Dietary carbohydrate deprivation increases 24-hour nitrogen excretion without affecting postabsorptive hepatic or whole body protein metabolism in healthy men

Resource allocation in mammalian systems

Cells execute biological functions to support phenotypes such as growth, migration, and secretion. Complementarily, each function of a cell has resource costs that constrain phenotype. Resource allocation by a cell allows it to manage these costs and optimize their phenotypes. In fact, the management of resource constraints (e.g., nutrient availability, bioenergetic capacity, and macromolecular machinery production) shape activity and ultimately impact phenotype. In mammalian systems, quantification of resource allocation provides important insights into higher-order multicellular functions; it shapes intercellular interactions and relays environmental cues for tissues to coordinate individual cells to overcome resource constraints and achieve population-level behavior. Furthermore, these constraints, objectives, and phenotypes are context-dependent, with cells adapting their behavior according to their microenvironment, resulting in distinct steady-states. This review will highlight the biological insights gained from probing resource allocation in mammalian cells and tissues.

Effects of energy loads on energy and nutrient absorption rates and gut microbiome in humans: A randomized crossover trial

OBJECTIVE

This study aimed to determine the effects of different energy loads on the gut microbiota composition and the rates of energy and nutrient excretion via feces and urine.

METHODS

A randomized crossover dietary intervention study was conducted with three dietary conditions: overfeeding (OF), control (CON), and underfeeding (UF). Ten healthy men were subjected to each condition for 8 days (4 days and 3 nights in nonlaboratory and laboratory settings each). The effects of dietary conditions on energy excretion rates via feces and urine were assessed using a bomb calorimeter.

RESULTS

Short-term energy loads dynamically altered the gut microbiota at the α-diversity (Shannon index), phylum, and genus levels (p < 0.05). Energy excretion rates via urine and urine plus feces decreased under OF more than under CON (urine -0.7%; p < 0.001, urine plus feces -1.9%; p = 0.049) and UF (urine -1.0%; p < 0.001, urine plus feces -2.1%; p = 0.031). However, energy excretion rates via feces did not differ between conditions.

CONCLUSIONS

Although short-term overfeeding dynamically altered the gut microbiota composition, the energy excretion rate via feces was unaffected. Energy excretion rates via urine and urine plus feces were lower under OF than under CON and UF conditions.

A ketogenic diet alters mTOR activity, systemic metabolism and potentially prevents collagen degradation associated with chronic alcohol consumption in mice

INTRODUCTION

A ketogenic diet (KD), which is a high fat, low carbohydrate diet has been shown to inhibit the mammalian target of rapamycin (mTOR) pathway and alter the redox state. Inhibition of the mTOR complex has been associated with the attenuation and alleviation of various metabolic and- inflammatory diseases such as neurodegeneration, diabetes, and metabolic syndrome. Various metabolic pathways and signalling mechanisms have been explored to assess the therapeutic potential of mTOR inhibition. However, chronic alcohol consumption has also been reported to alter mTOR activity, the cellular redox- and inflammatory state. Thus, a relevant question that remains is what effect chronic alcohol consumption would have on mTOR activity and overall metabolism during a KD-based intervention.

OBJECTIVES

The aim of this study was to evaluate the effect of alcohol and a KD on the phosphorylation of the mTORC1 target p70S6K, systemic metabolism as well as the redox- and inflammatory state in a mouse model.

METHODS

Mice were fed either a control diet with/without alcohol or a KD with/without alcohol for three weeks. After the dietary intervention, samples were collected and subjected towards western blot analysis, multi-platform metabolomics analysis and flow cytometry.

RESULTS

Mice fed a KD exhibited significant mTOR inhibition and reduction in growth rate. Alcohol consumption alone did not markedly alter mTOR activity or growth rate but moderately increased mTOR inhibition in mice fed a KD. In addition, metabolic profiling showed alteration of several metabolic pathways as well as the redox state following consumption of a KD and alcohol. A KD was also observed to potentially prevent bone loss and collagen degradation associated with chronic alcohol consumption, as indicated by hydroxyproline metabolism.

CONCLUSION

This study sheds light on the influence that a KD alongside alcohol intake can exert on not just mTOR, but also their effect on metabolic reprogramming and the redox state.

Physiopathology of Lifestyle Interventions in Non-Alcoholic Fatty Liver Disease (NAFLD)

Non-alcoholic fatty liver disease (NAFLD) is a major health problem, and its prevalence has increased in recent years. Diet and exercise interventions are the first-line treatment options, with weight loss via a hypocaloric diet being the most important therapeutic target in NAFLD. However, most NAFLD patients are not able to achieve such weight loss. Therefore, the requisite is the investigation of other effective therapeutic approaches. This review summarizes research on understanding complex pathophysiology underlying dietary approaches and exercise interventions with the potential to prevent and treat NAFLD.

Effect of a ketogenic diet on hepatic steatosis and hepatic mitochondrial metabolism in nonalcoholic fatty liver disease

Weight loss by ketogenic diet (KD) has gained popularity in management of nonalcoholic fatty liver disease (NAFLD). KD rapidly reverses NAFLD and insulin resistance despite increasing circulating nonesterified fatty acids (NEFA), the main substrate for synthesis of intrahepatic triglycerides (IHTG). To explore the underlying mechanism, we quantified hepatic mitochondrial fluxes and their regulators in humans by using positional isotopomer NMR tracer analysis. Ten overweight/obese subjects received stable isotope infusions of: [D7]glucose, [13C4]β-hydroxybutyrate and [3-13C]lactate before and after a 6-d KD. IHTG was determined by proton magnetic resonance spectroscopy (1H-MRS). The KD diet decreased IHTG by 31% in the face of a 3% decrease in body weight and decreased hepatic insulin resistance (-58%) despite an increase in NEFA concentrations (+35%). These changes were attributed to increased net hydrolysis of IHTG and partitioning of the resulting fatty acids toward ketogenesis (+232%) due to reductions in serum insulin concentrations (-53%) and hepatic citrate synthase flux (-38%), respectively. The former was attributed to decreased hepatic insulin resistance and the latter to increased hepatic mitochondrial redox state (+167%) and decreased plasma leptin (-45%) and triiodothyronine (-21%) concentrations. These data demonstrate heretofore undescribed adaptations underlying the reversal of NAFLD by KD: That is, markedly altered hepatic mitochondrial fluxes and redox state to promote ketogenesis rather than synthesis of IHTG.

The Effect of a Low GI Diet on Truncal Fat Mass and Glycated Hemoglobin in South Indians with Type 2 Diabetes-A Single Centre Randomized Prospective Study

Background: There has been no previous study that has investigated the effect of a low glycemic index (LGI) diet with local recipes of South Indian cuisine on the body fat composition using dual-energy X-ray absorptiometry (DXA). Truncal obesity has been associated with the risk of metabolic disorders and cardiovascular diseases. Aim: The aim of this study was to examine the effect of a low GI diet on glycemic control and body composition in people with type 2 diabetes in South India. Method: This was a prospective and randomized controlled study that was conducted over a period of 24 weeks. A total of 40 participants were recruited from the Department of Endocrinology and Diabetes Outpatient in Kerala, South India. All the patients had type 2 diabetes and were randomly assigned and given advice and instructions to follow either a low GI diet plan (n = 18) or their usual diet, which served as control (n = 18). The advice was reinforced throughout the study period. Dietary compliance was evaluated based on a 24 h dietary recall at weeks 3, 11, 12, 18, 23, and 24. The age of the subjects ranged from 35 to 65 years. Anthropometric, body composition, and cardio-metabolic parameters were measured according to standard procedures. T-tests were conducted to compare differences between intervention and control groups and the Pearson correlation coefficient was used to evaluate associations between the variables. Results: There were significant reductions (p < 0.05) in the low GI diet compared to the control group with respect to weight, body mass index (BMI), and triceps skinfold thickness. Similarly, significant reductions were observed in the low GI diet group with respect to region, total fat, android, and gynoid fat mass and the differences between the groups were significant at p < 0.05. There was also a positive correlation between BMI and android fat mass (r = 0.745), total fat mass (r = 0.661), total truncal mass (r = 0.821), and truncal fat (r = 0.707). There was a significant reduction in glycated hemoglobin in the low GI diet group compared to the control group at p < 0.05. Conclusion: This study has demonstrated that there was a significant reduction (p < 0.05) of truncal obesity and glycated hemoglobin in patients with type 2 diabetes on a local diet of South Indian cuisine with low GI compared with the control.

Albumin synthesis rates are not responsive to hyperglycemic hyperinsulinemia in postoperative patients

BACKGROUND

Insulin regulates albumin synthesis in vitro and in various experimental models. The current study was undertaken to determine the effects of a physiologic hyperinsulinemia on albumin synthesis in postoperative patients in whom plasma albumin concentrations are decreased.

METHODS

Studies were performed in postabsorptive patients after major abdominal operations. Mass spectrometry techniques were used to directly determine the incorporation rate of 1-[(13)C]-leucine into albumin. Consecutive blood samples were taken during a continuous isotope (D-Glc) infusion (0.16 µmol/kg/min). Isotopic enrichments were determined at baseline (period I) and after a 4-hour D-glucose (D-Glc) infusion at currently recommended rates (170 mg/kg/h, n = 10) or after infusion of saline (control group, n = 8) (period II).

RESULTS

After D-Glc infusion, plasma insulin concentrations increased significantly (period I, 6.6 ± 1.8 µU/mL; period II, 21.4 ± 2.1 µU/mL; P < .01). In contrast, plasma insulin concentration remained constant in control patients (period I, 3.8 ± 0.9 µU/mL(-1); period II, 5.9 ± 1.1 µU/mL; not significant vs period I, but P < .005 vs the corresponding value at the end of period II in the control group). Hyperinsulinemia was without effect on fractional albumin synthesis (period I, 12.8% ± 1.9%/d; period II, 11.9% ± 1.9%/d; not significant), and synthesis rates corresponded to those measured in controls (period I, 13.0% ± 1.2%/d; period II, 12.1% ± 0.1%/d; not significant vs period I and vs D-Glc infusion).

CONCLUSIONS

A standard D-Glc infusion is insufficient to increase albumin synthesis in postoperative patients.

Dietary protein regulates hepatic constitutive protein anabolism in rats in a dose-dependent manner and independently of energy nutrient composition

We had previously observed that drastic increases in protein consumption greatly modified hepatic protein anabolism in rats, but the confounding effects of other macronutrient changes or a moderate protein increase to generate the same modifications have not yet been established. This study examined the metabolic and hormonal responses of rats subjected to 14-day isoenergetic diets containing normal, intermediate, or high-protein levels (NP: 14% of energy, IP: 33%, HP: 50%) and different carbohydrate (CHO) to fat ratios within each protein level. Fasted or fed rats ( n = 104) were killed after the injection of a flooding dose of 13C-valine. The hepatic protein content increased in line with the dietary protein level ( P < 0.05). The hepatic fractional synthesis rates (FSR) of protein were significantly influenced by both the protein level and the nutritional state (fasted vs. fed) ( P < 0.0001) but not by the CHO level, reaching on average 110%/day, 92%/day, and 83%/day in rats fed the NP, IP, and HP diets, respectively. The FSR of plasma albumin and muscle did not differ between diets, while feeding tended to increase muscle FSR. Proteolysis, especially the proteasome-dependent system, was down-regulated in the fed state in the liver when protein content increased. Insulin decreased with the CHO level in the diet. Our results reveal that excess dietary protein lowers hepatic constitutive, but not exported, protein synthesis rates, independently of the other macronutrients, and related changes in insulin levels. This response was observed at the moderate levels of protein intake (33%) that are plausible in a context of human consumption.

Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism

BACKGROUND

Intermittent fasting (IF) was shown to increase whole-body insulin sensitivity, but it is uncertain whether IF selectively influences intermediary metabolism. Such selectivity might be advantageous when adapting to periods of food abundance and food shortage.

OBJECTIVE

The objective was to assess effects of IF on intermediary metabolism and energy expenditure.

DESIGN

Glucose, glycerol, and valine fluxes were measured after 2 wk of IF and a standard diet (SD) in 8 lean healthy volunteers in a crossover design, in the basal state and during a 2-step hyperinsulinemic euglycemic clamp, with assessment of energy expenditure and phosphorylation of muscle protein kinase B (AKT), glycogen synthase kinase (GSK), and mammalian target of rapamycine (mTOR). We hypothesized that IF selectively increases peripheral glucose uptake and lowers proteolysis, thereby protecting protein stores.

RESULTS

No differences in body weight were observed between the IF and SD groups. Peripheral glucose uptake and hepatic insulin sensitivity during the clamp did not significantly differ between the IF and SD groups. Likewise, lipolysis and proteolysis were not different between the IF and SD groups. IF decreased resting energy expenditure. IF had no effect on the phosphorylation of AKT but significantly increased the phosphorylation of glycogen synthase kinase. Phosphorylation of mTOR was significantly lower after IF than after the SD.

CONCLUSIONS

IF does not affect whole-body glucose, lipid, or protein metabolism in healthy lean men despite changes in muscle phosphorylation of GSK and mTOR. The decrease in resting energy expenditure after IF indicates the possibility of an increase in weight during IF when caloric intake is not adjusted. This study was registered at www.trialregister.nl as NTR1841.

Alterations in hepatic glucose and energy metabolism as a result of calorie and carbohydrate restriction

Carbohydrate restriction is a common weight-loss approach that modifies hepatic metabolism by increasing gluconeogenesis (GNG) and ketosis. Because little is known about the effect of carbohydrate restriction on the origin of gluconeogenic precursors (GNG from glycerol [GNG(glycerol)] and GNG from lactate/amino acids [GNG(phosphoenolpyruvate (PEP))]) or its consequence to hepatic energy homeostasis, we studied these parameters in a group of overweight/obese subjects undergoing weight-loss via dietary restriction. We used (2)H and (13)C tracers and nuclear magnetic resonance spectroscopy to measure the sources of hepatic glucose and tricarboxylic acid (TCA) cycle flux in weight-stable subjects (n = 7) and subjects following carbohydrate restriction (n = 7) or calorie restriction (n = 7). The majority of hepatic glucose production in carbohydrate restricted subjects came from GNG(PEP). The contribution of glycerol to GNG was similar in all groups despite evidence of increased fat oxidation in carbohydrate restricted subjects. A strong correlation between TCA cycle flux and GNG(PEP) was found, though the reliance on TCA cycle energy production for GNG was attenuated in subjects undergoing carbohydrate restriction. Together, these data imply that the TCA cycle is the energetic patron of GNG. However, the relationship between these two pathways is modified by carbohydrate restriction, suggesting an increased reliance of the hepatocyte on energy generated outside of the TCA cycle when GNG(PEP) is maximal.

CONCLUSION

Carbohydrate restriction modifies hepatic GNG by increasing reliance on substrates like lactate or amino acids but not glycerol. This modification is associated with a reorganization of hepatic energy metabolism suggestive of enhanced hepatic beta-oxidation.

South Beach Diet associated ketoacidosis: a case report

INTRODUCTION

It has been previously unclear whether a "mild" degree of low carbohydrate or "starvation" ketonemia and acidosis induced by a low carbohydrate diet is clinically relevant to a patient.

CASE PRESENTATION

A 30-year-old Caucasian male on a low carbohydrate diet presented with nausea, vomiting and abdominal pain. The patient's bicarbonate level was 12 and he had hyperglycemia and ketonemia. He was felt to be in diabetic ketoacidosis and was started on intravenous insulin and isotonic saline infusions and responded well. Following cessation of insulin therapy, the patient remained normoglycemic for the remainder of his hospital stay. He later admitted to having been on the South Beach Diet, which is a low carbohydrate diet, for the three weeks prior to his presentation and during which time he had lost 16 pounds. On admission his BMI was 27.1. On presentation, the patient was felt to be in diabetic ketoacidosis but, interestingly, he was subsequently euglycemic without therapy. Following discharge, the patient discontinued the diet plan and he has remained asymptomatic and euglycemic over the following two years.

CONCLUSION

The hyperglycemic ketoacidosis in this patient may have been caused by increased concentrations of free fatty acids in the absence of carbohydrate-induced inhibition of beta-oxidation of fatty acids and in the presence of an abnormally high ratio of glucagons to insulin. Given the present day popularity of low-carbohydrate diet plans, healthcare providers should be aware of the apparent association between such diets and symptomatic ketoacidosis. In a patient with ketoacidosis suspected to be secondary to a low carbohydrate diet, all other causes of high anion gap acidosis should be ruled out before attributing the acidosis to the low carbohydrate diet.

Glutamine and carbohydrate supplements reduce ammonemia increase during endurance field exercise

Blood ammonia concentration increases during endurance exercise and has been proposed as a cause for both peripheral and central fatigue. We examined the impact of glutamine and (or) carbohydrate supplementation on ammonemia in high-level runners. Fifteen men in pre-competitive training ran 120 min (~34 km) outdoors on 4 occasions. On the first day, the 15 athletes ran without the use of supplements and blood samples were taken every 30 min. After that, each day for 4 d before the next 3 exercise trials, we supplemented the athletes’ normal diets in bolus with carbohydrate (1 g·kg–1·d–1), glutamine (70 mg·kg–1·d–1), or a combination of both in a double-blind study. Blood ammonia level was determined before the run and every 30 min during the run. During the control trial ammonia increased progressively to approximately 70% above rest concentration. Following supplementation, independent of treatment, ammonia was not different (p > 0.05) for the first 60 min, but for the second hour it was lower than in the control (p < 0.05). Supplementation in high-level, endurance athletes reduced the accumulation of blood ammonia during prolonged, strenuous exercise in a field situation.

Low-carbohydrate nutrition and metabolism

The persistence of an epidemic of obesity and type 2 diabetes suggests that new nutritional strategies are needed if the epidemic is to be overcome. A promising nutritional approach suggested by this thematic review is carbohydrate restriction. Recent studies show that, under conditions of carbohydrate restriction, fuel sources shift from glucose and fatty acids to fatty acids and ketones, and that ad libitum-fed carbohydrate-restricted diets lead to appetite reduction, weight loss, and improvement in surrogate markers of cardiovascular disease.

Energy deficit without reducing dietary carbohydrate alters resting carbohydrate oxidation and fatty acid availability

Reduced carbohydrate (CHO) availability after exercise has a potent influence on the regulation of substrate metabolism, but little is known about the impact of fat availability and/or energy deficit on fuel metabolism when dietary CHO availability is not reduced. The purpose of this study was to determine the influence of a postexercise energy deficit, independent of CHO availability, on plasma substrate concentrations and substrate oxidation. Seven moderately trained men (peak oxygen uptake: 56 ± 2 ml·kg−1·min−1) performed exhaustive cycling exercise on two separate occasions. The two trials differed only by the meals ingested after exercise: 1) a high-fat diet designed to maintain energy balance or 2) a low-fat diet designed to elicit energy deficit. The CHO and protein contents of the diets were identical. The next morning, we measured plasma substrate and insulin concentrations and CHO oxidation, and we obtained muscle biopsies from the vastus lateralis for measurement of pyruvate dehydrogenase kinase (PDK)-2 and PDK-4 mRNA expression by using RT-PCR. Despite identical blood glucose (5.0 ± 0.1 and 4.9 ± 0.1 mM) and insulin (7.9 ± 1.1 and 8.4 ± 0.9 μU/ml) concentrations, plasma fatty acid and glycerol concentrations were elevated three- to fourfold during energy deficit compared with energy balance and CHO oxidation was 40% lower ( P < 0.01) the morning after energy deficit compared with energy balance (328 ± 69 and 565 ± 89 μmol/min). The lower CHO oxidation was accompanied by a 7.3 ± 2.5-fold increase in PDK-4 mRNA expression after energy deficit ( P < 0.05), whereas PDK-2 mRNA was similar between the trials. In conclusion, energy deficit increases fatty acid availability, increases PDK-4 mRNA expression, and suppresses CHO oxidation even when dietary CHO content is not reduced.

Adaptative increase of ornithine production and decrease of ammonia metabolism in rat colonocytes after hyperproteic diet ingestion

Chronic high-protein consumption leads to increased concentrations of NH(4)(+)/NH(3) in the colon lumen. We asked whether this increase has consequences on colonic epithelial cell metabolism. Rats were fed isocaloric diets containing 20 (P20) or 58% (P58) casein as the protein source for 7 days. NH(4)(+)/NH(3) concentration in the colonic lumen and in the colonic vein blood as well as ammonia metabolism by isolated surface colonic epithelial cells was determined. After 2 days of consumption of the P58 diet, marked increases of luminal and colonic vein blood NH(4)(+)/NH(3) concentrations were recorded when compared with the values obtained in the P20 group. Colonocytes recovered from the P58 group were characterized at that time and thereafter by an increased capacity for l-ornithine and urea production through arginase (P < 0.05). l-Ornithine was mostly used in the presence of NH(4)Cl for the synthesis of the metabolic end product l-citrulline. After 7 days of the P58 diet consumption, however, the ammonia metabolism into l-citrulline was found lower (P < 0.01) when compared with the values measured in the colonocytes recovered from the P20 group despite any decrease in the related enzymatic activities (i.e., carbamoyl-phosphate synthetase I and ornithine carbamoyl transferase). This decrease was found to coincide with a return of blood NH(4)(+)/NH(3) concentration in colonic portal blood to values close to the one recorded in the P20 group. In response to increased NH(4)(+)/NH(3) concentration in the colon, the increased capacity of the colonocytes to synthesize l-ornithine is likely to correspond to an elevated l-ornithine requirement for the elimination of excessive blood ammonia in the liver urea cycle. Moreover, in the presence of NH(4)Cl, colonocytes diminished their synthesis capacity of l-citrulline from l-ornithine, allowing a lower cellular utilization of this latter amino acid. These results are discussed in relationship with an adaptative process that would be related to both interorgan metabolism and to the role of the colonic epithelium as a first line of defense toward luminal NH(4)(+)/NH(3) concentrations.