Synthesis of linoleate and alpha-linolenate by chain elongation in the rat

A detailed guide to lipid therapy in intestinal failure

Intravenous lipid emulsions are an integral part of nutrition therapy in the intestinal failure patient. In addition to being a concentrated source of non-protein calories, they provide the essential fatty acids necessary for growth and development. Depending upon the oil source used in these products, complications such as intestinal failure associated liver disease (IFALD) can occur. This review will discuss the risks and benefits associated with these products, especially as they relate to the pediatric intestinal failure patient.

Metabolic fate (absorption, β-oxidation and deposition) of long-chain n-3 fatty acids is affected by sex and by the oil source (krill oil or fish oil) in the rat

The effects of krill oil as an alternative source of n-3 long-chain PUFA have been investigated recently. There are conflicting results from the few available studies comparing fish oil and krill oil. The aim of this study was to compare the bioavailability and metabolic fate (absorption, β-oxidation and tissue deposition) of n-3 fatty acids originating from krill oil (phospholipid-rich) or fish oil (TAG-rich) in rats of both sexes using the whole-body fatty acid balance method. Sprague–Dawley rats (thirty-six male, thirty-six female) were randomly assigned to be fed either a krill oil diet (EPA+DHA+DPA=1·38 mg/g of diet) or a fish oil diet (EPA+DHA+DPA=1·61 mg/g of diet) to constant ration for 6 weeks. The faeces, whole body and individual tissues were analysed for fatty acid content. Absorption of fatty acids was significantly greater in female rats and was only minimally affected by the oil type. It was estimated that most of EPA (>90 %) and more than half of DHA (>60 %) were β-oxidised in both diet groups. Most of the DPA was β-oxidised (57 and 67 % for female and male rats, respectively) in the fish oil group; however, for the krill oil group, the majority of DPA was deposited (82–83 %). There was a significantly greater deposition of DPA and DHA in rats fed krill oil compared with those fed fish oil, not due to a difference in bioavailability (absorption) but rather due to a difference in metabolic fate (anabolism v. catabolism).

What Is the Most Effective Way of Increasing the Bioavailability of Dietary Long Chain Omega-3 Fatty Acids--Daily vs. Weekly Administration of Fish Oil?

The recommendations on the intake of long chain omega-3 polyunsaturated fatty acids (n-3 LC-PUFA) vary from eating oily fish ("once to twice per week") to consuming specified daily amounts of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) ("250-500 mg per day"). It is not known if there is a difference in the uptake/bioavailability between regular daily consumption of supplementsvs. consuming fish once or twice per week. In this study, the bioavailability of a daily dose of n-3 LC-PUFA (Constant treatment), representing supplements, vs. a large weekly dose of n-3 LC-PUFA (Spike treatment), representing consuming once or twice per week, was assessed. Six-week old healthy male Sprague-Dawley rats were fed either a Constant treatment, a Spike treatment or Control treatment (no n-3 LC-PUFA), for six weeks. The whole body, tissues and faeces were analysed for fatty acid content. The results showed that the major metabolic fate of the n-3 LC-PUFA (EPA+docosapentaenoic acid (DPA) + DHA) was towards catabolism (β-oxidation) accounting for over 70% of total dietary intake, whereas deposition accounted less than 25% of total dietary intake. It was found that significantly more n-3 LC-PUFA were β-oxidised when originating from the Constant treatment (84% of dose), compared with the Spike treatment (75% of dose). Conversely, it was found that significantly more n-3 LC-PUFA were deposited when originating from the Spike treatment (23% of dose), than from the Constant treatment (15% of dose). These unexpected findings show that a large dose of n-3 LC-PUFA once per week is more effective in increasing whole body n-3 LC-PUFA content in rats compared with a smaller dose delivered daily.

Natural carbon isotope abundance of plasma metabolites and liver tissue differs between diabetic and non-diabetic Zucker diabetic fatty rats

Background

‘You are what you eat’ is an accurate summary for humans and animals when it comes to carbon isotope abundance. In biological material, natural¹³C/¹²C ratio is subject to minute variations due to diet composition (mainly from ingestion of C₃ and C₄ metabolism plants) and to the discrimination between ‘light’ and ‘heavy’ isotopes during biochemical reactions (isotope effects and isotopic fractionation).

Methodology/Principal Findings

Carbon isotopic abundance was measured in ZDF (fa/+) and ZDF (fa/fa), (lean and obese-diabetic rats respectively) fed the same diet. By analysing plasma metabolites (glucose and non-esterified fatty acids), breath and liver tissue by high-precision isotope ratio mass spectrometry, we demonstrate for the first time statistically distinguishable metabolic carbon isotope abundance between ZDF (fa/+) and ZDF (fa/fa) rats based on plasma glucose, palmitic, oleic, linoleic, arachidonic acids and bulk analysis of liver tissue (P<0.005) resulting into clear isotopic fingerprints using principal component analysis. We studied the variation of isotopic abundance between both groups for each metabolite and through the metabolic pathways using the precursor/product approach. We confirmed that lipids were depleted in ¹³C compared to glucose in both genotypes. We found that isotopic abundance of linoleic acid (C18: 2n-6), even though both groups had the same feed, differed significantly between both groups. The likely reason for these changes between ZDF (fa/+) and ZDF (fa/fa) are metabolic dysregulation associated with various routing and fluxes of metabolites.

Conclusion/Significance

This work provides evidence that measurement of natural abundance isotope ratio of both bulk tissue and individual metabolites can provide meaningful information about metabolic changes either associated to phenotype or to genetic effects; irrespective of concentration. In the future measuring the natural abundance δ¹³C of key metabolites could be used as endpoints for studying in vivo metabolism, especially with regards to metabolic dysregulation, and development and progression of metabolic diseases.

Linoleic acid: between doubts and certainties

Linoleic acid is the most abundant polyunsaturated fatty acid in human nutrition and represents about 14 g per day in the US diet. Following the discovery of its essential functions in animals and humans in the early 1920's, studies are currently questioning the real requirement of linoleic acid. It seems now overestimated and creates controversy: how much linoleic acid should be consumed in a healthy diet? Beyond the necessity to redefine the dietary requirement of linoleic acid, many questions concerning the consequences of its excessive consumption on human health arise. Linoleic acid is a direct precursor of the bioactive oxidized linoleic acid metabolites. It is also a precursor of arachidonic acid, which produces pro-inflammatory eicosanoids and endocannabinoids. A majority of the studies on linoleic acid and its derivatives show a direct/indirect link with inflammation and metabolic diseases. Many authors claim that a high linoleic acid intake may promote inflammation in humans. This review tries to (i) highlight the importance of reconsidering the actual requirement of linoleic acid (ii) point out the lack of knowledge between dietary levels of linoleic acid and the molecular mechanisms explaining its physiological roles (iii) demonstrate the relevance of carrying out further human studies on the single variable linoleic acid.

Plasma incorporation, apparent retroconversion and β-oxidation of 13C-docosahexaenoic acid in the elderly

BACKGROUND

Higher fish or higher docosahexaenoic acid (DHA) intake normally correlates positively with higher plasma DHA level, but recent evidence suggests that the positive relationship between intake and plasma levels of DHA is less clear in the elderly.

METHODS

We compared the metabolism of 13C-DHA in six healthy elderly (mean - 77 y old) and six young adults (mean - 27 y old). All participants were given a single oral dose of 50 mg of uniformly labelled 13C-DHA. Tracer incorporation into fatty acids of plasma triglycerides, free fatty acids, cholesteryl esters and phospholipids, as well as apparent retroconversion and β-oxidation of 13C-DHA were evaluated 4 h, 24 h, 7d and 28d later.

RESULTS

Plasma incorporation and β-oxidation of 13C-DHA reached a maximum within 4 h in both groups, but 13C-DHA was transiently higher in all plasma lipids of the elderly 4 h to 28d later. At 4 h post-dose, 13C-DHA β-oxidation was 1.9 times higher in the elderly, but over 7d, cumulative β-oxidation of 13C-DHA was not different in the two groups (35% in the elderly and 38% in the young). Apparent retroconversion of 13C-DHA was well below 10% of 13C-DHA recovered in plasma at all time points, and was 2.1 times higher in the elderly 24 h and 7d after tracer intake.

CONCLUSIONS

We conclude that 13C-DHA metabolism changes significantly during healthy aging. Since DHA is a potentially important molecule in neuro-protection, these changes may be relevant to the higher vulnerability of the elderly to cognitive decline.

The essentiality of arachidonic acid and docosahexaenoic acid

OBJECTIVE

The purpose of this review is to correlate the clinical finding that patients receiving parenteral nutrition with a fish oil-based lipid emulsion do not develop essential fatty acid deficiency (EFAD) with an experimental murine model, thus showing that arachidonic acid (AA) and docosahexaenoic acid (DHA) are likely to be the essential fatty acids.

BACKGROUND

Conventional belief is that linoleic acid (LA, omega-6) and alpha-linolenic acid (ALA, omega-3) are the essential fatty acids (EFAs). We have shown that a fish oil-based lipid emulsion containing AA (omega-6) and docosahexaenoic acid (omega-3) and insignificant quantities of LA and ALA is efficacious in the treatment of parenteral nutrition-associated liver disease (PNALD), a major cause of liver-related morbidity and mortality. The prospect of using a fish oil-based lipid emulsion as monotherapy has raised concerns of EFAD development, hindering its adoption into clinical practice.

DESIGN

Data from patients in our institution who received PN with a fish oil-based lipid emulsion was reviewed for clinical and biochemical evidence of EFAD, defined as an elevated triene-tetraene ratio (Mead acid/AA>0.2). We also investigated the minimum amount of fish oil required to prevent EFAD in a murine model and determined whether DHA and AA alone can prevent EFAD.

RESULTS

No patients receiving PN with a fish oil-based lipid emulsion in our institution have developed biochemical or clinical evidence of EFAD such as an elevated triene-tetraene ratio, growth retardation or dermatitis. This observation parallels our previously published animal studies, which demonstrated prevention of EFAD when 13% of total calories were from fish oil. Moreover, current work in our laboratory shows that AA and DHA provision alone is sufficient to prevent biochemical and physiologic evidence of EFAD in a murine model.

CONCLUSIONS

When dosed appropriately, fish oil-based lipid emulsions contain sufficient EFAs to prevent EFAD. Furthermore, AA and DHA alone may be the true EFAs.

Metabolic pathway that produces essential fatty acids from polymethylene-interrupted polyunsaturated fatty acids in animal cells

Sciadonic acid (20:3 Delta-5,11,14) and juniperonic acid (20:4 Delta-5,11,14,17) are polyunsaturated fatty acids (PUFAs) that lack the Delta-8 double bond of arachidonic acid (20:4 Delta-5,8,11,14) and eicosapentaenoic acid (20:5 Delta-5,8,11,14,17), respectively. Here, we demonstrate that these conifer oil-derived PUFAs are metabolized to essential fatty acids in animal cells. When Swiss 3T3 cells were cultured with sciadonic acid, linoleic acid (18:2 Delta-9,12) accumulated in the cells to an extent dependent on the concentration of sciadonic acid. At the same time, a small amount of 16:2 Delta-7,10 appeared in the cellular lipids. Both 16:2 Delta-7,10 and linoleic acid accumulated in sciadonic acid-supplemented CHO cells, but not in peroxisome-deficient CHO cells. We confirmed that 16:2 Delta-7,10 was effectively elongated to linoleic acid in rat liver microsomes. These results indicate that sciadonic acid was partially degraded to 16:2 Delta-7,10 by two cycles of beta-oxidation in peroxisomes, then elongated to linoleic acid in microsomes. Supplementation of Swiss 3T3 cells with juniperonic acid, an n-3 analogue of sciadonic acid, induced accumulation of alpha-linolenic acid (18:3 Delta-9,12,15) in cellular lipids, suggesting that juniperonic acid was metabolized in a similar manner to sciadonic acid. This PUFA remodeling is thought to be a process that converts unsuitable fatty acids into essential fatty acids required by animals.

The conditional nature of the dietary need for polyunsaturates: a proposal to reclassify ‘essential fatty acids’ as ‘conditionally-indispensable’ or ‘conditionally-dispensable’ fatty acids

The term essential fatty acid no longer clearly identifies the fatty acids it was originally used to describe. It would be more informative if the concept of essentiality shifted away from the symptoms arising from the lack of de novo synthesis of linoleate or α-linolenate and towards the adequacy of the capacity for synthesis and conservation of both the parent and the derived long-chain polyunsaturates. For instance, despite the existence of the pathway for synthesis of docosahexaenoate from α-linolenate, the former would be more correctly classified as ‘conditionally indispensable’ because the capacity of the pathway appears insufficient during early development, although it may be sufficient later in life in healthy individuals. Similarly, despite the inability to synthesize linoleate de novo, abundant linoleate stores and its relatively slow turnover in healthy adults probably makes linoleate ‘conditionally dispensable’ for long periods. There are two other anomalies with the terms essential and non-essential fatty acids: (1) under several different experimental circumstances, the C-skeleton of essential fatty acids is avidly used in the synthesis of non-essential fatty acids; (2) to function normally, the brain is required to endogenously synthesize several non-essential fatty acids. As with essential amino acids, which have been reclassified as indispensable or conditionally indispensable, such a change in terminology should lead to an improved understanding of the function and metabolism of polyunsaturates in particular, and long-chain fatty acids in general.

Problems with essential fatty acids: time for a new paradigm?

The term 'essential fatty acid' is ambiguous and inappropriately inclusive or exclusive of many polyunsaturated fatty acids. When applied most rigidly to linoleate and alpha-linolenate, this term excludes the now well accepted but conditional dietary need for two long chain polyunsaturates (arachidonate and docosahexaenoate) during infancy. In addition, because of the concomitant absence of dietary alpha-linolenate, essential fatty acid deficiency is a seriously flawed model that has probably led to significantly overestimating linoleate requirements. Linoleate and alpha-linolenate are more rapidly beta-oxidized and less easily replaced in tissue lipids than the common 'non-essential' fatty acids (palmitate, stearate, oleate). Carbon from linoleate and alpha-linolenate is recycled into palmitate and cholesterol in amounts frequently exceeding that used to make long chain polyunsaturates. These observations represent several problems with the concept of 'essential fatty acid', a term that connotes a more protected and important fatty acid than those which can be made endogenously. The metabolism of essential and non-essential fatty acids is clearly much more interconnected than previously understood. Replacing the term 'essential fatty acid' by existing but less biased terminology, i.e. polyunsaturates, omega3 or omega6 polyunsaturates, or naming the individual fatty acid(s) in question, would improve clarity and would potentially promote broader exploration of the functional and health attributes of polyunsaturated fatty acids.

What is the role of alpha-linolenic acid for mammals?

This review examines the data pertaining to an important and often underrated EFA, alpha-linolenic acid (ALA). It examines its sources, metabolism, and biological effects in various population studies, in vitro, animal, and human intervention studies. The main role of ALA was assumed to be as a precursor to the longer-chain n-3 PUFA, EPA and DHA, and particularly for supplying DHA for neural tissue. This paper reveals that the major metabolic route of ALA metabolism is beta-oxidation. Furthermore, ALA accumulates in specific sites in the body of mammals (carcass, adipose, and skin), and only a small proportion of the fed ALA is converted to DHA. There is some evidence that ALA may be involved with skin and fur function. There is continuing debate regarding whether ALA has actions of its own in relation to the cardiovascular system and neural function. Cardiovascular disease and cancer are two of the major burdens of disease in the 21st century, and emerging evidence suggests that diets containing ALA are associated with reductions in total deaths and sudden cardiac death. There may be aspects of the action and, more importantly, the metabolism of ALA that need to be elucidated, and these will help us understand the biological effects of this compound better. Additionally, we must not forget that ALA is part of the whole diet and should be seen in this context, not in isolation.

Eicosapentaenoic and docosapentaenoic acids are the principal products of alpha-linolenic acid metabolism in young men*

The capacity for conversion of alpha-linolenic acid (ALNA) to n-3 long-chain polyunsaturated fatty acids was investigated in young men. Emulsified [U-13C]ALNA was administered orally with a mixed meal to six subjects consuming their habitual diet. Approximately 33 % of administered [13C]ALNA was recovered as 13CO2 on breath over the first 24 h. [13C]ALNA was mobilised from enterocytes primarily as chylomicron triacylglycerol (TAG), while [13C]ALNA incorporation into plasma phosphatidylcholine (PC) occurred later, probably by the liver. The time scale of conversion of [13C]ALNA to eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA) suggested that the liver was the principal site of ALNA desaturation and elongation, although there was some indication of EPA and DPA synthesis by enterocytes. [13C]EPA and [13C]DPA concentrations were greater in plasma PC than TAG, and were present in the circulation for up to 7 and 14 d, respectively. There was no apparent 13C enrichment of docosahexaenoic acid (DHA) in plasma PC, TAG or non-esterified fatty acids at any time point measured up to 21 d. This pattern of 13C n-3 fatty acid labelling suggests inhibition or restriction of DHA synthesis downstream of DPA. [13C]ALNA, [13C]EPA and [13C]DPA were incorporated into erythrocyte PC, but not phosphatidylethanolamine, suggesting uptake of intact plasma PC molecules from lipoproteins into erythrocyte membranes. Since the capacity of adult males to convert ALNA to DHA was either very low or absent, uptake of pre-formed DHA from the diet may be critical for maintaining adequate membrane DHA concentrations in these individuals.

Lauric acid is desaturated to 12:1n-3 by hepatocytes and rat liver homogenates

Lauric acid desaturation was investigated and described in liver homogenates and in cultured rat hepatocytes. The identification of the desaturated product of lauric acid has been performed using the oxidative cleavage method, and we showed that the obtained monoene was mainly 12:1n-3. This result suggests that lauric acid desaturation could be the first step in the biosynthesis of alpha-linolenic acid in animal cells.

Beta-oxidation of linoleate in obese men undergoing weight loss

BACKGROUND

In animals, the whole-body content and accumulation of linoleate can be measured and compared with its intake to determine linoleate beta-oxidation. This method can also provide quantitative information about the beta-oxidation of linoleate in humans.

OBJECTIVES

The objectives of the study were to 1) use the wholebody fatty acid balance method to quantify whole-body concentrations of linoleate in humans, 2) estimate the distribution of linoleate between adipose and lean tissue, and 3) assess the effect of weight loss on linoleate stores and beta-oxidation in obese humans.

DESIGN

Nine healthy obese men underwent supervised weight loss for 112 d (16 wk). Magnetic resonance imaging data and fatty acid profiles from fat biopsies were both used to determine linoleate stores in adipose and lean tissue and in the whole body. Linoleate beta-oxidation was calculated as intake - (accumulation + excretion).

RESULTS

Mean weight loss was 13 kg and linoleate intake was 24 +/- 6 mmol/d over the study period. Whole-body loss of linoleate was 37 +/- 18 mmol/d, or 28% of the level before weight loss. Combining the intake and whole-body loss of linoleate resulted in linoleate beta-oxidation exceeding intake by 2.5-fold during the weight-loss period.

CONCLUSIONS

All dietary linoleate is beta-oxidized and at least an equivalent amount of linoleate is lost from the body during moderate weight loss in obese men. The method studied permits the assessment of long-term changes in linoleate homeostasis in obese humans and may be useful in determining the risk of linoleate deficiency in other conditions.

Monounsaturated fatty acid diets improve glycemic tolerance through increased secretion of glucagon-like peptide-1

Diets enriched in monounsaturated fatty acids (MUFA)s have been shown to benefit glycemic control. Furthermore, MUFAs specifically stimulate secretion of the antidiabetic hormone, Glucagon-like peptide-1 (GLP-1) in vitro. To determine whether the MUFA-induced benefit in glycemic tolerance in vivo is due to increased GLP-1 release, lean Zucker rats were pair-fed a synthetic diet containing 5% fat derived from either olive oil (OO; 74% MUFA) or coconut oil (CO; 87% saturated fatty acids; SFA) for 2 weeks. Food intake and body weight gain were similar for both groups over the feeding period. The OO group had improved glycemic tolerance compared with the CO group in both oral and duodenal glucose tolerance tests [area under curve (AUC) 121 +/- 61 vs. 290 +/- 24 mM.120 min, P < 0.05; and 112 +/- 28 vs. 266 +/- 65 mM.120 min, P < 0.05, respectively]. This was accompanied by increased secretion of gut glucagon-like immunoreactivity (gGLI; an index of GLP-1 levels) in the OO rats compared with the CO rats (402 +/- 96 vs. 229 +/- 33 pg/ml at t = 10 min, P < 0.05). Tissue levels of GLP-1 and plasma insulin and glucagon levels were not different between the two groups. To determine the total contribution of GLP-1 to the enhanced glycemic tolerance in OO rats, the GLP-1 receptor antagonist exendin(9-39) (Ex(9-39)) was infused 3 min before a duodenal glucose tolerance test. Ex(9-39) abolished the benefit in glycemic tolerance conferred by OO feeding (OO+Ex(9-39) vs. CO+Ex(9-39), P = NS), and resulted in a deterioration of glycemic tolerance in the OO+Ex(9-39) group when compared with the OO controls (AUC 331 +/- 21 vs. 112 +/- 28 mM.120 min, P < 0.05). To probe the mechanism by which the OO diet enhanced GLP-1 secretion, a GLP-1-secreting L cell line was incubated for 24 h with either 100 microM oleic acid (MUFA) or 100 microM palmitic acid (SFA) and subsequently challenged with GIP, a known stimulator of the L cell. Preexposure to oleic acid but not to palmitic acid significantly increased GIP-induced GLP-1 secretion when compared with controls (55 +/- 12% vs. 34 +/- 9%, P < 0.01). These results demonstrate that the benefit in glycemic tolerance obtained with MUFA diets occurs in association with increased GLP-1 secretion, through a mechanism of enhanced L cell sensitivity. These results suggest that diet therapy with MUFAs may be useful for the treatment of patients with impaired glucose tolerance and/or type 2 diabetes through increased GLP-1 secretion.

Fatty acid carbon isotope ratios in humans on controlled diets

Carbon stable isotope ratios for six serum fatty acids (FA) are reported for human subjects on controlled fat diets to determine the range of natural isotope abundance and to demonstrate the leveling effect of a well-controlled diet. Twenty-nine subjects were randomly assigned to one of three controlled diets containing high, medium, or low fat. Diets were consumed for 8 wk. Serum samples were collected at baseline (0), 5, 6, 7, and 8 wk. FA were extracted and methylated. Isotope ratios were analyzed by high-precision gas chromatography combustion-isotope ratio mass spectrometry. At baseline, mean delta 13C for 16:0b, 16:1a, 18:0a, 18:1c,18:2n-6d and 20:4n-6bc were -24.1, -21.7, -21.6, -25.6, -29.6, and -25.0/1000, respectively, with an average standard deviation of 1.9/1000. Most delta 13C decreased during the diet period and appeared to have stabilized by week 5 at -25.3, -21.9, -22.3, -26.5, -30.1, and -24.5/1000, respectively. Between-subjected variability decreased from 1.74 to 1.20/1000 on the controlled diets. Measurement variability was 0.53/1000. The within-subject variability during weeks 5-8 was 0.57/1000 (range of 0.32-0.84/1000), showing a minimum biological fluctuation on controlled diets. There was no diet group effect on delta 13C of serum FA. Except for 18:2, the delta 13C of experimental diets was lower than that of serum FA, consistent with observations in animals. These data show that carbon isotope ratios stabilize in response to controlled diets within 5 wk, reflecting the isotope ratio of their dietary source, and establish isotope ratio fluctuations for endogenous compounds for future studies.

Recent advances in the biology of n-6 fatty acids

The intensive research carried out in the last 10 years on the unique biological functions of n-3 fatty acids (FA), has promoted comparative investigations on various aspects (metabolic, functional) of the biology of n-6 FA. The involvement of peroxisomes in fatty acid metabolism, initially described for the n-3 acids, has now been shown also for the n-6 FA (formation of 22 carbon delta 4 unsaturated FA, formation of newly identified products of beta-oxidation of arachidonic acid, AA). Additional pathways of AA conversion, beyond the classical eicosanoids, give rise to a series of biologically active products, such as the epoxides, involved in the modulation of vascular functions, through the cytochrome p450 system, and to the AA-ethanolamide, anandamide, an endogenous ligand of the cannabinoid receptors, through a phospholipase-mediated process. Finally, nonenzymatic oxidation products of AA, the isoprostanes, isomers of prostaglandins, also endowed of potent biological activities, are generated both in in vitro-induced lipid oxidation and in vivo, being considered as reliable markers of in vivo oxidative stress. As to the nutritional aspects of the n-6 FA, attention is now paid to the intake of preformed long-chain polyunsaturated FA (PUFA) in the n-6 series, mainly AA, through the diet, in analogy with the intake of the long-chain n-3 FA, in fish-eating populations. The importance of the dietary intake of preformed AA is now recognized in newborns, through maternal milk. The ranges of the intakes of AA in population groups, not currently adequately estimated, appear to be wider than generally assumed, and the elevated intakes in some population groups, in the order of several hundred milligrams per day, may be partly responsible of yet unexplored population-based differences in physiologic variables. Recent research on the functional effects of n-6 FA has confirmed their lipid-lowering effects, which can be observed also in neonates, and has shown that, in cooperation with the n-3, they directly and indirectly contribute to modulate functional parameters at the cellular level, such as receptor function, ion channels, and gene expression. From a nutritional point of view, it is clear that PUFA represent the biologically most active component of dietary fat, and the n-6 are quantitatively the most relevant fraction in our diet. In the light of the diversified activities of n-6 and n-3 PUFA, a correct balance between the various fatty acids is recommended.

High-precision continuous-flow isotope ratio mass spectrometry

Although high-precision isotope determinations are routine in many areas of natural science, the instrument principles for their measurements have remained remarkably unchanged for four decades. The introduction of continuous-flow techniques to isotope ratio mass spectrometry (IRMS) instrumentation has precipitated a rapid expansion in capabilities for high-precision measurement of C, N, O, S, and H isotopes in the 1990s. Elemental analyzers, based on the flash combustion of solid organic samples, are interfaced to IRMS to facilitate routine C and N isotopic analysis of unprocessed samples. Gas/liquid equilibrators have automated O and H isotopic analysis of water in untreated aqueous fluids as complex as urine. Automated cryogenic concentrators permit analysis at part-per-million concentrations in environmental samples. Capillary gas chromatography interfaced to IRMS via on-line microchemistry facilitates compound-specific isotope analysis (CSIA) for purified organic analytes of 1 nmol of C, N, or O. GC-based CSIA for hydrogen and liquid chromatography-based interfaces to IRMS have both been demonstrated, and continuing progress promises to bring these advances to routine use. Automated position-specific isotope analysis (PSIA) using noncatalytic pyrolysis has been shown to produce fragments without appreciable carbon scrambling or major isotopic fractionation, and shows great promise for intramolecular isotope ratio analysis. Finally, IRMS notation and useful elementary isotopic relationships derived from the fundamental mass balance equation are presented.