ω3 and ω6 fatty acids (FA) may have divergent effects on the development of obesity. We examined the association of baseline erythrocyte ω3 and ω6 FA composition with body weight change and the risk of becoming overweight or obese in the Women's Health Study (WHS) participants.

We identified 534 women who had baseline erythrocyte FA measured and a baseline body mass index (BMI) of 18.5-<25 kg/m(2). Body weight was updated at a total of six time points during follow-up.

Weight gain during a mean of 10.4-year follow-up increased with increasing quartiles of baseline erythrocyte cis ω6 FA, ω6/ω3 ratio, and trans FA while decreased with increasing cis ω3 FA. After multivariable adjustment including total energy intake and physical activity, the weight gain (kg) in the highest versus the lowest quartile was 3.08 versus 2.32 for erythrocytecis ω6 FA (p trend 0.04), 2.07 versus 2.92 for cis ω3 FA (p trend 0.08), 2.93 versus 2.05 for ω6/ω3 ratio (p trend 0.046), and 3.03 versus 2.27 for trans FA (p trend 0.06). Among individual FA, the associations were significant for 18:2ω6, 18:3ω6, and trans 18:1 and marginally significant for 20:3ω6 and trans 18:2. The risk of becoming overweight or obese (defined as BMI ≥25 kg/m(2) at any follow-up time point) increased across increasing ω6/ω3 ratio (multivariable model p trend 0.04).

In this prospective study, we found suggestive evidence that erythrocyte cis ω6 FA may be positively associated, and cis ω3 FA inversely associated with weight gain in initially normal-weight women.