Dietary fatty acid composition influences energy accretion in rats.

To study the influence of dietary fatty acid composition on energy metabolism, forty male rats were fed elemental diets containing 42% of energy as fish oil, safflower oil, olive oil or beef tallow for 12 wk. Food intakes and body weights were measured daily. Energy expenditure and body composition were determined using doubly labeled water on the final 2 d. Pooled fecal energy losses differed in response to dietary manipulation, with losses greatest in rats fed beef tallow and least in those fed olive oil. Higher lean body mass gains and lower fat mass gains were observed in rats fed diets containing fish oil, compared with rats fed olive oil or beef tallow. Total energy gains in rats fed olive oil (3632 +/- 145 kJ) and beef tallow (3850 +/- 136 kJ) were higher than those in rats fed fish oil (2905 +/- 196 kJ). Energy efficiency in both the olive oil and beef tallow groups was also higher than that in the fish oil group. There were no differences in energy expenditure measured by doubly labeled water technique among the four groups. The data suggest that dietary fatty acid composition alters the efficiency of energy substrate accretion in rats.