Whey protein supplements help athletes maintain or increase muscle mass when they train, or during rest periods when they tend to decondition. The supplements are particularly effective for maintaining muscle mass in aging people, which helps prevent chronic disease and improves functional capacity and the quality of life. A review of literature by Michaela Devries and Stuart Phillips concluded that whey protein supplements promote gains in muscle mass, particularly when combined with weight training. They spare muscle mass during calorie-restricted dieting. Supplements reduce the loss of muscle mass that occurs naturally during aging. Whey protein stimulates protein synthesis greater than soy or casein protein. Consuming whey protein supplements regularly is a good way of optimizing muscle protein synthesis and slowing the aging process.
Source: Journal of Food Science, 80: a8-a15, 2015