Volume 79, Number 3 May/June 2008
In recent years, considerable research has focused on protein’s role in maintaining or increasing skeletal muscle mass and improving body composition (increasing skeletal muscle and decreasing body fat). This is particularly important for physically active people to maximize physical performance and for older adults to help prevent sarcopenia (age-related skeletal muscle wasting). Both resistance exercise (e.g., weight training, using weight machines and resistance band workouts) and an adequate intake of dietary protein (defined by the Acceptable Macronutrient Distribution Range as 10 to 35% of total energy) are strategies to maintain and build muscle mass. However, not all proteins are alike or equally effective in improving body composition. Emerging research indicates that dairy protein has beneficial effects on body composition.
Resistance exercise promotes muscle hypertrophy (i.e., an increase in muscle fiber size), but a net gain in muscle mass is only possible if an adequate intake of high-quality protein is also consumed. Dairy foods are an excellent source of the highest-quality protein (80% casein and 20% whey) providing all of the essential amino acids that humans cannot synthesize and in proportions resembling amino acid requirements. Whey protein in particular is the richest source of leucine. This branched chain amino acid has been shown to stimulate new muscle protein synthesis and inhibit muscle protein degradation after resistance exercise. Casein and whey also differ in their digestion or delivery of amino acids to skeletal muscle. Casein is described as a “slow” protein, whereas whey is regarded as a “fast” protein.
Recent studies examining the effect of different protein sources on skeletal muscle growth in young men participating in resistance exercise support the benefits of consuming milk as a source of protein. In a 12-week randomized controlled trial in 56 healthy young men who participated in a weight lifting program five days/week, those who consumed two cups of fat-free milk immediately and then again one hour after exercise (four cups total) gained more muscle mass and lost more body fat than those who consumed a soy protein beverage or a carbohydrate only beverage.
Studies of individual milk proteins, casein and particularly whey, support an increase in skeletal muscle amino acid uptake, protein synthesis, or muscle mass when consumed in the hours surrounding resistance exercise. Also, whey protein has been shown to reduce body fat under similar conditions.
For older adults, adequate intake (i.e., moderately above the RDA) of high-quality protein along with resistance exercise may help reduce the risk of sarcopenia. Preserving or increasing muscle mass may reduce older adults’ risk of chronic diseases such as cardiovascular disease and type 2 diabetes, as well as promote greater independence and quality of life.
Additional research is needed to clarify dairy protein’s benefits for physically active adults and its role in helping to prevent sarcopenia. However, findings to date provide another reason to consume three daily servings of dairy foods, as recommended by the 2005 Dietary Guidelines for Americans.
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