Relationship between protein intake and resistance training–induced muscle hypertrophy in middle-aged women: A pilot study

•A 16-wk resistance exercise training program caused 1.5% muscle hypertrophy (MH).•Daily total protein intake throughout the intervention was not associated with MH.•Protein intake at baseline during breakfast was negatively associated with MH.•Increased protein intake at breakfast during resistance exercise training was positively associated with MH. The aim of this study was to observe the relationship of protein intake at each meal and daily total with change in lean tissue mass with progressive resistance exercise training (RET) in healthy middle-aged women. Twenty-two healthy Japanese women were recruited from Shiga Prefecture, Japan, and a supervised whole body RET program was conducted twice a week for 16 wk. The dietary intake was assessed using 3-d dietary records. Dual-energy x-ray absorptiometry was used to measure the whole body lean soft tissue mass (WLTM). Multiple regression analysis was performed to examine the relationship between the protein intake and RET-induced changes in the WLTM after adjusting for age, sleep quality, physical activity, and energy intake. The 16-wk RET program caused a significant gain in the WLTM (1.46 ± 0.45%, P = 0.004). Multiple regression analysis showed that the baseline protein intake at breakfast was negatively associated with the percent change in the WLTM (β = –1.598; P = 0.022). Additionally, the percent change (β = 0.624; P = 0.018) in protein intake at breakfast was positively associated with the percent change in WLTM. Increasing protein intake at breakfast may contribute to RET-induced muscle hypertrophy in middle-aged women, especially among those who habitually consume low-protein levels at breakfast. However, future studies with larger sample sizes are required to confirm the importance of protein intake at breakfast.