Aging leads to a reduction in muscle mass, although the reduction can be attenuated with exercise. Increased protein intake also can help to preserve muscle mass. Whey protein has been studied for its effects on muscle protein synthesis in college-aged athletes. It has been shown to increase myofibrillar protein synthesis (muscle protein synthesis), likely because whey protein has a greater amount of the branched chain amino acid, leucine. It has been shown that increased leucine will lead to a greater increase in mammalian target of rapamycin (mTOR), which leads to increased myofibrillar protein synthesis (e.g., greater increase in muscle fiber size) (1,2). However, milk protein, which also contains casein, also may be effective to increase muscle protein synthesis. In this Nutritionist's View, I will discuss some research on the effects of different proteins on muscle protein synthesis and metabolic health in older adults.
PROTEIN INTAKE AND MYOFIBRILLAR PROTEIN SYNTHESIS IN OLDER ADULTS
Mitchell et al. (3) studied the difference between milk protein and whey protein on muscle protein synthesis in 16 healthy men aged 40 to 65 years. Two groups of men (8 per group) were asked to consume 20 g of milk protein or 20 g of whey protein. The researchers took muscle biopsies from the vastus lateralis at 120 minutes before protein ingestion and then 90 and 210 minutes after consumption. There were no differences in muscle protein synthesis between the two groups of men.
Although there were improvements in muscle mass in healthy men regardless of protein type, Hector et al. (4) evaluated the difference between whey protein and soy protein on myofibrillar protein synthesis during a short-term weight loss program in overweight and obese men (n = 19) and women (n = 21). The participants were aged between 25 and 65 years, with a body mass index between 28 and 50 kg/m2, and consumed a hypoenergetic diet (750 kcal per day deficit) for 14 days. This was a randomized controlled trial, where the participants received either 27 g of isolated whey protein or 26 g of soy protein twice per day, or 25 g of a maltodextrin (carbohydrate) supplement. The protein supplements provided 1.3 ± 0.1 g/kg/day, and the carbohydrate supplement provided 0.7 ± 0.1 g/kg of protein per day.
Muscle biopsies were taken from the vastus lateralis before and 3 hours after consumption of the respective macronutrients. The whey protein supplement resulted in a 9% ± 1% decrease in myofibrillar protein synthesis, compared with a 28% ± 5% and 31% ± 5% reduction in the soy and carbohydrate groups, respectively (P < 0.05). Hector et al. (4) reported that whey protein supplementation attenuated the decrease in muscle protein synthesis after short-term weight loss. This study sheds light on the importance of protein type and amount during weight loss diets; however, long-term studies would need to be conducted to determine if the whey protein would maintain a steady preservation of muscle loss through time.
PROTEIN INTAKE AND METABOLIC HEALTH IN OLDER ADULTS
Although protein intake typically is studied to evaluate its impact on muscle protein synthesis, some researchers have examined its impact on metabolic health. Coker et al. (5) assessed if essential amino acids, combined with whey protein and phytosterols (plant sterols) would improve blood lipid concentrations and insulin sensitivity in adults who had high blood triglyceride concentrations. They had a small sample size of nine participants, aged 50 years and older, with plasma triglyceride concentrations greater than 150 mg/dL and who refrained from taking any statin medications in the last 6 weeks. This was a 4-week trial, where all participants had an oral glucose tolerance test at the beginning and end of the 4 weeks. Coker et al. (5) reported a significant decrease in total plasma cholesterol and low-density lipoprotein cholesterol (LDL-C) concentrations (P < 0.02). In addition, in six of the nine participants, plasma triglyceride concentrations significantly decreased by 95 ± 13 mg/dL (P = 0.007). In addition, the researchers reported a significant improvement in insulin sensitivity (P = 0.008). Although this was a small study, it sheds light on the fact that a protein supplement, combined with phytosterols, can improve blood lipid concentrations and insulin sensitivity, both predictors of heart disease and/or diabetes mellitus.
Could different proteins added to a high-fat meal effect postprandial change in plasma lipid, amino acid, and glucose concentrations, as well as oxidative stress markers and vascular function? Mariotti et al. (6) included 15% of casein, whey, or alpha-lactalbumin–enriched whey protein in a high-fat meal, in a three-time point crossover study to try to determine the answer to this question. There were 10 healthy, overweight men (waist circumference > 94 cm). The only significant finding was that casein significantly decreased postprandial plasma triglyceride concentrations by approximately 22% (P < 0.05), with no changes seen from whey protein or alpha-lactalbumin–enriched whey protein.
Based on the few studies presented in this Nutritionist's View article, it seems that whey and casein can improve muscle protein synthesis or metabolic health in older adults. Epidemiologic studies have shown that protein-rich dairy foods can decrease the risk of developing type 2 diabetes mellitus; however, most of the clinical trials in this area have been short-term (7). In addition, some researchers studied only men or overweight/obese individuals. Long-term studies are required to determine definitively the effects of various types of protein intake on metabolic health.
Finally, it is important that individuals of all ages consume a healthy and varied diet, incorporating appropriate amounts of healthy proteins, carbohydrates, and fats. Protein supplementation should be just that: supplementation.
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