Elsevier

Nutrition

Volume 15, Issue 9, September 1999, Pages 656-660
Nutrition

Applied Nutritional Investigations
Attenuation of the protein wasting associated with bed rest by branched-chain amino acids

https://doi.org/10.1016/S0899-9007(99)00120-3Get rights and content

Abstract

Bed rest is generally accepted as being an appropriate ground-based model for human spaceflight. The objectives of this study were to test the hypothesis that increasing the amount of branched-chain amino acids (BCAAs) in the diet could attenuate the protein loss associated with bed rest. Nineteen healthy subjects were randomized into two groups according to diet. During the 6 d of bed rest, the diets were supplemented with either 30 mmol/d each of three non-essential amino acids, glycine, serine, and alanine (control group), or with 30 mmol/d each of the BCAAs, leucine, isoleucine, and valine (BCAA group). Nutrition was supplied as a commercially available defined formula diet at a rate of 1.3 × REE. Nitrogen (N) balance and urinary 3-MeH excretion were determined for the 6 d. In our results, the urine-based estimate of N balance was 22.2 ± 14.4 (n = 9) mg N · kg−1 · d−1 and 60.5 ± 10.1 mg (n = 8) N · kg−1 · d−1 for the control and BCAA-supplemented groups, respectively (P < 0.05). Urinary 3-MeH excretion was unchanged in both groups with bed rest. We conclude that BCAA supplementation attenuates the N loss during short-term bed rest.

Introduction

Bed rest is associated with decreased nitrogen balance, with skeletal muscle being the major site of the protein loss. The protein loss associated with bed rest is a problem common to all ages, although the consequences are most serious for the elderly because their recuperative powers are weakest.1 Protein loss is always of concern because a loss of 35–40% protein is fatal.2 With intermediate losses, resistance to infection increases as protein depletion increases.3, 4, 5 In the case of bed rest there is also the loss of physical strength that follows a period of immobility.6, 7

Magnetic resonance imaging (MRI) and dual photon x-ray absorptiometry (DPA) measurements on subjects after periods of bed rest ranging from 1–17 wk have shown that most of the protein loss is from the legs and lower back.7, 8, 9, 10 Recovery after 17 wk of bed rest was not rapid; it took 8 wk.8 Thus, it is important in the treatment and rehabilitation of patients with a variety of conditions to institute specific measures to reduce the protein loss that occurs with bed rest. Clearly physical activity would be effective, but physical activity by bed-rest patients is usually not practical.

A bed-rest study by Stuart found that increasing the protein content of the diet from 0.6 g protein kg/d to 1.0 g protein kg/d prevented both the nitrogen loss associated with 7 d of bed rest and any decrease in the whole body protein synthesis rate.11 May and Buse suggested that many of the advantages of giving high protein doses were in fact due to the increased amounts of the branched-chain amino acids (BCAAs) provided.12 The objective of this study was to test this hypothesis by adding supplemental BCCAs to diet during bed rest.

Section snippets

Materials and methods

A total of 19 healthy subjects (17 males, 2 females, age range 22–37 y) were recruited from the surrounding community. Subject characteristics are given in Table I For the protection of human subjects, informed consent for this study was obtained in accordance with the policies of the Temple University Health Science Center and the University of Medicine and Dentistry of New Jersey—School of Osteopathic Medicine Institutional Review Boards.

Results

The data, including subject characteristics, are summarized in Table I. Five subjects were dropped from the study, two because of non-compliance and three because their dietary intakes fell outside the desired limits. Because of the need to use a pair-feeding approach, actual activity factors ranged between 1.02 and 1.45 with a mean of 1.28. Activity factors and their variance were the same for both groups (Table I).

Overall N retention was greater for the BCAA-fed group (P < 0.03, Table I),

Energy balance

The subjects in this study were probably in positive energy balance during bed rest. Energy intake was about 28 kcal · kg−1 · d−1 (Table I). Using the doubly labeled water method to measure the energy expenditure on nine subjects during bed rest, Gretebeck et al.15 found the mean energy expenditure to be 24.2 + 0.8 kcal · kg−1 · d−1. A similar study by Goran et al.16 measured the energy expenditure of five healthy men who although not confined to bed were severely limited in the amount of

Acknowledgements

We would like to thank our subjects for their participation and the staff of the GCRC at Temple University Health Sciences Center for their excellent care of the subjects.

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    This study was supported by NIH grants RO1 AG14078 (TPS), RO1 AG 07988 (GB), R01 AA 10221 (GB), and RR-349 (GB, GCRC branch of the NCRR).

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