Although the energy stored in the lean tissue (LT) and adipose tissue (AT) is well known, the energy required to synthesize 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 synthesize 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 ad libitum access to normal (N, n = 6), high-fat (HF, n = 7), or high-protein (HP, n = 8) diets for 4 weeks. The ΔLT/ΔBW was 0.77 in the N group, 0.70 in the HF, and 0.87 in the HP groups, respectively. The average energy required to gain BW was 8.8 kJ/g in the N group, 7.0 kJ/g in the HF group, and 11.3 kJ/g in the HP group. We observed a positive correlation between ΔLT/ΔBW and energy required for BW gain. The regression line demonstrated that the energy expended to synthesize LT was 13.9 kJ/g and AT was -7.9 kJ/g. Therefore, combined with the energy stored in LT, the energy required to accumulate LT is approximately 19 kJ/g, whereas the energy to accumulate AT could not be elucidated.