What’s protein good for? The relationship between protein intake, strength and muscle mass

The association between protein intake, strength, and muscle mass is widely recognized in scientific literature and deserves special attention, especially in light of recent evidence reinforcing the importance of adequate protein intake, particularly among individuals engaged in strength training and older adults.

Initially, it is relevant to highlight that various muscular adaptations occur in response to strength training – commonly referred to as weight training – considered the primary stimulus for maintaining and increasing muscle mass. This type of exercise, when properly prescribed by a qualified professional, promotes an increase in the rate of muscle protein synthesis for up to 48 hours, in addition to intensifying the sensitivity of skeletal muscle to amino acids for up to 24 hours. In other words, strength training conditions the muscle to absorb and utilize amino acids, which are derived from protein intake, more efficiently, favoring recovery and hypertrophy processes.

Given this, three essential questions arise:
(1) What is the recommended daily protein intake?
(2) How should this intake be distributed?
(3) What is the best protein source?

Protein intake recommendations for sedentary adults and older adults range between 0.6 and 1.0 g/kg/day, depending on specific clinical conditions. However, in physically active individuals, particularly those who engage in strength exercises, these needs are increased. For physically active adults, an intake between 1.6 and 2.2 g/kg/day is recommended, while for older adults, the recommended values range between 1.2 and 1.6 g/kg/day.

Nevertheless, beyond the total quantity, the distribution of protein intake throughout the day is a determining factor for optimizing muscle protein synthesis. Evidence suggests that young adults achieve a maximum stimulus for protein synthesis with doses of approximately 0.2 to 0.3 g/kg of protein per meal. However, older adults exhibit a phenomenon known as anabolic resistance, characterized by lower efficiency in the uptake and utilization of amino acids by skeletal muscle. For this reason, a protein intake per meal of at least 0.4 g/kg is recommended for this age group.

Additionally, aging is often accompanied by a condition known as anorexia of aging, a multifactorial state involving physiological, biochemical, social, and environmental changes, which results in a reduction in total caloric intake, including proteins. This reduced intake compromises muscle anabolic stimulus, hindering the maintenance and increase of muscle mass, and favoring the development of sarcopenia – a musculoskeletal condition characterized by the progressive reduction of muscle mass, strength, and function.

Sarcopenia, or muscle loss, is one of the most studied syndromes in the context of aging due to its association with functional disability, increased dependence, and a higher risk of mortality. Its prevention and treatment are primarily based on adequate protein consumption and regular physical exercise, with an emphasis on strength training.

Another relevant aspect concerns the quality and origin of dietary protein. Traditionally, animal-derived proteins are considered more efficient for maintaining and increasing muscle mass and strength due to their high biological value and essential amino acid profile. However, it is recommended to prioritize lean sources with low saturated fat content, such as patinho (round steak), filet mignon, and maminha (bottom sirloin/tri-tip).

Plant-based proteins, in turn, have received increasing attention for containing fibers and bioactive compounds with anti-inflammatory and antioxidant properties. Despite these benefits, their fibers can reduce appetite – which is particularly concerning in the face of anorexia of aging – and their higher proportion of carbohydrates can pose a challenge for older adults with metabolic disorders, such as insulin resistance, type 2 diabetes, hypertriglyceridemia, and obesity. Table 1 presents isolated animal protein sources and combined plant-based protein sources, targeting 20 g of protein per meal:

Table 1 |Food Portions Equivalent to 20g of Protein: Comparison Between Animal and Plant-Based Sources with Brown Rice.

Source: Brazilian Food Composition Table (TBCA)

Of course, this combination is simplistic and does not necessarily represent a real dish, which would be composed of other vegetables and food sources that would facilitate reaching the daily protein target; however, it reveals the disparity in protein content when considering different protein sources. Therefore, vegetarians need to pay attention to adequate quantities, seeking a qualified professional (Nutritionist).

Thus, adopting a mixed diet that combines animal and plant-based protein sources appears to be the most appropriate strategy to preserve muscle health in older adults. Animal proteins, especially lean red meat, provide critical nutrients such as iron, while plant proteins contribute to gut microbiota health and glycemic control. The balanced combination of both allows for combining nutritional quality, metabolic safety, and functional sustainability.

References:

Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults [published correction appears in Br J Sports Med. 2020 Oct;54(19):e7. doi: 10.1136/bjsports-2017-097608corr1.]. Br J Sports Med. 2018;52(6):376-384. doi:10.1136/bjsports-2017-097608

Nunes EA, Colenso-Semple L, McKellar SR, et al. Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults. J Cachexia Sarcopenia Muscle. 2022;13(2):795-810. doi:10.1002/jcsm.12922

Tabela Brasileira de Composição de Alimentos (TBCA). Universidade de São Paulo (USP). Food Research Center (FoRC). Versão 7.2. São Paulo, 2023.