n-3 fatty acid metabolism in women.

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.