tables, particularly low-energy-density traditional foods. A new simple general factor system is possible based on NME, yet the minimal experimental methodology is no more than that required for ME. By accounting for unavailable carbohydrate the new factor system appears as specific to foods as the USA's food-specific Atwater system, while it is more representative of energy supply from food components. The NME content of foods is readily calculable as the sum from fat (37 kJ/g), protein (13 kJ/g), available carbohydrate (16 kJ/g), fully-fermentable carbohydrate (8 kJ/g), alcohol (26 kJ/g) and other components. Obstacles to the implementation of NME appear to be subjective and minor. In conclusion, the ME standard is at best an approximate surrogate for NME, and inadequately approximates food energy values for the purpose of informing the consumer about the impact on energy balance of the energy supply for equal intake of individual foods. NME is superior to ME for nutrition labelling and other purposes.

Tolerance of low-digestible carbohydrates: a general view

The tolerance of low-digestible carbohydrates (LDCs) may be measured as the potential to cause abdominal symptoms and laxation. Tolerance of any one LDC is determined by its concentration in the food product eaten, the amount of the food product eaten, the frequency of eating the food and the consumption of other foods (increasing tolerance) and water decreasing tolerance). Added to these, individuals vary considerably in their response to low-digestible carbohydrates in the reporting of gastrointestinal symptoms. A precise maximum no-response dose is sometimes difficult to obtain because some dose--response curves are distinctly sigmoidal. Food regulators hoping to set a trigger level at which laxation may occur have been unable to take account of all these factors because the necessary information matrices are not available for any one LDC. Nevertheless analysis of the data shows consistent trends and for circumstances when food is consumed throughout the day it now seems feasible to assign specific tolerances to specific low-digestible carbohydrates, especially the polyols for which most is known. The method by which the no-effect dose or laxative threshold is expressed is critical to its application to individual foods.

Evidence for altered control of glucose disposal after total colectomy

Colonic fermentation of organic matter to short-chain fatty acids has been implicated in the improvement in insulin sensitivity achieved by feeding diets rich in complex carbohydrates. The present study assessed the potential role of the colon in determining postprandial glucose kinetics. Metabolic responses to a complex-carbohydrate test meal were determined in conjunction with a primed continuous infusion of D-[6,6-2H]glucose in a group of ileostomists and sex-matched controls. Glucose disposal (GD) was computed using Steele's (1959) non-steady-state kinetics on a single compartment model. Insulin sensitivity was derived using cumulative GD as the dependent variable, and time and the integrated insulin concentration as independent variables. The ileostomist group had a significantly higher postprandial plasma insulin concentration ( P = 0.034) compared with the control group, but no difference in the plasma glucose concentration. Total GD was similar in each group, although the insulin-dependent GD was substantially lower in the ileostomists (0.46 v. 0.13 mg glucose/min per pmol, P = 0.015). The ileostomist group also showed a 50% lower rate of glucose oxidation in the postprandial period (p = 0.005), although the rate of non-oxidative GD was not significantly affected. The present study indicates that loss of the colon is associated with several characteristics of the insulin resistance syndrome, and favours a view that the colon has a role in the control of postprandial glucose.

The influence of the colon on postprandial glucagon-like peptide 1 (7-36) amide concentration in man

Glucagon-like peptide (7-36) amide (GLP-1) is an incretin hormone of the enteroinsular axis released rapidly after meals despite the fact that GLP-1 secreting cells (L-cells) occur predominantly in the distal gut. The importance of these colonic L-cells for postprandial GLP-1 was determined in healthy control subjects and in ileostomy patients with minimal small bowel resection (<5 cm). Subjects were fed a high complex carbohydrate test meal (15.3 g starch) followed by two carbohydrate-free, high fat test meals (25 g and 48.7 g fat respectively). Circulating levels of glucose, insulin, glucagon, glucose insulinotrophic peptide (GIP) and GLP-1 were measured over a 9-h postprandial period. For both subject groups the complex carbohydrate test meal failed to elicit a rise in either GIP or GLP-1. However, both hormones were elevated after the fat load although the GLP-1 concentration was significantly reduced in the ileostomist group when compared with controls (P=0.02). Associated with this reduction in circulating GLP-1 was an elevation in glucagon concentration (P=0.012) and a secondary rise in the plasma glucose concentration (P=0.006). These results suggest that the loss of colonic endocrine tissue is an important determinant in the postprandial GLP-1 concentration. Ileostomists should not be assumed to have normal enteroinsular function as the colon appears to have an important role in postprandial metabolism.

Energy balance and thermogenesis in rats consuming nonstarch polysaccharides of various fermentabilities

BACKGROUND

The equivalents of dietary protein, fat, and available carbohydrate as fuels for maintenance (kJ apparent metabolizable energy/kJ maintenance requirement) are known from classical experiments and are similar across species; that for nonstarch polysaccharide (NSP) is undetermined.

OBJECTIVES

Our objectives were to determine the energy equivalent of NSP and the thermic responses to NSP.

DESIGN

In a randomized block design, 120 rats were treated in groups of 10 for 28 d with a basal diet (control) supplemented with starch and 10 different NSP treatments in amounts between 38 and 92 g/kg basal diet. Cellulose and starch were references. Thermic responses, deduced from body-composition changes and modeling of energy disposition, and energy and substrate excretion were determined.

RESULTS

NSP had fermentabilities between 0.01 and 0.93 g/g intake. Fermentability, partial digestible energy, and net metabolizable energy values of NSP were closely related. Generally, 51% of apparent metabolizable energy from NSP (fermentable gross energy) met maintenance requirements. Diet (energy)-induced thermogenesis (DIT) was evident from whole diets. Fermentable NSP supplied net metabolizable energy and caused DIT. After DIT and fermentation were accounted for, NSP-induced thermogenesis was generally -2+/-4% (x+/-SEM) of gross NSP energy, except for an outlying pectic preparation, which was 33% (P< 0.1).

CONCLUSIONS

The energy equivalent of NSP was 196 (100/51) kJ/kJ, compared with 128, 105, and 100 for protein, fat, and glucose, respectively, from the classical experiments. With the exception of pectic NSP, NSP does not induce thermogenesis in excess of that associated with DIT and fermentation.

Energy expenditure and net substrate utilization in men ingesting usual and high amounts of nonstarch polysaccharide

BACKGROUND

Diets intrinsically high in nonstarch polysaccharides (NSPs) are frequently advised for body weight regulation and health, but the consequences for energy expenditure and fuel selection are undetermined.

OBJECTIVE

We determined whether energy expenditure and fuel selection differ when men consume a diet intrinsically higher in NSP than a usual mixed diet.

DESIGN

A randomized crossover design was used in which 12 healthy men were fed a maintenance diet for approximately 3 wk in a metabolic suite. By judicial choice of food exchanges, the usual- and high-NSP diets were similar in protein, fat, and carbohydrate contents. Twenty-four-hour, indirect, open-circuit calorimetry was performed, including measurements of total hydrogen gas and methane. Participants were weight stable (within 2 kg for 3 wk), entered an 11-m3 calorimetry chamber for 36 h with measurements taken in the last 24 h, and underwent a strictly controlled program of moderate physical activity (1.3 x basal metabolic rate).

RESULTS

The mean total 24-h energy expenditure and percentages from protein, fat, and carbohydrate metabolism were 10 MJ/d and 16%, 35%, and 48%, respectively. Differences (mean+/-SEM) between the 2 diets were only -0.005+/-0.130 MJ/d, -0.3+/-1.3%, -0.2+/-2.0%, and 0.6+/-2.2%, respectively, and were nonsignificant (P> 0.2).

CONCLUSIONS

There was no thermogenic response to the high-NSP diet, which would be advantageous for body weight control, and no short-term influence on body composition, as may be judged from a lack of change in protein, fat, or carbohydrate metabolism.

Simultaneous time-varying systemic appearance of oral and hepatic glucose in adults monitored with stable isotopes

The rates (and extent) of appearance of glucose in arterialized plasma from an oral glucose load and from liver (RaO, RaH) can be estimated in humans using radioisotopes, but estimates vary among laboratories. We investigated the use of stable isotopes and undertook 22 primed intravenous infusions of D-[6,6-2H2]glucose with an oral load including D-[13C6]glucose in healthy humans. The effective glucose pool volume (VS) had a lower limit of 230 ml/kg body weight (cf. 130 ml/kg commonly assumed). This VS in Steele's one-compartment model of glucose kinetics gave a systemic appearance from a 50-g oral glucose load per 70 kg body weight of 96 +/- 3% of that ingested, which compared with a theoretical value of approximately 95%. Mari's two-compartment model gave 100 +/- 3%. The two models gave practically identical RaO and RaH at each point in time and a plateau in the cumulative RaO when absorption was complete. Less than 3% of 13C was recycled to [13C3]glucose, suggesting that recycling errors were practically negligible in this study. Causes of variation among laboratories are identified. We conclude that stable isotopes provide a reliable and safe alternative to radioactive isotopes in these studies.

Splanchnic retention of intraduodenal and intrajejunal glucose in healthy adults

Estimates of the spanchnic retention and appearance in the systemic circulation of orally administered glucose vary among laboratories even after recently identified sources of error have been accounted for [Livesey, G., P. D. G. Wilson, J. R. Dainty, J. C. Brown, R. M. Faulks, M. A. Roe, T. A. Newman, J. Eagles, F. A. Mellon, and R. Greenwood. Am. J. Physiol. 275 (Endocrinol. Metab. 38): E717-E728, 1998]. We questioned whether, in healthy humans, D-glucose delivered intraluminally to the midjejunum appeared systemically as extensively as that delivered intraduodenally. Subjects were infused over a period of 90 min with 50 g of glucose in 1 liter of isotonic saline (incorporating 0.5 g D-[13C6]glucose) per 70 kg of body weight. Infusions were via enteral tubes terminating approximately 15 and 100 cm postpylorus. The systemic appearance of glucose was monitored by means of a primed-continuous intravenous infusion of D-[6,6-2H2]glucose. Whereas 98 +/- 2% (n = 7) of the duodenally infused glucose appeared in the systemic circulation, only 35 +/- 9% (n = 7) of midjejunally infused glucose did so, implying that 65 +/- 9% was retained in the splanchnic bed. Either glucose was less efficiently absorbed at the midintestinal site or hepatic glucose sequestration was increased 10-fold, or both. The proximal intestine plays a key role in the delivery of glucose to the systemic circulation, and the distal intestine potentially delivers more glucose to the liver.

Metabolizable energy of high non-starch polysaccharide-maintenance and weight-reducing diets in men: experimental appraisal of assessment systems

We have examined the reliability of several food energy assessment systems for healthy men. The predictions of metabolizable energies were compared with determinations made in energy balance studies with three maintenance diets (12 MJ/d); one of the diets was moderate in non-starch polysaccharide (NSP; 2.1% of gross energy) and two were higher in NSP (3.5-4.6% of gross energy). A fourth diet was a submaintenance (6 MJ/d) high NSP (7% gross energy) diet. Discrepancies between the different food energy assessment systems and the determined metabolizable energy values ranged between 0 and 15%. With the maintenance diets, the Atwater specific factor system had errors generally within 6% of the determined value and a limit of agreement (bias +2SD) for diets of 10%. This accuracy compares with errors of 2% for both the originally published assessment of this system and a more recent general formula; both systems were without bias with increasing NSP content of the diets but the latter had limits of agreement within 3%. In contrast, the Atwater general, the European general and a recent FDA general formula showed increasing bias with increasing NSP intake. All of the general energy assessment systems overpredicted metabolizable energy from the high NSP submaintenance diet, which shows that even the least biased and most reliable energy assessment system that we identified applies to maintenance diets only; thus a correction has to be made for submaintenance diets.

Is the rat a suitable model for humans on studies of cereal digestion?

OBJECTIVES

To compare the extent of starch digestion from barley flake and flour in humans and rat. Is the rat a useful model?

DESIGN

Four healthy male ileostomy volunteers consumed, in random order, flapjacks containing flaked barley on one occasion and barley flour on another. Ileostomy fluid was collected for analysis of starch and nonstrach polysaccharide (NSP) hourly for 12 h. The ratio of starch to NSP in ileostomy fluid was compared with that in the terminal ileum of eight randomly selected male rats consuming the flaked barley flapjack and another eight rats consuming the barley flour flapjack.

RESULTS

After consuming flaked barley the terminal ileal starch-NSP ratio was 20 times lower in rats than in ileostomists and was 0.04 (s.e.m. 0.006) g/g compared with 0.89 (s.e.m. 0.05) g/g respectively. By contrast the starch-NSP ratio was very low after consuming the barley flour in both species; however the ratio was still lower in rats than in ileostomists and was 0.02 (s.e.m. 0.01) g/g compared with 0.05 (s.e.m. 0.01) g/g respectively. Using the NSP as an indigestible marker, starch from barley flake that resisted small intestinal digestion was calculated to be 0.7 (s.e.m. 0.02)% in rats compared with a higher value of 17 (s.e.m. 1)% in ileostomists.

CONCLUSION

The cell walls of barley flake limit the extent of starch digestion in humans but in rats this appears not to be a limiting factor. In the present instance it was not possible to extrapolate results from rats to humans.

Circadian patterns of total 24-h hydrogen and methane excretion in humans ingesting nonstarch polysaccharide (NSP) diets and the implications for indirect calorimetric and D2 18O methodologies

OBJECTIVES

To quantify the relationship between substrate fermentation and total 24-h H2 and CH4 excretion on mixed diets and to assess errors incurred in the calculation of energy expenditure and fuel selection by the exclusion of these gases from standard calculations.

DESIGN AND SUBJECTS

Twelve healthy, lean men were studied over two consecutive dietary periods of 3 weeks. Measurements of total H2 and CH4 excretion were made during 24h within a whole body calorimeter. Subjects were fed a diet containing 16 g or 38 g of nonstarch polysaccharide (NSP) and 16 g or 19 g resistant starch (RS). Colonic fermentation was measured by balance techniques during the two dietary treatments.

RESULTS

There was an inverse non-linear relationship between H2 and CH4 excretion on both diets (r2 = 0.53; P < 0.001), but absolute excretion did not increase significantly as the intake of NSP and RS (from 28 +/- 3 and 48 +/- 4 g/day) increased. No relationship was found between daytime and 24-h measurements of H2 and CH4. H2 and CH4 excretion introduces an error of less than 0.2% and 1% in calculations of energy expenditure and CO2 production from the standard human equations used in indirect calorimetric (IC) and doubly labelled water (DLW) methodologies respectively, and less than 2% in fuel utilisation calculated as % non-protein energy expenditure from IC.

CONCLUSIONS

This study provides evidence that neither daytime nor total 24-h rates of H2 or CH4 excretion accurately predict degree of fermentation of NSP+RS in either individual subjects or groups of subjects, probably because of changes in the stochiometry of the fermentation process.

D-tagatose is a bulk sweetener with zero energy determined in rats

The ketohexose D-tagatose is readily oxidized but contributes poorly to lipid deposition. We therefore examined whether this sugar contributes to energy requirements by determining its net metabolizable energy value in rats. All substrate-induced energy losses from D-tagatose, with sucrose as reference standard, were determined as a single value accounting for the sum of the energy losses to feces, urine, gaseous hydrogen and methane and substrate-induced thermogenesis. A randomized parallel design involving two treatment periods (adaptation to D-tagatose and subsequent energy balance) and two control groups (to control for treatment effects in each period) was used. Rats consumed 1.8 g test carbohydrate daily as a supplement to a basal diet for a 40- or 41-d balance period after prior adaptation for 21 d. Growth, protein and lipid deposition were unaffected by supplementary gross energy intake from D-tagatose compared with an unsupplemented control, but sucrose significantly (P < 0.05) increased all three. Based on the changes induced in protein and fat gain during the balance period it was calculated that D-tagatose contributed -3 +/- 14% of its heat of combustion to net metabolizable energy, and therefore this ketohexose effectively has a zero energy value. D-Tagatose would potentially be helpful in body weight control, especially in diabetic subjects because of its antidiabetogenic effects.

Effects of the type and level of dietary fibre supplements on nitrogen retention and excretion patterns

The hypothesis was tested that fermentable dietary fibre (DF) sources elevate faecal N excretion at the expense of urinary N without affecting N retention. DF that substantially increase fermentation (pectin, sugarbeet and soya bran) or are poorly fermented (crystalline cellulose and maize bran) were fed as supplements to a basal DF-free diet at three dose levels: 0, 50 and 100 g supplement/kg basal diet. The diets were fed to juvenile male Wistar rats for 2 weeks before a 7 d period when faeces and urine were collected. Faecal excretion of N was significantly increased, dose-dependently, by all DF supplements and was positively correlated to faecal bulking. Urinary excretion of N was lower at the high doses of the DF supplements but reached significance only with the highly fermentable (0.68) sugarbeet-supplemented diets. Regression analysis showed that the major part (0.75) of the increase in faecal N excretion due to DF supplementation was balanced by a reduction in urinary excretion; N retention was therefore, at the dose levels used, only affected to a small extent. Only in the maize-bran-supplemented diets were the reductions in N retention significant. The shift in N excretion from urine to faeces can be explained largely by the degree of microbial fermentation in the large intestine caused by the addition of DF supplements and emphasizes the modifying role that certain DF supplements may have on the enterohepatic cycle of N. Possible implications of these findings for patients with liver or renal failure or for conditions when the intake of dietary protein is marginal are discussed.

Whole body metabolism is not restricted to D-sugars because energy metabolism of L-sugars fits a computational model in rats

Can L-sugars contribute to whole body energy metabolism? Energy balance studies were undertaken in rats fed L-sugars at a rate of 10 g/100 g basal diet. Partial metabolizable energy values (MEVp) during the last 28 of 56 d while consuming the L-sugar diets showed that L-glucose contributed no energy (0.3 +/- 0.9 kJ/g, P > 0.5, mean +/- SEM), whereas L-fructose and L-glucose contributed 6.9 +/- 0.9 (P < 0.001) and 8.8 +/- 1.8 (P < 0.001) kJ/g, respectively. Over periods from 0 to 28 and 0 to 56 d of L-sugar treatment, measurements were made of energy intake, the average lean mass of animals, fat and lean mass deposition. Using these measurements and a computational model, estimates were made of each supplement's net metabolizable energy value for maintenance (NEVs). These estimates confirmed the supply of energy from L-fructose, with NEVs of 5.6 +/- 3.2 (P < 0.32) and 6.1 +/- 1.7 (P < 0.01) kJ/g over 28 and 56 d, respectively, and L-gulose with NEVs of 10.2 +/- 3.4 (P < 0.01) and 11.4 +/- 1.8 (P < 0.001) kJ/g over 28 and 56 d, respectively. A lack of energy supply from L-glucose was confirmed with NEVs of -2.6 +/- 3.2 (P > 0.5) and 0.0 +/- 1.6 (P > 0.5) kJ/g over 28 and 56 d, respectively. As reference substrates, sucrose and cellulose gave expected energy values in all determinations. Methods comparison analysis showed no differences between MEVp and NEVS. The assimilation of energy from L-fructose and L-gulose is probably via large bowl microorganisms. These observations show that L-gulose and L-fructose contribute to whole body energy metabolism while L-glucose does not.

Metabolizable energy of macronutrients

I describe recent advances in assessing the amount of energy available from diets of varied composition. Empirical models of food energy prediction have grown in preference to factorial models over the past 20 y and knowledge of the quantitative aspects of energy salvage in the colon have modified our thoughts on how to best calculate food energy values. In contrast, food regulatory practices have been limited mostly to changes in the way in which carbohydrate is defined or measured and there has been little change in the basic approach to energy evaluation or the energy conversion factors used with food components. The empirical systems have advantages over factorial models of food energy assessment and it is proposed that such empirical systems should be used in food regulations, food tables and databases, and in nutritional studies in which knowledge of metabolizable energy intake is desirable but cannot be determined directly.

Determination of digestible energy values and fermentabilities of dietary fibre supplements: a European interlaboratory study in vivo

The performance of methods to determine energy conversion factors for dietary fibre (DF) supplements and fermentability (D) values of their non-starch polysaccharides (NSP) was investigated. Heats of combustion, digestible energy (DE) and D values were determined on five DF supplements in five European laboratories on five separate occasions. In each instance the DF supplements were fed to juvenile male Wistar rats at two doses, 50 and 100 g/kg basal diet, for 3 weeks with food and faeces collected in the 3rd week. Among-laboratory variations in heats of combustion (delta Hc) were < 2%. DE values (kJ/g dry weight) at the upper and lower doses respectively were: 10.4 and 9.9 for a high-methoxyl apple pectin, 9.5 and 9.4 for a sugar-beet DF supplement, 12.2 and 12.7 for soyabean DF supplement, 3.8 and 4.0 for maize bran, and 0.3 and 0.3 for Solka-floc cellulose. Variations among laboratories, among occasions and among animals were < 1, < 2 and < 2.5 kJ/g respectively. The among-occasion: among-laboratory variance ratio for DE was 0.5, suggesting the method performed equally well in all laboratories. There was no evidence of learning of fatigue or fatigue in the performance of the method. D values were also independent of dose and at the high and lower doses were: pectin 0.92 and 0.95, sugar-beet NSP 0.68 and 0.68, soyabean NSP 0.86 and 0.88, maize bran 0.17 and 0.18, cellulose 0.07 and 0.06. Among-laboratory variance tended to increase with decreasing fermentability and ranged from 0.03 to 0.18. The DE and D data were not significantly different from a previously proposed relationship DE = 0.7 x delta Hc x D, where delta Hc is the heat of combustion of the supplement. We conclude that while the among-laboratory variation in the D of difficult-to-ferment NSP is too large for the reliable prediction of energy value the method for the direction determination of DE is both reproducible and repeatable, that DE is independent of dosage of DF supplement up to 100 g/kg diet, and that it is safe to discriminate between energy values with a precision of 3 kJ/g. The conversion of both DE and D to net metabolizable energy for the purpose of food labelling, tables and databases is described.

Influence of the physical form of barley grain on the digestion of its starch in the human small intestine and implications for health

It has been suggested that incomplete digestion of cereal starch explains the low energy values of certain cereals of large particle size. We used human subjects with ileostomies to investigate the digestion of barley and to determine whether the physical form of barley affects stomal excretion of starch, glucooligosaccharides, nitrogen, fat, and calculated energy. Only 2 +/- 1% of starch remained undigested after finely milled barley was eaten, but after flaked barley was eaten 17 +/- 1% resisted digestion, partly as oligosaccharides (G1-G10) but largely as intact unpitted starch granules bound by intact cell walls. The calculated energy excretion from the stoma was three times higher after flaked than after milled barley [51.5 decreasing to 15.3 kJ/g nonstarch polysaccharide (NSP, P < 0.001]. NSP, starch, and fat made almost equal contributions to the higher energy excretion. It is concluded that possibly the botanical source of cereals and certainly processing, other than retrogradation of the starch, are important determinants of starch digestibility and energy value. Possible clinical implications are introduced.

Energy balance and expenditure while consuming guar gum at various fat intakes and ambient temperatures

We devised a new model to evaluate whether dietary composition affects whole-body energy metabolism in rats. Dietary guar gum, but not corn starch, elevates both fecal energy excretion and energy expenditure. Neither ambient temperature (21 vs 28 degrees C) nor fat content of the basal diet (15% vs 33% metabolizable energy) has a marked effect on guar gum's thermogenic stimulus. Starch and guar gum each contribute 17.5 kJ/g to gross energy intake. This energy is fully available from the starch but guar gum contributes only 9-13 kJ/g to digestible energy intake on the low-fat diet (15% of energy), only 6 kJ/g to digestible energy on the higher-fat diet (33% of energy) (because of a fat-gum interaction), decreases urinary energy loss by 1.4-2.7 kJ/g gum, and elevates energy expenditure by 11.6-14.8 kJ/g. The thermogenic effect of the guar gum is highly reproducible. Several potential mechanisms to explain such elevated energy expenditure are considered. The model used to quantify the thermogenic stimulus is highly sensitive and could be applied to the trial of potentially thermogenic drugs as well as dietary ingredients.

'Determination' of sugar alcohol and Polydextrose absorption in humans by the breath hydrogen (H2) technique: the stoichiometry of hydrogen production and the interaction between carbohydrates assessed in vivo and in vitro

The production of hydrogen from substrates and substrate mixture of sugar alcohols and Polydextrose was determined, both in vivo using the breath hydrogen test, and in vitro, using human faecal microorganisms in anaerobic culture. One objective was to test a previous assumption that the stoichiometry of hydrogen production from different alternative carbohydrates is similar. Another objective was to discover whether hydrogen responses from mixtures of substrates were simply additive, or whether interactions occurred. The breath tests were performed in a 10 subject x 10 substrate factorial design with substrates and substrate mixtures (5-11 g) administered in 42 g chocolate confectionery. Incorporation of the alternative carbohydrates lactitol (L), Isomalt (I) and Polydextrose (P) into otherwise conventional confectionery increased breath hydrogen production by approximately 112, 73 and 11%/g respectively. There was no interaction between L and I or between P and I, but a combination of L and P approximately doubled the breath hydrogen anticipated from their individual contributions (P < 0.05). Anaerobic cultures showed a sixfold range in the efficiency of converting individual substrates and mixtures to hydrogen gas (0.003-0.018 kJ H2 per kJ carbohydrate). The positive interaction between L and P, and the lack of interaction between L and I, and between P and I, found in vivo were reproduced in vitro. The work showed that interpretation of the hydrogen breath test is confounded by differing stoichiometries for hydrogen production, by interaction between substrates and by an uncertain extent to which small intestinal hydrolysis yielding species with a fermentation stoichiometry that differs from the parent substrate.

Influence of mild cold on the components of 24 hour thermogenesis in rats

1. The influence of two weeks' acclimation to either 28 degrees C (thermal neutrality) or 21 degrees C (mild cold) on 24 h heat production and motor activity has been investigated in male Wistar rats. Food intake was controlled and provided as a single meal of approximately 170 kJ per day. Mathematical modelling was used to relate metabolic rate to measured movement and time of day. 2. For animals at thermal neutrality it was clear that metabolic rate increased during periods of substantial measured movement and returned to baseline during periods of minimal activity. Total heat production could therefore be divided into two components: underlying and movement-induced thermogenesis. 3. At 21 degrees C, a more complex model was needed. During periods of substantial activity, the relation between metabolic rate and movement was similar to that at 28 degrees C and total heat production could be divided into the same two components of underlying and movement-induced thermogenesis. However, during periods of prolonged inactivity, a different model was required, which included a component of extra metabolic activity, termed supplementary thermogenesis. By fitting this model to data at 28 and 21 degrees C, it was possible to partition 24 h heat production into the three possible sources of underlying, movement-induced and supplementary thermogenesis. 4. Total 24 h heat production was approximately 25% higher for rats at 21 compared with 28 degrees C (P less than 0.01) and underlying thermogenesis was approximately 20% higher for those in the mild cold (P less than 0.01). Measured movement was significantly reduced in the mild cold (P less than 0.05) although it was energetically less efficient since there was no difference in movement-induced thermogenesis, which accounted for 18 and 15% of total heat production at 28 and 21 degrees C respectively. Supplementary thermogenesis was observed only in the mild cold and it accounted for approximately 6% of 24 h heat production, while the peak value accounted for 20% of total heat production. Circadian variations in thermogenesis were also different at 28 compared with 21 degrees C. Possible mechanisms accounting for the components of underlying thermogenesis and supplementary thermogenesis are discussed.

Energy values and energy balance in rats fed on supplements of guar gum or cellulose

In five experiments where guar gum (GG) or Solka-floc cellulose (SF) supplemented a semi-synthetic diet (100 g/kg) for male Wistar rats at 21 degrees, it was found that GG acutely depressed both ad lib. and meal-fed food intakes by 40-50%. The effect was temporary, with the GG having no effects on food intake in the longer term. Dietary energy balance over 28 d with animals fed on equal amounts of a basal ration showed partial digestible energy values, calculated from the intake and faecal loss of energy, for the supplements which averaged 0 kJ/g SF and 10 kJ/g GG, so that GG contributed substantial amounts of absorbable energy. Despite this additional energy from GG, there was very often no additional gain of body fat. Rather, in some experiments, fat deposition was actually decreased by supplementation with GG. GG is inferred to have a putative thermogenic effect which is often greater than the energy it supplies. This effect occurred to a greater extent in circumstances associated with a higher 'energy status', indicated by higher efficiencies of conversion of gross dietary energy to retained body energy, higher fat:lean tissue deposition ratios and the occurrence of larger deposits at the epididymal fat pad site. There was some evidence that deposition at this site was more extensively affected by GG than deposition of fat in the body as a whole. Possible implications of the present findings are discussed.

Calculating the energy values of foods: towards new empirical formulae based on diets with varied intakes of unavailable complex carbohydrates

A statistical analysis has been made by generalised linear regression of the relationship between the availability of digestible energy (DE) and the intakes of dietary gross energy (E) and of dietary 'fibre' or unavailable complex carbohydrate (U), for published observations on 43 human diets with varied intakes and sources of U (2-93 g daily) and varied intakes of E (7598-15104 kJ or 1816-3610 kcal daily). Simple formulae were devised also for the availability of metabolizable energy (ME) assuming urinary energy losses of 30 kJ or 7 kcal/g nitrogen (N) intake. For the calculation of DE and ME in mixed human diets with accuracy greater than obtained with previously published methods, the following linear formulae are proposed and their limitations discussed: DE (kJ) = 0.96E(kJ)-9U(g) ME (kJ) = 0.96E(kJ)-9U(g)-30N(g) DE (kcal) = 0.96E(kcal)-2U(g) ME (kcal) = 0.96E(kcal)-2U(g)-7N(g) The equations devised are used to identify atypical observations on faecal energy excretion, and their possible causes are considered. In particular diets high in whole-grain cereal cause faecal energy losses greater than predicted by these equations which is possibly explained by more carbohydrate reaching the colon than expected from current compositional analytical methods.

The biological effects and digestible energy value of a sugar-beet fibre preparation in the rat

A sugar-beet fibre preparation (SBF) was incorporated into a semi-synthetic rat diet at a level of 100 g/kg. The material caused no feed aversion, and gain of live weight was unimpaired. SBF appeared to be slightly more fermentable than wheat bran and only marginally less effective as a faecal bulking agent when compared with equalized intakes of non-starch polysaccharide. SBF did not stimulate mucosal cell turnover in the small intestine. Some enlargement of the caecum was observed in animals given SBF, but it was no greater than that of animals given wheat bran. Animals given SBF had a lower serum cholesterol concentration than both the fibre-free controls and those given wheat bran. This hypocholesterolaemic effect was less than that of guar gum however. The partial digestibility of energy for SBF was 0.64, and its partial digestible energy value was 11.3 kJ (2.7 kcal)/g. The partial digestibility of energy for non-starch polysaccharide in SBF was estimated to be 0.53 and its partial digestible energy value was 9.1 kJ (2.2 kcal)/g. This value was not significantly different from that expected for unavailable carbohydrate in mixed human diets.

Energy balance and energy values of alpha-amylase (EC 3.2.1.1)-resistant maize and pea (Pisum sativum) starches in the rat

Apparent and partial digestible energy values for alpha-amylase (EC 3.2.1.1)-resistant, retrograde starches, isolated from cooked maize and pea starches (RMS and RPS respectively), were determined in male Wistar rats (about 180 g) during a 28-29 d balance period with ten animals per treatment. The starches were provided as supplements (100 g/kg diet) to a semi-synthetic basal diet (B), and their effects on the apparent digestibilities of nitrogen and fat, and on gains of live weight, fat and lean tissue were examined. Diet B alone was the control; sucrose (Su) and Solka-floc cellulose (SFC) were also examined for reference. Apparent digestibilities for Su, SFC, RMS and RPS were 1.0, 0.16, 0.98 and 0.89 respectively. Whereas the apparent digestibilities of gross energy, N and fat in the diet were unaffected by supplementation with Su, each was decreased by supplementation with SFC, RMS and RPS. Partial digestible energy values calculated from the intakes and faecal losses of energy in the basal and supplemented diets were 15, 12.4 and 0.8 kJ/g for RMS, RPS and SFC respectively. These values were smaller than corresponding apparent digestible energy values calculated from the apparent digestibility of the supplement and its gross energy value. Only the Su and starch supplements increased the intake of apparent digestible energy and the gain of live weight. Both starches and Su increased total energy (and fat) deposition to almost similar extents. It is concluded that the resistant starches contribute significant dietary energy, enhance growth and elevate fat deposition to extents almost similar to Su.

Energy values of unavailable carbohydrate and diets: an inquiry and analysis

To determine digestible-energy values (DEVs) for unavailable carbohydrate (UC), an analysis was made of published data on 29 human diets with UC from several sources and intakes ranging from 4 to 93 g/d. A distinction was made between apparent DEVs, derived from UC intake and fecal loss, and partial DEVs, derived from energy intake and fecal loss. By use of a proposed calculation, partial DEVs ranged from -20 to +10 kJ/g (-4.8 to +2.4 kcal/g) in different diets; all values were below the corresponding apparent DEVs. Factors explaining this range, including analytical problems, are considered. Rather than finding discrepancies, both the partial DEVs and the energy available from the whole diet were found to be related to the apparent digestibility of UC (r = 0.88), the proportion of the diet estimated as UC (r = 0.73), and both combined (r = 0.98). Several food-energy evaluation systems are also assessed for accuracy and the implications of these observations for food-energy evaluation are discussed.

Utilization of alpha-amylase (EC 3.2.1.1) resistant maize and pea (Pisum sativum) starch in the rat

1. The extent of utilization of alpha-amylase (EC 3.2.1.1)-resistant retrograded starches in vivo was assessed in male Wistar rats (about 100 g body-weight). All animals were given a fibre-free semi-synthetic basal diet (SS) containing sucrose as the only carbohydrate source, ad lib., for 13 d. On day 14, after an overnight fast, rats were allocated to one of five dietary treatments (n 30):1, fibre-free basal SS diet; 2-5, basal SS diet supplemented with 100 g sucrose, Solka floc (cellulose), resistant maize starch (RCS) or resistant pea (Pisum sativum) starch (RPS)/kg diet. Animals allocated to each dietary treatment were divided into three groups of ten rats which were given the appropriate diet for 8 or 9, 17 or 18 and 29 or 30 d (8/9, 17/18 or 29/30 d groups respectively). Rats were fed on 12 g diet/d (treatment 1) or 13.2 g diet/d (treatments 2-5) for the first 20 d, and 14 or 15.4 g/d respectively until the end of the experiment. Rats fed on the supplemented basal diets were thus given 10% more food to allow for the addition of the test carbohydrate. Faecal carbohydrate excretion was determined at intervals in the 17/18 d groups. At the end of each experimental period animals were killed after consuming their daily food ration and small intestinal length, weight of caecal and ileal contents and tissue, and pH of caecal contents measured. The amount of carbohydrate in the caecal and ileal contents from the 8/9 and 17/18 d groups was determined. 2. Weights of ileal and caecal contents, caecal tissue and faecal output were significantly greater at all time points for rats fed on the resistant starches compared with those fed on basal and sucrose-supplemented diets. Values were higher for RPS-fed rats than for RCS-fed rats. 3. The quantity of carbohydrate recovered from ileal and caecal contents showed that both RCS and RPS were partially digested and absorbed as carbohydrate, but that RPS was digested to a lesser extent. 4. The concentration of carbohydrate decreased between the ileal and caecal sites when RPS and RCS were given but was essentially unchanged when cellulose was given. This is consistent with rapid fermentation of a fraction of these starches. 5. Faecal carbohydrate elimination in the 17/18 d groups fed on RCS and RPS declined with time, which suggested an adaptive response resulting in increased utilization of the starches. This adaptive response was slower in the RPS-fed rats than the RCS-fed rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Digestible energy value of gums in the rat--data on gum arabic

Male Wistar rats were fed a defined formula diet free from non-starch polysaccharides and either containing no additive or supplemented with gum arabic or cellulose or starch. Food and faeces were analysed by bomb calorimetry both to assess the effect of these substances on the apparent digestibility of dietary gross energy and to ascribe digestible energy values to the supplements. The former was not affected by starch and was decreased more by cellulose than by gum arabic. The energy values obtained were 17.4 +/- 0.4 kJ/g starch, 1.7 +/- 0.6 kJ/g cellulose and 14.7 +/- 0.5 kJ/g gum arabic. The latter is consistent with the high apparent digestibility of gum arabic in vivo and casts doubt on the validity of growth assay procedures that suggest a near-zero energy value for gum arabic.

Estimation of energy expenditure, net carbohydrate utilization, and net fat oxidation and synthesis by indirect calorimetry: evaluation of errors with special reference to the detailed composition of fuels

Sources of error in the interpretation of respiratory data are evaluated and reviewed with special reference to the detailed composition of foods. Estimates of fuel utilization or synthesis are 12-fold more sensitive to errors in the nonprotein respiratory quotient than is the heat equivalent of oxygen. Estimates of protein oxidation from nitrogen excretion can be in error from +14 to -39% of the true value. Heat equivalents of oxygen, respiratory quotients, and urinary nitrogen-to-oxygen conversion ratios are considered for 60 artificial and 101 conventional food proteins, 36 artificial and 125 conventional food fats, and the different carbohydrates contained in these foods. It is concluded that there is considerable uncertainty when the mix of fuels utilized is assessed accurately. Accuracy is best within 5% of the true values. This analysis is completed with descriptions of some physiological sources of error in an appendix.

Theory and validity of indirect calorimetry during net lipid synthesis

A critical examination is made of the validity of indirect calorimetry when the nonprotein respiratory quotient is greater than 1. The different published stoichiometries for lipogenesis from glucose are excluded as a source of uncertainty in the interpretation of gaseous exchange measurements. The validity of indirect calorimetry is proved independently by an algebraic approach which, in contrast to previous attempts, makes minimal assumptions about stoichiometries. Although equations relating the respiratory quotient to the heat equivalent of oxygen are found valid, there is uncertainty in using these equations to predict accurately carbohydrate utilization and fat oxidized or synthesized. Reference tables interrelating respiratory data, the heat equivalent of oxygen, and net fuel utilization or synthesis for specified fuels are provided. A suggested framework for calculating energy expenditure in terms of ATP gain is given as an appendix.

The metabolizable energy value of Polydextrose in a mixed diet fed to rats

The digestible energy (DE) and metabolizable energy (ME) values of a commercial Polydextrose product and the polymer that it contained were determined by metabolic energy balance in male Wistar rats and compared with values obtained by radiochemical balance using a radiochemical analogue. The energy values of the whole preparations and of the polymer fractions were estimated. In the energy-balance study of 6 d duration, 100 g maize starch/kg control diet were replaced by Polydextrose to provide a test diet. Polydextrose had no significant effects on food intake, body-weight gain, digestibility of nitrogen and N retention but significantly increased the water consumption to 143% of the control value (P less than 0.05) and the water content of fresh faecal pellets from 548 (SE 10) to 646 (SE 15) g/kg wet weight (P less than 0.01). By energy balance the DE in the Polydextrose product and in the polymer that it contained were 13.5 and 12.8 (SE 1.9) kJ/g respectively. The corresponding ME values were 12.7 and 12.1 (SE 1.8) kJ/g respectively. These values were higher (P less than 0.05) than the corresponding values obtained by the radiochemical balance procedure: DE 8.8 and 8.6 (SE 0.4) kJ/g polymer respectively and ME 8.0 and 7.8 (SE 0.5) kJ/g polymer respectively. These findings indicate relatively high energy values for Polydextrose by comparison with previously published values and illustrate a potential difficulty when using energy values obtained by certain radiochemical methods to estimate the energy values of a mixed diet given to rats. Several alternative explanations of the discrepancies are advanced.

The energy cost of triglyceride-fatty acid recycling in nonobese subjects after an overnight fast and four days of starvation

The basal blood glycerol concentration was determined and the rate of glycerol turnover was assessed by a nonradioactive infusion technique in six healthy nonobese adults after an overnight fast and again after four days of total starvation. Simultaneously, estimates of total energy expenditure and net fat oxidation were made from measurements of oxygen consumption, carbon dioxide production, and urinary nitrogen excretion. The data were combined to provide quantitative estimates of the activity of the triglyceride/fatty acid cycle. The basal concentration of glycerol in venous blood rose from a mean value of 54 +/- 8 mumol/L (SEM) before starvation to 154 +/- 5 mumol/L on day 4 of starvation. Glycerol turnover rates correlated well with the basal blood glycerol concentration (r = .95) and increased from a mean value of 115 +/- 17 mumol/min before starvation (equivalent to mobilization of about 3.95 kJ triglyceride/min) to 304 +/- 20 mumol/min (equivalent to mobilization of about 18.41 kJ/min). The estimated rate of net fat oxidation was 3.00 +/- 0.47 kJ/min before starvation and 4.00 +/- 0.14 kJ/min on day +4 of starvation. The rate of triglyceride energy recycling or rate of deposition of triglyceride energy into fat stores was calculated from the difference in the rate of fat energy mobilization and the rate of energy released during net fat oxidation. The values were found to be 0.94 +/- 0.26 kJ/min before starvation and 6.29 +/- 0.54 kJ/min on day +4 of starvation.(ABSTRACT TRUNCATED AT 250 WORDS)

Food energy values of dietary fibre components and decreased deposition of body fat

The effects of guar gum (GG) and solka floc cellulose (SF) on food intake, stomach emptying and body fat deposition in rats were assessed together with their food energy values. Voluntary food intake in meal-fed rats (ca 100 g, male Wistar kept at 21C) was depressed initially by 40 per cent on adding 10 g GG to 100 g fibre free (FF) semisynthetic diet. This effect was not sustained, intake doubling within 15 d. Similar effects were not observed with SF. In pair-fed animals neither GG nor SF affected the rate of stomach emptying after 28 days. Digestible energy (DE) intake per rat over the 28 days was GG diet = 4616, SF diet = 4410, FF diet = 4373 kJ but body fat was lower in GG (25%) and SF (16%) fed rats than in FF fed rats. The calculated DE values of the fibre components were 10.1 kJ/g GG and 1.5 kJ/g SF (sem 1 kJ/g). After including body fat into the equations for metabolizable energy, the energy values were -7.1 kJ/g GG and -4.8 kJ/g SF (sem 2 kJ/g). This is equivalent to an increased energy expenditure of 17.2 kJ/g GG and 6.3 kJ/g SF. For GG an increased intestinal mucosal mass and cell turnover explains part of the apparent increase in energy demand. Our overall conclusion is that under certain circumstances and with regard to fat deposition some fibre components can be attributed negative energy values.