Yuhong Lin

n-3 fatty acid metabolism in women.

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.The extent to which women of reproductive age are able to convert the n-3 fatty acid alpha-linolenic acid (ALNA) to eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA) was investigated in vivo by measuring the concentrations of labelled fatty acids in plasma for 21 d following the ingestion of [U-13C]ALNA (700 mg). [13C]ALNA excursion was greatest in cholesteryl ester (CE) (224 (sem 70) micromol/l over 21 d) compared with triacylglycerol (9-fold), non-esterified fatty acids (37-fold) and phosphatidylcholine (PC, 7-fold). EPA excursion was similar in both PC (42 (sem 8) micromol/l) and CE (42 (sem 9) micromol/l) over 21 d. In contrast both [13C]DPA and [13C]DHA were detected predominately in PC (18 (sem 4) and 27 (sem 7) micromol/l over 21 d, respectively). Estimated net fractional ALNA inter-conversion was EPA 21 %, DPA 6 % and DHA 9 %. Approximately 22 % of administered [13C]ALNA was recovered as 13CO2 on breath over the first 24 h of the study. These results suggest differential partitioning of ALNA, EPA and DHA between plasma lipid classes, which may facilitate targeting of individual n-3 fatty acids to specific tissues. Comparison with previous studies suggests that women may possess a greater capacity for ALNA conversion than men. Such metabolic capacity may be important for meeting the demands of the fetus and neonate for DHA during pregnancy and lactation. Differences in DHA status between women both in the non-pregnant state and in pregnancy may reflect variations in metabolic capacity for DHA synthesis.Using a stable-isotope tracer technique, Burdge & Wootton (2002) noted that women of child-bearing age (about 28 years) had a much greater capacity to convert a-linolenic acid (18 : 3n-3) to docosahexaenoic acid (22 : 6n-3) compared with men of a similar age (Burdge et al. 2002). Based on their analysis of the area under the curve of the time course plot of the plasma C-labelled fatty acids, they reported that there was approximately a 9 % excursion of C-labelled 18 : 3n-3 into C-labelled 22 : 6n-3 in women. In men, however, they found essentially no excursion of the label into plasma 22 : 6n-3 (0 % excursion). However, they did show excursion of the tracer into both eicosapentaenoic (20 : 5n-3) and docosapentaenoic (22 : 5n3) acids in both groups of subjects (Burdge et al. 2002). Using similar isotope tracer procedures, we previously reported that both men (n 4) and women (n 4) were capable of converting H5-labelled 18 : 3n-3 ethyl ester into C20 and C22 polyunsaturated fatty acids, including H5-labelled 22 : 6n-3 (Pawlosky et al. 2001). Moreover, we found that both male (n 5) and female (n 5) subjects were able to synthesize 22 : 6n-3 from 18 : 3n-3 when they subsisted on various diets (fishor beef-based, or ad libitum) that had different concentrations of long-chain polyunsaturated fatty acids (Pawlosky et al. 2003). We derived the in vivo rate constant coefficients for the individual transformations of n-3 fatty acids beginning with 18 : 3n-3 and calculated the percentage utilization of each precursor n-3 fatty acid for product formation (e.g. the percentage of 20 : 5n-3 converted to 22 : 5n-3 was calculated using the rate constant coefficients describing this transformation) using a compartmental modelling procedure. The differences between men and women in their capacities to utilize 18 : 3n-3 for 22 : 6n-3 production observed by Burdge & Wootton (2002) prompted us to analyse data from our dietary study (Pawlosky et al. 2003) in respect of gender. In evaluating the effects of the beefor fish-based or ad libitum diets on the kinetics of n-3 fatty acid metabolism, we observed that gender exerted a profound influence in the determination of a rate constant coefficient involved in one of the steps in the biosynthesis of 22 : 6n-3. During the period when subjects subsisted on a beef-based diet, the rate constant coefficient for the conversion of 22 : 5n-3 to 22 : 6n-3 was much greater (P1⁄40·001) in women (k 0·041 (SD 0·007)) compared with men (k 0·012 (SD 0·004)). The larger rate constant coefficient in women led to a nearly 3-fold greater amount of 22 : 5n-3 utilized for 22 : 6n-3 synthesis compared with men (Fig. 1). This was also observed during the ad libitum dietary phase of study, but did not reach statistical significance (P 1⁄4 0·08). Very interestingly, while subjects subsisted on the fish-based diet, both groups showed about equal capability in their utilization of 22 : 5n-3 for 22 : 6n-3 synthesis (Fig. 1). The effect of gender on the percentage excursion of labelled n-3 fatty acids observed by Burdge & Wootton (2002) in human subjects may be largely explained by the differences in the magnitude of the rate constant coefficient that describes the synthesis of 22 : 6n-3 from 22 : 5n-3 in male and female subjects. Gender appears to be a strong determinant that influences the synthesis of 22 : 6n-3 in human subjects. It is possible that Burdge & Wootton (2002) failed to detect excursion of the tracer into plasma 22 : 6n-3 in male subjects because of the lower conversion rate of 22 : 5n-3 to 22 : 6n-3, demanding highly sensitive analytical procedures. It is noteworthy that the beef-based diet did not appear to stimulate the synthesis of 22 : 6n-3 in male subjects compared with a fish-based diet (Fig. 1). This suggests that the regulation of n-3 fatty acid metabolism in women is more sensitive to dietary alterations and this may possibly be due to hormonal factors.

Distribution of omega-6 and omega-3 polyunsaturated fatty acids in the whole rat body and 25 compartments

The steady state compositions of omega-6 and omega-3 polyunsaturated fatty acids (PUFA) throughout the various viscera and tissues within the whole body of rats have not previously been described in a comprehensive manner. Dams consumed diets containing 10wt% fat (15% linoleate and 3% α-linolenate). Male offspring (n=9) at 7-week of age were euthanized and dissected into 25 compartments. Total lipid fatty acids for each compartment were quantified by GC/FID and summed for the rat whole body; total n-6 PUFA was 12wt% and total n-3 PUFA was 2.1% of total fatty acids. 18:2n-6 accounted for 84% of the total n-6 PUFA, 20:4n-6 was 12%, 18:3n-3 was 59% of the total n-3 PUFA, 20:5n-3 was 2.1%, and 22:6n-3 was 32%. The white adipose tissue contained the greatest amounts of 18:2n-6 (1.5g) and 18:3n-3 (0.2g). 20:4n-6 was highest in muscle (60mg) and liver (57mg), while 22:6n-3 was greatest in muscle (46mg), followed by liver (27mg) and carcass (20mg). In terms of fatty acid composition expressed as a percentage, 18:2n-6 was the highest in the heart (13wt%), while 18:3n-3 was about 1.3wt% for skin, white adipose tissue and fur. 20:4n-6 was highest (21-25wt%) in the circulation, kidney, and spleen, while 22:6n-3 was highest in the brain (12wt%), followed by the heart (7.9wt%), liver (5.9wt%), and spinal cord (5.1wt%). Selectivity was greatest when comparing 22:6n-3 in brain (12%) to white adipose (0.08%) (68-fold) and 22:5n-6 in testes (15.6%) compared to white adipose (0.02%), 780-fold.

Differences in long chain polyunsaturates composition and metabolism in male and female rats

Human studies and some animal work have shown more docosahexaenoic acid (DHA) and arachidonic acid (ARA) was accumulated or converted from precursors in females compared to males. This study explored in-depth the effect of gender on fatty acid composition and polyunsaturated fatty acid metabolism in rats fed one of two well-defined diets containing 10% total fat. One diet contained 15% of linoleic acid (LA) and 3% of α-linolenic acid (ALA) of the total fatty acids (LA+ALA diet), while the other diet contained 15% LA and 0.05% ALA (LA diet). At the age of 20 weeks, all animals were orally administered a single dose of a mixture of deuterium-labeled LA and ALA. Caudal venous blood was then drawn at 0, 2, 4, 8, 12, 24, 48, 96 and 168h. The concentrations of the deuterated precursors and their metabolites in plasma total lipids were quantified by GC/MS negative chemical ionization. Endogenous fatty acids were quantified by GC/FID analysis. When expressed as the percentage of oral dosage, female rats accumulated more precursors and more products, deuterated DHA and deuterated n-6 docosapentaenoic acid (2H5-DPAn-6), in plasma than did male rats in both the LA+ALA diet and the LA diet. For the endogenous non-labeled PUFA, greater concentrations of DHA and DPAn-6 were similarly observed in female rats compared to males within each diet. A lower concentration of non-labeled ARA was observed only in female rats fed the LA+ALA diet. In summary, greater endogenous and exogenous DHA and DPAn-6 was observed in female rat plasma and this was independent of dietary ALA status.

Quantitation of Human Whole-Body Synthesis-Secretion Rates of Docosahexaenoic Acid and Eicosapentaenoate Acid from Circulating Unesterified α-Linolenic Acid at Steady State

The rate at which dietary α-linolenic acid (ALA) is desaturated and elongated to its longer-chain n-3 polyunsaturated fatty acid (PUFA) in humans is not agreed upon. In this study, we applied a methodology developed using rodents to investigate the whole-body, presumably hepatic, synthesis-secretion rates of esterified n-3 PUFA from circulating unesterified ALA in 2 healthy overweight women after 10 weeks of low-linoleate diet exposure. During continuous iv infusion of d5-ALA, 17 arterial blood samples were collected from each subject at -10, 0, 10, 20, 40, 60, 80, 100, 120, 150, 180, and 210 min, and at 4, 5, 6, 7, and 8 h after beginning infusion. Plasma esterified d5-n-3 PUFA concentrations were plotted against the infusion time and fit to a sigmoidal curve using nonlinear regression. These curves were used to estimate kinetic parameters using a kinetic analysis developed using rodents. Calculated synthesis-secretion rates of esterified eicosapentaenoate, n-3 docosapentaenoate, docosahexaenoic acid, tetracosapentaenate, and tetracosahexaenoate from circulating unesterified ALA were 2.1 and 2.7; 1.7 and 5.3; 0.47 and 0.27; 0.30 and 0.30; and 0.32 and 0.27 mg/day for subjects S01 and S02, respectively. This study provides new estimates of whole-body synthesis-secretion rates of esterified longer-chain n-3 PUFA from circulating unesterified ALA in human subjects. This method now can be extended to study factors that regulate human whole-body PUFA synthesis-secretion in health and disease.