Limited information is available on the role of iron in fatty acid metabolism in humans. We hypothesized that iron supplementation will increase desaturase activity, and so, the aim of this study was to determine the effect of iron supplementation on fatty acid desaturase activity in young women. Participants were randomly assigned to a control group (CG) or supplementation group (SG) who were provided with 37.4 mg of elemental iron daily for 12 weeks. Forty women completed the trial, n = 19 in CG and n = 21 in SG. The mean ages were 25.2 and 24.6 years, and body mass indices were 21.8 and 21.2 (kg/m(2)) in CG and SG, respectively. Serum ferritin concentrations increased significantly (P < .01) in subjects assigned to SG but were unchanged in CG. Serum iron concentrations were not significantly changed. Plasma zinc concentrations at the end of the intervention were similar to baseline values for individuals in CG but were decreased significantly (P = .004) in SG. Plasma fatty acids, phospholipid fatty acids, and desaturase activities, expressed as precursor-to-product ratios, were not significantly affected by the intervention, although in SG the concentration of serum ferritin was correlated positively (P < .05) with Δ6-desaturase activity. Supplementing non-anemic women with low dose iron improves iron status but has no significant effect on desaturase activity. The lack of a clear effect on an indirect indicator of desaturase activity may be related to the antagonism between iron and zinc, as illustrated by the decrease in plasma zinc concentrations in women who were supplemented with iron.