Substitution of dietary polyunsaturated fatty acids (PUFAs) for saturated fatty acids (SFAs) reduces the amount of abdominal fat in broilers (Villaverde et al., 2005, Wongsuthavas et al., 2007). The mechanism of the effect of PUFAs is not known (Villaverde et al., 2006), but we have put forward (Wongsuthavas et al., 2007) that PUFAs versus SFAs are preferentially oxidized (Beynen and Katan, 1985) and thereby yield ATP so that carbohydrates are shifted from the oxidative into the lipogenic pathway. The conversion of glucose into body fat is less efficient in terms of energy deposition than is the conversion of fatty acids into fat (Newsholme and Leech, 1984). Monounsaturated fatty acids (MUFAs) are also preferentially oxidized (Beynen and Katan, 1985) and thus should also lower the deposition of abdominal fat. In this study, we tested whether dietary MUFAs indeed reduce abdominal fat mass. Three strains of chickens were fed on diets containing either SFAs in the form of beef tallow, MUFAs in the form of olive oil or PUFAs in the form of soybean oil. De-novo fatty acid synthesis was assessed indirectly by using the concentration of plasma triacylglycerols as indicator.