BACKGROUND & AIMS:The activity of stearoyl-CoA desaturase-1 (SCD1), the central enzyme in the synthesis of monounsaturated fatty acids (MUFA), has been associated with de novo lipogenesis. In experimental models SCD1 is down-regulated by polyunsaturated fatty acids (PUFA), but clinical studies are scarce. The effect of long-chain n-3 PUFA (LCn-3PUFA) supplied by the regular diet, in the absence of fatty fish or fish oil supplementation, remains to be explored. METHODS:We related 1-y changes in plasma SCD1 index, as assessed by the C16:1n-7/C16:0 ratio, to both adiposity traits and nutrient intake changes in a sub-cohort (n = 243) of non-hypertriglyceridemic subjects of the PREDIMED (PREvención con DIeta MEDiterranea) trial. RESULTS:After adjustment for confounders, including changes in fasting triglycerides, plasma SCD1 index increased in parallel with body weight (0.221 [95% confidence interval, 0.021 to 0.422], P = 0.031) and BMI (0.115 [0.027 to 0.202], P = 0.011). Additionally, dietary LCn-3PUFA (but not MUFA or plant-derived PUFA) were associated with decreased plasma SCD1 index (-0.544 [-1.044 to -0.043], P = 0.033, for each 1 g/d-increase in LCn-3PUFA). No associations were found for other food groups, but there was a trend for fatty fish intake (-0.083 [-0.177 to 0.012], P = 0.085, for each 10 g/d-increase). CONCLUSIONS:Our data add clinical evidence on the down-regulation of plasma SCD1 index by LCn-3PUFA in the context of realistic changes in fish consumption in the customary, non-supplemented diet. CLINICAL TRIAL REGISTRATION:http://www.Controlled-trials.com/ISRCTN35739639.