Assessing the validity of plasma phospholipid fatty acids as biomarkers of dairy fat intake using data from a randomized controlled intervention trial

Background Plasma phospholipid pentadecanoic acid (C15:0), heptadecanoic acid (C17:0), and trans-palmitoleic acid (trans-C16:1n-7) are correlates of dairy fat intake. However, their relative concentrations may be influenced by other endogenous factors, such as liver fat content, and their validity as biomarkers of dairy fat intake has yet to be established. Objectives We investigated whether liver fat content modifies relations between concentrations of C15:0, C17:0, and trans-C16:1n-7 (alone and in combination with iso-C17:0) and known dairy fat intake in the context of a randomized controlled intervention study. We further examined the proportion of dairy fat intake explained by these fatty acids on their own and when considering liver fat content. Methods We used data from a 12-wk intervention trial in which participants (n = 62) consumed diets limited in dairy (0.3 g/d of dairy fat), rich in low-fat dairy (8.7 g/d of dairy fat), or rich in full-fat dairy (28.5 g/d of dairy fat). We used linear regression models to examine relations between relative fatty acid concentrations and grams per day of dairy fat intake, liver fat percentage, and their interaction. Results Only trans-C16:1n-7 in isolation (beta: 0.0004 +/- 0.0002, P = 0.03) and combined with iso-C17:0 (beta: 0.002 +/- 0.0005, P < 0.0001) were consistently positively associated with dairy fat intake regardless of liver fat content. Trans-C16:1n-7 combined with iso-C17:0 also explained the greatest proportion of variation (35.4%) in dairy fat intake. C15:0 and C17:0 were not associated with dairy fat intake after adjusting for liver fat and were predicted to be higher in relation to increased dairy fat intake only among individuals with elevated liver fat. Conclusions The potential for liver fat to affect relative plasma phospholipid concentrations of C15:0 and C17:0 raises questions about their validity as biomarkers of dairy fat intake. Of the fatty acid measures tested, trans-C16:1n-7 combined with iso-C17:0, especially with adjustment of liver fat, age, and sex, may provide the most robust estimate of dairy fat consumption.