Elsevier

Nutrition

Volume 28, Issue 5, May 2012, Pages 566-571
Nutrition

Basic nutritional investigation
Spreading intake of a leucine-rich fast protein in energy-restricted overweight rats does not improve protein mass

https://doi.org/10.1016/j.nut.2011.08.020Get rights and content

Abstract

Objective

Energy restriction decreases fat mass and fat-free mass. Our aim was to prevent the latter using type and timing of protein nutrition as tools.

Methods

Young male Wistar rats were given a high-energy diet for 5 wk and then energy restricted and fed a high-protein diet containing caseins, milk-soluble proteins (MSP), or a casein–MSP mixture (n = 9 per group) as the only source of protein for 3 wk. Food intake was spread over 12 h, whereas in a previous experiment rats consumed their daily ration within 2 to 3 h. Weight and food intake were recorded. The body composition was measured by dual-energy x-ray absorptiometry before and after energy restriction. After 3 wk, the hind-limb muscles, the kidney, intestine, liver, and spleen weights, metabolic plasma parameters, and the liver and extensor digitorum longus muscle protein synthesis rates were measured in the postprandial state.

Results

The food intake was similar in all groups. Energy restriction induced a significant decrease in body weight and fat mass (P < 0.001) and stopped the slow growth of lean body mass, with no differences between groups. Among all tissues, a significant effect was detected only for the intestine (P = 0.0012), with a higher weight in the casein group. Postprandial liver and muscle protein synthesis rates were not different between groups.

Conclusion

When using a high-protein diet spread over 12 h, the nature of the protein intake has no influence on the sparing of lean body mass during energy restriction in young overweight rats.

Introduction

Obesity has reached epidemic proportions: more than 1 billion adults are overweight, and at least 300 million are clinically obese [1]. Obesity is a risk factor for cardiovascular diseases and diabetes and contributes strongly to the global burden of the associated health costs. Obese individuals seeking weight loss often use restrictive diets, which lead to a decrease in adiposity but also to a loss of fat-free mass [2]. This loss of fat-free mass and in particular muscle mass should be prevented because muscle is an emergency store of amino acids that can be used during stresses, allowing an organism to maintain homeostasis. This loss can be limited by including a sufficient amount of protein in the energy-restricted diet [3]. Previously we compared the capacity of caseins (slowly digested milk proteins) with that of milk-soluble proteins (MSP; rapidly digested leucine-rich proteins) to maintain lean body mass in overweight, energy-restricted rats [4]. In the present study, we investigated whether the timing of the intake of these proteins could have an influence on the sparing of lean body mass.

Indeed, in our previous experiment [4], rats consumed their daily ration within 2 to 3 h. We found that, although the regulations of liver and muscle protein metabolisms were not the same, the final nitrogen balance (and thus whole-body protein mass) was not different between groups. In that experiment, postabsorptive muscle protein synthesis rates were higher in the casein-fed group than in the MSP-fed groups [4]. Given the results obtained in test-meal studies in humans [5], [6], [7], we postulated that the muscle protein balance (i.e., protein synthesis minus proteolysis) was probably lower in the casein-fed group than in the MSP-fed group in the postprandial state and that the reverse occurred in the postabsorptive state, so that the overall 24-h balance was the same. To expand on these recent findings, we postulated that, by spreading protein intake over 12 h, a high postprandial muscle protein balance could be maintained in the MSP-fed group by its high leucine content, a key amino acid for protein synthesis stimulation [8], and by limiting the duration of the postabsorptive period to limit postabsorptive muscle losses. This would be the equivalent of four meals a day in humans, whereas in our previous experiment it was more equivalent to one meal a day.

Thus, in the present experiment, young male Wistar rats were fed ad libitum for 5 wk with a high-energy diet [9] and then energy restricted and fed a high-protein diet containing caseins, MSP, or a casein–MSP mixture for 3 wk. Food intake was spread over 12 h: four equal meals were distributed by an automatic delivery system. We used dual-energy x-ray absorptiometry (DEXA) to assess the changes in lean body mass, and we measured muscle mass and in vivo postprandial protein synthesis rates.

Section snippets

Animals and diets

This study was performed in accordance with current legislation on animal experimentation in France. Male Wistar rats (n = 27; Harlan, Gannat, France) weighing 325.0 ± 1.3 g (mean ± standard error) were housed individually in cages under controlled environmental conditions (temperature 20 ± 1°C, humidity 50 ± 5%) with a 12-h inverse light/dark cycle (light on at 17:00 h) and free access to tap water. On arrival, rats were acclimated to the animal facilities for 5 d and fed ad libitum with

Evolution of body weight, food intake, and body composition during energy restriction

Before energy restriction, initial body weights were similar among the three groups (casein 501 ± 15 g, MSP 477 ± 9 g; mixture 488 ± 13 g, P > 0.05, no significant differences). During the high-fat feeding period, animals gained 5.3 ± 0.2 g/d. Control rats fed a normal diet during the same period gained 4.1 ± 0.2 g/d [9]. Energy restriction not only stopped this slow growth of body weight but also induced a significant weight loss in all groups (Fig. 1). Casein-fed rats tended to maintain a

Discussion

Our aim was to compare the capacity of different milk protein fractions to minimize the loss of lean body mass induced by energy restriction in previously overfed male Wistar rats. Caseins (slowly digested dietary proteins), MSP (rapidly digested leucine-rich proteins), and a mixture of both (caseins/MSP, 50/50 w/w) were compared. Previously we showed that when food was consumed within 2 to 3 h/day, despite a positive effect on postabsorptive muscle protein synthesis rates and no difference in

Conclusion

Despite the high leucine content of MSP and the shortness of the postabsorptive period, there was no difference in the body protein mass evolution among the groups. Thus, when protein intake is high, the nature and the timing of protein intake have no influence on lean body mass changes during energy restriction.

Acknowledgments

The authors thank Christian Lafarge and Philippe Lhoste for animal management and Corinne Pouyet and Estelle Pujos for mass spectrometric measurements.

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This project was supported by French National Research Agency and by the Centre National Interprofessionnel de l’Economie Laitière.

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