Consumption of the marine long chain polyunsaturated fatty acid docosahexanoic acid (DHA, C22:6 (n-3)) and eicosapentanoic acid (EPA, C20:5 (n-3)) are associated with health benefits. In particular, EPA and DHA consumption is associated with a reduction in certain lifestyle diseases including cardiovascular disease and dementia. Because of the potential health benefits of marine fatty acids, researchers are interested in reliable predictive tests for fatty fish intake. The phospholipid membranes of erythrocytes and thrombocytes are useful in order to determine the intake of fish in the preceding 18 weeks, because both EPA and DHA are stored in blood. However, for longer term assessment of fatty fish consumption, researcher cannot rely on phospholipid membranes in plasma because the turnover of cells is too high. Instead, adipose tissue provides a slower turnover of cells (1 to 3 years) and may provide a better measure of long-term intake.
Researchers1 have investigated the association between the fatty acid composition of the diet and the fatty acid composition of the subcutaneous adipose tissue. Food diaries (7-day weighed food records) were kept by 24 healthy volunteers at baseline, at 5 months and at 8 months, and tissue biopsies were taken on the subcutaneous adipose tissue from the gluteal muscles after 1 and 8 months. From the food records it was reported that the average intake of fish was 24g/d and that marine fish oil consumption was 0.56g/d, which was equivalent to 0.2% of total energy intake. The total consumption of fish and n-3 fatty acids from fish were significantly associated with the total subcutaneous adipose tissue concentration of DHA. However, both EPA and docosapentanoic acid (DPA, C22:5 (n-6)) showed no association with fish intake or fish oil intake.
Eicosapentanoic acid has been shown to be the short-term marker of choice in plasma phospholipid membranes. However, in this study DHA was shown to be a better determinant of longer-term fish intakes in adipose tissue. Interestingly, the adipose tissue of the subjects had lower levels of EPA compared to DHA, despite similar intakes of the two fatty acids from the diet. This may suggest that EPA is selectively metabolised and used for eicosanoid synthesis, whereas DHA is more readily stored. This is supported by evidence that shows higher concentrations of DHA in breast milk compared to EPA. Even though this study used a small number of subjects, the stable fish intakes and stable body weights as well as the use of 7-day weighed food diaries at 3 points in the study, give strength to the statistical analysis.