Selection of carbohydrate to protein ratio and correlations with weight gain and body fat in rats allowed three dietary choices.

In order to determine the spontaneous macronutrient choice of a normal rat population, we allowed growing animals to choose their food among three isocaloric diets: 5% protein - 40% carbohydrate; 45% protein - 40% carbohydrate and 5% protein - 70% carbohydrate, and we measured macronutrient intake, carbohydrate to protein (C/P) ratio and weight gain twice weekly during 9 weeks. At the end of the study, final body weight gain and fat accretion as well as brain serotonin content were measured. Protein consumption increased and fat intake decreased steadily during the assay (from 18.8 to 29 kcal% vs. 35.7 to 25.2 kcal%) whereas carbohydrate consumption remained constant (approx. 46 kcal%) ("carbohydrate homeostasis"), and as a consequence the C/P ratio dropped. There were great variations in total protein consumption between animals (13-32 kcal%) and an inverse relationship with fat intake because carbohydrate consumption was constant. Daily weight gain was negatively correlated with the corresponding C/P ratio, as were final weight gain and body fat content with the global C/P ratio characterizing the food choice of each rat. The concentrations of brain serotonin and 5-hydroxyindole acetic acid were rather uniform and did not show any correlations with any of the other variables. The percent food protein chosen by the animal determined the outcome of the trial at 9 weeks, i.e. food intake, weight gain and body fat accretion. In this experimental model, body fat accumulation is, therefore, directly linked to the protein content of the diet, inversely proportional to the fat level and not related to the carbohydrate content. Since the animals increase the proportion of protein in their food with age above basic nutritional requirements, it is postulated that the macronutrient choice of the rat is also determined by the need of the brain for a proper and balanced supply of neurotransmitter precursors adapted to the situation of each animal.