Many clinical trials showed favorable effects of high-doses supplemental n-3 polyunsaturated fatty acids (PUFA) on cardio-metabolic risk factors. However, limited studies examined the prospective associations of circulating n-3 PUFA with body fat and its distribution, metabolic syndrome (MS), carotid atherosclerosis, and nonalcoholic fatty liver disease (NAFLD) in subjects with habitual diets containing low levels of n-3 PUFA.

This community-based prospective study enrolled 4048 participants (40–75 years) at baseline (2008–2010, 2013) from Guangzhou, China. They were followed-up approximately once every 3 years. Fatty acids in erythrocyte membranes were measured at baseline. We determined metabolic syndrome factors, body fat by DXA scanning, carotid intima-media thickness (IMT) and NAFLD by ultrasound at the visits. General information, anthropometric indices, habitual dietary intake and other covariates were assessed at each visit.

Among the total 4048 subjects, 3075 and 2671 subjects had erythrocyte n-3 PUFA data and completed the first and second follow-ups. Generally, erythrocyte n-3 PUFA were favorably associated with body fat (particularly at abdomen) and its changes, and with the presence and incidence of MS, type 2 diabetes, carotid IMT thickening. The participants with the highest (vs lowest) quartile of n-3 PUFA were associated with -5.81% fat mass (p < 0.001) and -2.11% of fat mass change at the abdomen (Android) area. The adjusted hazards ratios (95% CI) for the highest (vs. lowest) group were 0.74 (0.61, 0.89) (total n-3 PUFA), 0.71 (0.59, 0.86) (docosahexaenoic acid, DHA), 0.78 (0.65, 0.95) (docosapentaenoic acid, DPA), 1.96 (1.60, 2.40) (gamma-linolenic acid, GLA) for MS; 0.70(0.55, 0.90) (total n-3 PUFA), 0.67(0.52,0.87) (DHA) and 0.73(0.57,0.93) (DPA) for bifurcation IMT thickening, 0.57(0.38, 0.86) (eicosapentaenoic acid, EPA) and 0.63 (0.41, 0.95) (DPA) for type 2 diabetes, and 1.18 (1.09, 1.33) (DHA) for alleviated NAFLD. Both higher levels of total and individual marine n-3 PUFAs (DHA, EPA and DPA) were associated with lower blood pressure at baseline and lower changes in diastolic and systolic blood pressure over the follow-up period. Plant n-3 PUFA (α-linolenic acid, ALA) largely had less significant association with the above-mentioned indices as compared with marine n-3 PUFAs.

Higher proportions of erythrocyte n-3 PUFA (particularly marine sources) was associated with lower body fat, blood pressure and their changes, and lower risks of MS, type 2 diabetes and bifurcation IMT thickening, but higher chance of alleviated NAFLD in middle-aged and older adults.