Effects of High Protein Diet on the Large Intestine in Overweight Humans

High protein diets are widely used for their slimming effect. Those diets increase the quantity of undigested protein and peptides reaching the large intestine. Microbiota utilizes protein and peptides as substrates and produces amino-acid derived metabolites such as short chain fatty acids, branched chain fatty acids, lactate, formate, succinate, ammonium, hydrogen sulfide and phenolic compounds. Some of those metabolites have been shown to be beneficial to the host whereas other could be deleterious. High protein diets may have different effects whether based on animal or plant proteins since those two kinds of protein have different amino acids composition and slightly different digestibility. The objective of this study is to evaluate in overweight volunteers the consequences of high protein diets (animal vs vegetal) vs isocaloric-normoproteic diet at the level of the large intestine and metabolism. Firstly, composition and activity of the microbiota and the metabolites produced in the large intestine lumen will be determined. Then, to evaluate the consequences of the tested diets on mucosal physiology, gene expression will be analyzed in rectal biopsies and fecal water will be tested on colonocytes cell lines. Lastly, the consequences of the diets on host metabolism will be evaluated.

Dietary intervention

Before the study, volunteers (n=42) will provide a 3 day food record that will be utilized by a dietician to calculate their usual energy intake. The study will start with a 2 weeks normalization period during which the volunteers will have to eat according to menus (Fig. 1). The diet will provide to the volunteer their usual individual energy intake; and protein, carbohydrates and fat will respectively provide 15, 50 and 35 % of the energy intake.

At the end of this standardization period, volunteers will be randomly assigned into 3 groups (n=14) for 3 weeks (intervention period) (Fig. 1). Volunteers of the 3 groups will follow a diet with a reduced digestible carbohydrates intake (15% of energy intake) while undigestible carbohydrate intake will remain unchanged. The volunteers will consume 3 times per day a dietary supplement powder that they will dissolve in water and then drink. This supplement will provide them 15% of their energy intake. The first group will receive a casein (animal protein) supplement so that they will have a high protein diet (protein, carbohydrates and fat will respectively provide 30, 35 and 35 % of the energy intake). The second group will receive a soy protein(vegetal) supplement so that they will also consume a high protein diet (protein, carbohydrates and fat will respectively provide 30, 35 and 35 % of the energy intake). The third group will receive a maltodextrin (digestible carbohydrate, placebo control) supplement so that they will ingest a normoproteic diet, similar in macronutrient composition to the standardization diet (protein, carbohydrates and fat will respectively provide 15, 50 and 35 % of the energy intake). The dietary supplement will be given in a double-blinded protocol. There will be no energy restriction between the standaridization and intervention periods.

Clinical protocol