The invention pertains to the use of a composition comprising per serving between 50-300 kcal; between 10 g and 35 g proteinaceous matter; and at least 2.5 microgram vitamin D, for the manufacture of a nutritional product for (a) the treatment or prevention of muscle mass decrease, (b) stimulation of muscle mass increase; or (d) stimulating muscle mass preservation, in obese or overweight adults of at least 40 years of age participating in a weight loss program. The inventors surprisingly found that nutritional supplementation with a low-caloric composition rich in protein, particularly whey protein and leucine, compared to an iso-caloric control supplement without protein, synergistically leads to preservation of muscle mass during a weight loss program involving a hypocaloric diet and a resistance exercise program in overweight/obese older adult.