Submitted to: British Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/8/2012
Publication Date: 8/2/2012
Citation: Morris, M.S., Jacques, P.F. 2012. Total protein, animal protein, and physical activity in relation to muscle mass in middle-aged and older Americans. British Journal of Nutrition. 2:1-10. Interpretive Summary: We lose muscle mass in our arms and legs as we age. This can lead to disability and falls. Strength training is known as a good preventive strategy, and recent studies have suggested that consuming more than the recommend amount of protein might also be helpful. The roles that leisure-time physical activity and protein source might play are unclear. We wondered whether aerobic forms of exercise might also help to maintain muscle mass. This is an interesting question, because many aging Americans are biking, swimming, running, etc. to lose or maintain weight or to maintain or improve cardiovascular health, but there may be additional, unknown benefits. Also, strength training regimens are difficult to maintain over the long-term, because they are not as much fun as some other forms of physical activity. We used data collected on 2,427 participants aged >49 years in the US National Health and Nutrition Examination Survey (2003-2006). We estimated the subjects’ usual intakes of total protein and beef from 2 24-hour diet recalls and used a formula to calculate the skeletal muscle mass index from arm and leg measurements, including measures of girth and skinfold thickness. Participants self-reported their physical activity levels. In subjects who performed only moderate aerobic activity or no activity, intakes of beef and total protein were unrelated to muscle mass. In subjects who performed vigorous aerobic or muscle-strengthening activities, appendicular muscle mass increased with increasing beef intake. Total protein intake was related to muscle mass in subjects who did not exercise, and in those who performed muscle-strengthening activities. Subjects who performed such exercises but did not meet the Recommended Daily Allowance (RDA) for protein of 0.8 g/kg body weight actually had very low muscle mass. Our findings suggest that if aerobic activities or muscle-strengthening exercises are performed without proper nutritional support, the result may be loss, rather than gain or maintenance, of muscle mass. Specifically, people who want to obtain the maximum benefit to muscle from aerobic activity and strength training should make sure that they consume some complete protein, such as meat. However, we did not identify any benefit to exceeding the RDA for protein.
Technical Abstract: Resistance training is recognized as a good strategy for retarding age-related declines in muscle mass and strength. Recent studies have also highlighted the potential value of protein intakes in excess of current recommendations. The roles that leisure-time physical activity and protein quality might play in the preservation of skeletal muscle during aging, and how such influences might interact in free-living people, are unclear. We sought to clarify these issues using data collected on 2,427 participants aged greater than or equal to 50 years in the US National Health and Nutrition Examination Survey (2003-2006). We estimated subjects’ usual intakes of total protein and beef from 2 24-hour diet recalls and computed the appendicular skeletal muscle mass index from anthropometric measures. Participants self-reported their physical activity levels. Analyses were controlled for demographic factors, obesity, and smoking. In subjects who performed only moderate aerobic activity or no activity, intakes of beef and total protein were unrelated to muscle mass. In subjects who performed vigorous aerobic or muscle-strengthening activities, appendicular muscle mass increased with increasing beef intake. Total protein intake was related to muscle mass only in subjects who performed muscle-strengthening exercises, and the lowest mean value for the appendicular skeletal muscle mass index occurred in members of that group who failed to meet the Recommended Daily Allowance (RDA) for protein of 0.8 g/kg body weight. Our findings underscore the importance to people who undergo strength training of meeting the RDA for protein and generate the hypothesis that vigorous aerobic activity can help prevent sarcopenia – especially if supported by a diet rich in high-quality protein.