Rats self-select a constant protein-to-carbohydrate ratio rather than a constant protein-to-Energy Ratio and have low plasma FGF21 concentrations
Résumé
Under dietary self-selection (DSS), rats ingest 25–30% of energy as protein. This high level appears to
be explained by metabolic benefits related to reduced carbohydrate dependence and associated pathologies. However, the mechanisms underlying these choices remain largely misunderstood.
The aim was to test the hypothesis that in a DSS model, rats select a protein-to-energy (PE) ratio to maintain the protein-to-carbohydrate (PC) ratio constant and that fibroblast growth factor 21 (FGF21) is involved in this response.
Adult male Wistar rats were used in 3 experiments.
The first was to determine whether the PE ratio was influenced by changes in carbohydrate content. The second was to test whether the PE ratio was defended with a modified DSS model. The third was to determine whether the selected PE ratio was of metabolic interest compared with a standard 15%protein diet. Food intake, body weight, and energy expenditure were measured. After 3 wk, plasma was sampled and rats were killed to determine body composition and gene expression. Statistical analyses were mainly done by ANOVA tests and correlation tests.
The selected PE ratio increased from 20% to 35% when the carbohydrate content of the protein-free diet
increased from 30% to 75% (R2 =0.56; P<10−6). Consequently, the PC ratiowas constant (70%) in all groups (P=0.18).
In self-selecting rats, plasma FGF21 concentrations were 3 times lower than in rats fed the 5% protein diet (P < 10−4) and similar to those in rats fed a 30% diet.
This study showed that self-selecting rats established PE ratios larger than those considered sufficient
to achieve optimal growth in adult rats (10–15%), and the ratios were highly dependent on carbohydrates, apparently with the aim of maintaining a constant and high PC ratio. This was associated with a minimization of plasma FGF21.