BackgroundFood selection and ingestion both in humans and rodents, often is a critical factor in determining excess energy intake and its related disorders.MethodsTwo different concepts of high-fat diets were tested for their obesogenic effects in rats; in both cases, lipids constituted about 40% of their energy intake. The main difference with controls fed standard lab chow, was, precisely, the lipid content. Cafeteria diets (K) were self-selected diets devised to be desirable to the rats, mainly because of its diverse mix of tastes, particularly salty and sweet. This diet was compared with another, more classical high-fat (HF) diet, devised not to be as tasty as K, and prepared by supplementing standard chow pellets with fat. We also analysed the influence of sex on the effects of the diets.ResultsK rats grew faster because of a high lipid, sugar and protein intake, especially the males, while females showed lower weight but higher proportion of body lipid. In contrast, the weight of HF groups were not different from controls. Individual nutrient’s intake were analysed, and we found that K rats ingested large amounts of both disaccharides and salt, with scant differences of other nutrients’ proportion between the three groups. The results suggest that the key differential factor of the diet eliciting excess energy intake was the massive presence of sweet and salty tasting food.ConclusionsThe significant presence of sugar and salt appears as a powerful inducer of excess food intake, more effective than a simple (albeit large) increase in the diet’s lipid content. These effects appeared already after a relatively short treatment. The differential effects of sex agree with their different hedonic and obesogenic response to diet.

Scope: Eating large amounts of fat is usually associated with fat accumulation. However, different types of diets (not only lipids) elicit different metabolic responses.Methods and results: Male and female rats (10wk-old) were distributed in four groups and fed for onemonth a standard diet (SD), or this diet enriched with either lipid (high-fat diet, HF) or protein (highprotein diet, HP), or a cafeteria diet (CAF). Both HF and CAF diets shared the percentage of energy from lipids (40%) but these were different. Protein-derived energy in the HP diet was also 40%. Feeding SD, HF and HP diets did not result in differences in energy intake, energy expenditure, total body weight or lipid content. However, the CAF fed groups showed increases in these parameters, which were more marked in the male rats. The CAF diet increased the mass of adipose tissue while the HF diet did not.Conclusions: Different diets produced substantial changes in the fate of ingested nutrient energy. Dietary lipids were not essential for sustaining increases in body lipid (or adipose tissue) content. Body protein accrual was unrelated to dietary lipids and overall energy intake. Both protein and lipid accrual were more efficient in male than in female rats.