Although the energy stored in the lean tissue (LT) and adipose tissue (AT) is well known, the energy required to synthesise these tissues is obscure. Theoretically, the energy at the point at which ΔLT/Δ body weight (BW) reaches 100 % on a regression line, which indicates the relationship between ΔLT/ΔBW and the energy required for BW gain, is considered to be the energy expended to synthesise LT. Therefore, we investigated this relationship in rats. Rats were fed diets with different ratios of protein, fat and carbohydrates because their ΔLT/ΔBW values were expected to be different. Six-week-old male Sprague-Dawley rats had