High-protein diets are advocated to facilitate weight loss, and improve cardiovascular risk factors, but data on psychiatric effects are lacking. We analyzed data from 1947 men and 2909 women aged 25–74 years when examined in 1971–1975 as the baseline of the National Health and Nutrition Examination Follow-Up Study. The amounts of macronutrients were obtained from a 24-hour recall, and frequencies of eating protein-rich foods were estimated using a 3-month food frequency questionnaire. Severely depressed mood (SDM) was defined as Center for Epidemiologic Studies Depression Scale score ≥ 22 or taking anti-depression medication after an average of 10.6 years of follow-up. A significant gender difference was observed in the prevalence of SDM and its association with protein intake. The weighted prevalence of SDM was 11.45 (SE = 0.96) % and 17.45(1.05) % respectively among men and women. Among men, the relative risk (RRs) of SDM were 1.00, 0.46 (95% CI = 0.22–0.99) and 0.38 (0.16–0.92) respectively for the lowest, middle and highest third protein intake (p for trend = 0.0347). Among women, the RRs were 1.00, 1.93 (1.23–3.08) and 2.47 (1.24–4.90) respectively with lowest, middle and the highest third intakes (p for trend = 0.0023). These estimates were adjusted for cigarette smoking, alcohol consumption, BMI, socioeconomic status at baseline, and the history of cancer, stroke, heart attack and diabetes assessed at follow-up interview. The authors concluded that increased intake of protein demonstrated a protective effect among men but a deleterious effect among women.