Background: Although dietary protein and physical activity play essential roles in developing and preserving lean mass, studies exploring these relationships are inconsistent, and large-scale studies on sources of protein and lean mass are lacking. Accordingly, the present study examined the relationship between total protein intake, protein sources, physical activity, and lean mass in a representative sample of US adults. Methods: This cross-sectional study analyzed data from 2011–2016 US National Health and Nutrition Examination Survey and corresponding Food Patterns Equivalents Database (n = 7547). Multiple linear regression models were performed to examine the sex-specific associations between total protein intake, protein sources (Dairy, Total Protein Foods, Seafood, and Plant Proteins), physical activity, and lean mass adjusting for demographics, weight status, and total daily energy intake. Results: Total protein intake was inversely related to lean mass in females only (Lean mass index: β= −0.84, 95%CI: −1.06–−0.62; Appendicular lean mass index: β= −0.35, 95%CI: −0.48–−0.22). However, protein sources and physical activity was positively associated with lean mass in males and/or females (p \u3c 0.05). Conclusion. Study results suggest that consuming more protein daily had a detrimental influence on lean mass in females whereas eating high-quality sources of proteins and being physically active are important for lean mass for men and women. However, the importance of specific protein sources appears to differ by sex and warrants further investigation
