Biomarkers of dairy fat

Dear Editor:

There were 2 interesting articles in a recent issue of the Journal in which odd-chain fatty acids (15:0 and 17:0) were used as circulating biomarkers of dairy fat (1, 2). In one of the studies (1), serum pentadecanoic acid (15:0) was shown to be inversely associated with incident type 2 diabetes, and in the other study (2) the association of pentadecanoic acid and heptadecanoic acid (17:0) with the risk of incident stroke was studied and no significant association was found. These odd-chain fatty acids are considered to be validated biomarkers for dairy fat and they correlated with dairy consumption in many studies (1–4). However, the association between the intake of dairy fat and the relative serum content of heptadecanoic acid has not been clear in all studies (5, 6). In a large cohort study [EPIC (European Prospective Investigation into Cancer and Nutrition)], there was a strong positive ecologic correlation (r = 0.8, P ≤ 0.01) between the total intake of fish and plasma concentration of heptadecanoic acid, whereas there was no correlation between heptadecanoic acid or pentadecanoic acid and dairy products (6). Accordingly, we have seen in our studies (MA Lankinen et al., 2015) a positive correlation between pentadecanoic and heptadecanoinc acids with DHA in plasma phospholipids. The fatty acid heptadecanoic acid is present in the fat of fish (0.31–2.0% depending on fish species) (7, 8). Salmon contains ∼40 mg heptadecanoic acid and 20 mg pentadecanoic acid per 100 g (9). Therefore, we are a bit concerned if these odd-chain fatty acids are considered to be a valid biomarker for dairy fat intake in populations who consume considerable amounts of fish. In populations with a high consumption of dairy fat and a low consumption of fish, odd-chain fatty acids are probably valid biomarkers for dairy fat intake. In populations who consume fish, the presence of odd-chain fatty acids in fish should be taken into account to avoid misleading conclusions.