An adequate amount of dietary protein is necessary for the body to function properly and maintain cellular health. Protein is the major functional and structural component of every human cell. Dietary proteins are comprised of individual amino acids that act as building blocks and precursors for a variety of functions in the body, such as coenzymes, hormones, and nucleic acids. Protein is the structural component of collagen, neurotransmitters, enzymes, blood transporters, skeletal muscle tissue, membrane carriers, hair, fingernails, keratin, albumin, intracellular matrices, and many hormones.
Dietary protein is a foundational nutrient for maintaining muscle mass, mobility, and associated health outcomes, especially as we age. Protein-energy malnutrition is a major risk factor for the development of sarcopenia in older populations. Therefore, supporting muscle mass growth and preventing its loss can be a crucial element in promoting the mobility and health outcomes related to sarcopenia. We often focus on exercise as an important element in maintaining mobility in the elderly, but rarely do we consider the impact of protein on the maintenance of muscle mass.
According to the Institute of Medicine, the current recommended dietary allowance (RDA) for protein is 0.80 gram (g) per kilogram (kg) of body weight per day for both adult men and women; however, this may be inadequate to meet the needs of certain populations, such as older adults who are at a greater risk of age-related muscle loss. Some researchers suggest that the minimum daily protein intake for adults aged 65 and over should be raised to 1 g/kg of body weight per day to help reduce incident frailty. Recent studies suggest 1 to 1.3 g/kg/day is needed to support muscle protein synthesis (MPS) and to maintain muscle mass and functionality in older adults.
In most exercising individuals, an overall daily protein intake between 1.4 g/kg and 2.0 g/kg of body weight is considered sufficient for maintaining a positive muscle protein balance, according to the International Society of Sports Nutrition (ISSN). This value is within the Institute of Medicine’s Acceptable Macronutrient Distribution Range for protein; however, the ISSN provides evidence that a higher protein intake in the range of 2.3 g/kg/day to 3.1 g/kg/day (or possibly even higher) may be required to optimize lean body mass retention, to support MPS, and to promote loss of fat mass in resistance-trained individuals and athletes. What does the research say about protein intake in untrained and unhealthy individuals?
A 2021 meta-analysis and systematic review published in Nutrients investigated the effects of increased dietary protein intake on body weight in overweight and obese subjects. The studies identified that diets rich in protein (18% to 59% of total energy consumed) reduced body weight by 1.6 kg compared to isocaloric controls. Furthermore, compared to normoglycemic participants who only lost 0.7 kg more on a high protein diet, those with prediabetes lost 2.6 kg more on a high protein diet compared to those consuming the standard protein diet. Prediabetic subjects on the standard protein diet regained 5.8 kg more than those on the high protein diet. The researchers concluded that diets higher in protein may be more beneficial on body weight management compared to carbohydrates in overweight and obese individuals.
In a double-blind, randomized controlled trial, Lassen and colleagues investigated the effects of high protein supplementation plus mild caloric restriction on the gut microbiota and energy metabolism on overweight or obese subjects with metabolic syndrome. There were 107 participants (18 to 65 years of age) who were randomized to consume the high protein powder that contained 34 g of protein (from milk proteins and free amino acids), 2 g of fat, and 6 g of carbohydrates, or the isocaloric normoproteic mixture that contained only 7.3 g of protein (pea protein and calcium caseinate), 7.6 g of fat, and 24.5 g of carbohydrates. Along with a 50% carbohydrate, 35% fat, and 15% protein diet with a 600-kcal caloric restriction, participants took the protein supplements (360 kcal) twice daily for 12 weeks followed by another 4-week, diet-free period.
Both groups lost visceral fat mass; however, there was a greater reduction in the high protein supplement group (−20.8 ± 23.2 cm2) compared to the normoproteic comparator group (−14.5 ± 24.3 cm2). Protein supplementation was shown to have little effect on bacterial composition, but significantly stimulated bacterial amino acid metabolism; 90% amino acid synthesis functions were enriched with the high protein group compared to 13% in the normal protein group. Microbial diversity also increased in those with a baseline low bacterial gene count, independent of protein supplementation, and those whose bacterial diversity increased the most showed the greatest decreases in fat mass, visceral fat, and weight. There was a significant decrease in fat-free mass in the normal protein group, but there were no changes in the high protein group, demonstrating that higher protein intake can maintain muscle mass. Both groups experienced significant reductions in systolic and diastolic blood pressure, body mass index, waist circumference, fasting blood glucose, total cholesterol, low-density lipoprotein cholesterol, and triglycerides from baseline to 12 weeks. C-reactive protein and tumor necrosis factor-alpha were significantly lower in the high protein group compared to the normal protein group; markers commonly associated with inflammation in obesity. However, there were greater HbA1c reductions in the normal protein group. The researchers concluded that the higher protein intake along with a mild energy restriction activates gut microbial amino acid metabolism and induces visceral fat loss in overweight and obese individuals.
Numerous studies report the benefits of whey protein supplementation for promoting weight loss. Another systematic review and meta-analysis published in the Journal of the American College of Nutrition demonstrated that whey protein supplementation significantly reduced body weight and fat mass and improved markers of cardiovascular disease risk factors in overweight and obese patients. For those who may have sensitivities or intolerances to dairy-based proteins, pea protein and beef protein powders are great alternatives for augmenting protein intake and for supporting healthy body composition and MPS.
This research highlights the importance of consuming adequate dietary protein (either as part of a balanced diet or from supplementation) for those who are overweight, obese, or have metabolic dysregulation. However, more research is needed to determine optimal protein intake levels, sources, and duration for supporting healthy body composition in the context of higher body mass index and/or cardiometabolic conditions.
By Caitlin Higgins, MS, CNS