Dietary Protein Intake

Dietary protein is an essential macronutrient, derived from both plant and animal sources, that is required for muscle building, immune function and wound healing. ^[1] Intake is usually expressed in grams per kilogram of body weight per day, and one proposed classification defines low (below 1.25 g/kg/day), moderate (1.25 to 1.9 g/kg/day), high (1.9 to 3.4 g/kg/day) and very high (above 3.4 g/kg/day) protein diets. ^[2] The same analysis puts average protein intake at about 1.3 g/kg/day and conventional high-protein diets at about 1.8 to 3 g/kg/day. ^[2] The PROT-AGE Study Group, convened by the European Union Geriatric Medicine Society, states that older adults need more dietary protein than younger adults to support good health, promote recovery from illness and maintain functionality. ^[3] In the United States, 30% of men and 50% of women over 71 consume inadequate dietary protein, owing to factors such as changes in gut function, loss of appetite, tooth loss, financial concerns and social isolation. ^[1] A review of vegetarian diets concludes that protein-rich plant foods such as legumes, nuts and seeds are sufficient to achieve full protein adequacy in adults, while protein adequacy of vegetarian diets is more complex in older adults. ^[4]

Resistance exercise and protein ingestion both stimulate muscle protein synthesis, and the two are synergistic when protein is consumed before or after resistance exercise. ^[5] According to the International Society of Sports Nutrition, a positive muscle protein balance builds and maintains muscle mass, and a daily intake of 1.4 to 2.0 g/kg/day is sufficient for this in most exercising individuals. ^[5] Older people must compensate for age-related changes in protein metabolism, such as high splanchnic extraction and declining anabolic responses to ingested protein, and need more protein to offset the inflammatory and catabolic conditions of common chronic and acute diseases. ^[3] Aging is associated with significant decreases in postprandial muscle protein synthesis. ^[1] Rapidly digested proteins with high proportions of essential amino acids and adequate leucine are the most effective at stimulating muscle protein synthesis, and the International Society of Sports Nutrition advises acute doses containing 700 to 3000 mg of leucine. ^[5]

For people older than 65, the PROT-AGE Study Group recommends an average intake of at least 1.0 to 1.2 g/kg/day, at least 1.2 g/kg/day for those who exercise or are otherwise active, and 1.2 to 1.5 g/kg/day for most older adults with acute or chronic disease. ^[3] A recent review of protein and aging reaches the same ranges and adds that requirements can rise to 2.0 g/kg/day in more severe illness, malnutrition and chronic conditions. ^[1] In a meta-analysis of 49 randomized trials (1863 healthy adults) of prolonged resistance training, protein supplementation increased gains in one-repetition-maximum strength by 2.49 kg and fat-free mass by 0.30 kg, with no further fat-free mass gain beyond total intakes of 1.62 g/kg/day. ^[7] In that analysis, the effect of protein supplementation on fat-free mass gains decreased with increasing age and was greater in resistance-trained individuals. ^[7] In a meta-analysis of 74 randomized controlled trials (RCT), increasing daily protein may enhance lean body mass gains in the 62 studies enrolling subjects in resistance exercise training (SMD 0.22), significantly so in adults aged 65 or older ingesting 1.2 to 1.59 g/kg/day and in younger adults ingesting at least 1.6 g/kg/day, with only marginal effects on physical function tests.

In adults with overweight or obesity aiming for weight loss, increased protein intake prevented muscle mass decline (SMD 0.75) but not losses of strength or physical function, with intakes above 1.3 g/kg/day expected to increase muscle mass and intakes below 1.0 g/kg/day associated with a higher risk of decline. ^[11] Lean mass made up 25% to 39% of total weight lost with incretin-based therapies, and the authors of that meta-analysis conclude that resistance training, adequate protein intake and body composition monitoring can significantly preserve muscle mass during weight loss. ^[12] Compared with animal protein, plant protein resulted in slightly lower muscle mass in randomized trials, an effect stronger in adults younger than 60 and seen with non-soy plant proteins, whereas soy and milk protein did not differ. ^[13] PROT-AGE makes an exception for older people with severe kidney disease (estimated GFR below 30 mL/min/1.73 m2) who are not on dialysis, who may need to limit protein intake. ^[3]