This review discusses the factors involved in the aetiology of obesity. The effect of dietary macronutrient composition on the metabolic fate of fat (oxidation or storage) is emphasised. Available information on dietary intake in adults from food balance sheets or dietary surveys, show that females eat a relatively low amount of fat (50-70 g/d on average), although this seems to be rising. The increasing prevalence of obesity in our population could be related to the glycemic index of meals, their fatty acid composition and the time interval between meals. This paper compares the oxidation rates of different fatty acids, particularly polyunsaturated fatty acids (PUFA), and among these, the high oxidation rates found for a-linolenic and linoleic acids. Medium chain saturated fatty acids (C8 to C12) are more likely to be oxidised in comparison with PUFA and longer chain saturated fatty acids. The latter are more likely to be directed to fat storage, particularly when they are combined with highly glycemic carbohydrates (CHO) in the same meal. Given the large proportion of CHO in the usual diet, the relevance of de-novo lipogenesis from CHO is questioned. It is concluded that this route is not readily used in humans consuming normal diets. Therefore, this mechanism can not be responsible for the rising prevalence of obesity.