Authors: Laura Perez & Irene Molina-Gonzalez
Our muscles are made of muscle fibres rich in contractile proteins. Our skeletal muscle undergoes several cycles of protein synthesis (after feeding) and breakdown (when fasting) throughout the day, maintaining muscle mass if diet and physical activity are adequate (1). The balance between muscle synthesis and breakdown is required for muscle remodelling, adaptation and repair (2).
When we exercise, we break the fibres of our muscles, and muscle protein synthesis is inhibited while protein breakdown dominates. This is because muscle synthesis is a process that requires energy, and during exercise, this energy is needed for muscle contraction. However, post-exercise, energy is available for new protein synthesis. Protein synthesis enables muscle hypertrophy (growth). For this to happen, we need to synthesise more protein than what has been broken down (2).
1. Amino acids, the protein building blocks
We need amino acids, the building blocks to stimulate muscle protein synthesis. Infusing amino acids after resistance training significantly stimulates protein synthesis (3). Moreover, studies have compared the types of amino acids that are more important for protein muscle synthesis after exercise. Tipton and colleagues (1999) compared the effect of providing 40g of mixed amino acids or 40g of essential amino acids (4). They compared the effect on protein muscle synthesis to a placebo drink (a drink with the same flavour but without any proteins) after resistance training. They observed that providing essential amino acids had the most significant effect on protein synthesis.
In particular, the essential amino acid leucine has been extensively studied as it is critical for stimulating the initiation of muscle protein synthesis (5,6).
- Can you explain why essential amino acids may have a stronger effect on muscle protein synthesis and whether you agree we should focus more on certain types of amino acids for better muscle recovery?
Our body cannot produce essential amino acids; therefore, we need to obtain them through food. They are crucial for muscle protein synthesis, especially the branched-chain amino acids (BCAAs). Due to their branched structural form, they take this name and are only produced by plants, bacteria and fungi. However, animal products like dairy, eggs, meat, fish and chicken are also rich in this type of amino acids. In stores, you can typically find these announced in protein products.
As you mentioned before, particularly leucine has been of interest in the field as it can activate a critical mechanism within muscle cells, the rapamycin (mTOR) signalling pathway, which regulates muscle protein synthesis. However, after exercise, some studies have suggested that increasing leucine levels does not further increase protein synthesis (7), meaning that it can serve as an activator but not an enhancer of protein synthesis. It is an area that still needs further research.
2. Protein Ingestion Time
Studies have investigated the best time of protein ingestion after exercise and have found that even 24 hours post-exercise, there is an increase in protein synthesis after protein supplementation (4,8). Moreover, there has been a growing consensus that protein ingestion before sleep may improve skeletal muscle protein synthesis and adaptations to exercise (7-10). However, some studies lack proper controls, where placebos have a different caloric content than protein supplements or groups have different daily protein intake. This raises questions about whether the effects are due to the timing of protein intake or a higher daily protein intake.
- In your opinion and experience, when would it be more appropriate to supplement protein in physically active people?
In my opinion and based on experience, protein supplementation can be particularly beneficial for physically active individuals. Athletes may not be able to uptake all the recommended protein through diet. Therefore, supplementation may help meet their protein needs. It is important in this active population to obtain all the nutritional requirements, in addition to protein, to support muscle repair, recovery, and adaptations. This is particularly emphasised in periods of high training volume or when the goal is increasing muscle mass.
I also believe that the stronger effects of protein supplementation occur when taking in a protein-rich meal or protein supplementation shortly after exercise. However, this is not always doable. Therefore, snacks high in protein or a protein shake may be the best option to provide the necessary amino acids and ensure recovery.
3. Types of Protein
Digging into the different types of protein, several sources have been studied and compared to see whether some have more potent effects than others. For instance, whey protein has been extensively used and established as one of the most potent and fast-acting proteins. It can increase the levels of amino acids, particularly leucine, in the blood faster than soy and casein (11-13). However, casein is generally chosen as a protein before sleep due to its slower absorption (8). In the last years, there has been a change in protein plant resources in part due to ecological and ethical reasons, where soy products have become more popular.
Other types of proteins, like collagen, can be used to support repair of connective tissue. After exercise, there is a rapid increase in collagen in the tendon, peaking at 24h post-exercise (14). After exercise, collagen supplementation has been seen to improve joint pain, function, and muscle strength (15).
- What proteins do you think are the best to replenish stores and promote skeletal muscle recovery?
I don’t think there is a source that is better than others to promote muscle recovery. There are different sources, and people may choose one over others depending on their goals or dietary requirements.
Popular sources are whey protein and casein, which derive from milk products. As you mentioned, whey protein has been traditionally used as the classic protein supplementation. Whey is rich in leucine and quickly increases the available amino acids in the blood, which promotes a peak in protein synthesis quicker than other resources. Casein, on the other hand, is quite slowly absorbed. This is why overnight studies chose this source of protein, as it can prevent protein breakdown during fasting at night, and this may enhance recovery. Other animal resources are available, such as eggs, beef, and fish. The last two are particularly rich in collagen.
Regarding plant resources, which are chosen by vegetarians, vegans or intolerant to lactose, include soy, pea and fungus (mycoprotein) proteins. Soy and pea protein can be comparable to animal products. Mycoprotein is an interesting type of protein. There has been a growing interest in a bioavailable source of essential amino acids, high-quality protein, and rich in fibre. Some studies suggest that it could lower the levels of cholesterol and maintain glucose and insulin levels; however, is an area under development (16).
- Do you think taking whole proteins or their peptides is better to improve absorption?
The absorption of proteins can be influenced by various factors, including their form and composition.
Whole proteins are intact protein molecules made of the full sequence of amino acids. They require digestion by enzymes in the stomach and small intestine to break them down into smaller peptides and amino acids before absorption can occur. This process takes time and can result in a slower release of amino acids into the bloodstream. On the other side, we have protein peptides, smaller fragments of proteins that have been partially broken down through enzymatic hydrolysis or fermentation. These peptides consist of shorter sequence of amino acids, which are absorbed rapidly by the small intestine. This can lead to a more rapid increase in amino acid availability in the bloodstream. This immediate availability of amino acids can be beneficial for post-exercise recovery or for a quick supply of amino acids. Some proteins like creatine and collagen are often found as peptides.
It's worth noting that the overall impact of protein form on absorption may vary depending on individual factors, such as the specific protein source, unique digestive capabilities, and other nutrients or compounds that may affect absorption. Protein peptides may offer some advantages, yet whole proteins from high-quality sources, such as whey or casein, are still widely used and recognised for their benefits in promoting muscle recovery and growth. Whole proteins provide a complete amino acid profile, supporting various functions beyond muscle recovery.
4. Protein Intake in Different Populations
Early studies suggested that the amount of protein the muscles could absorb in one dose was 20g, where any excess protein was believed to be metabolised to urea or fat (17). However, these studies were limited to the use of one limb for exercise (18) or investigated slow-acting protein (17). More recent studies using whole-body exercise have discovered that muscles can absorb doses of between 40-70g of protein (19,20). However, most of these studies are generally done in male adults of similar weight, which may vary between leaner individuals, females or aged individuals.
- What would be your recommendations for the intake of proteins with physical activity? What about non-active populations?
Most studies use a similar population; protein requirements vary depending on age, sex, exercise level, and current health state.
Moreover, depending on the type of exercise performed, requirements may differ. The International Society of Sports Nutrition suggests that active populations should take a daily protein intake of 1.4-2 grams per kilogram of body weight per day (g/kg/d)21. In endurance athletes, like marathon runners, cyclists, or swimmers, increasing this recommended daily intake does not seem to increase the benefits further (21). Individuals involved in resistance training in periods of building muscle mass, daily intake may be as high as 2.3-3.1 g/kg/d (21).
As you describe, the amount of protein taken in one dose is higher than previously thought, and an absolute dose should consist of 20-40g. It is very important to distribute the protein intake evenly throughout the day, including before and after exercise, to help optimise muscle protein synthesis.
Regarding non-active populations, the recommended protein intake is typically lower. The Dietary Reference Intake (DRI) suggests a protein intake of around 0.8 g/kg/d for sedentary. Individuals with certain health conditions may benefit from slightly higher protein intake to support muscle maintenance and overall health.
However, these are general recommendations, and individual protein needs can vary. People should seek professional advice when unsure of their individual needs.
There is the belief that ageing populations should take, in addition to exercise, extra protein to prevent sarcopenia (the loss of muscle mass in ageing). The supplementation with whey protein after exercise in elderly males increases the amount of amino acids circulating in the blood (22). A Meta-Analysis by Thomas and colleagues concluded that in aged individuals, protein supplementation does not seem to improve the effects of resistance training (23). However, the studies analysed consisted of a low dietary protein intake, and ageing populations may require a higher daily protein intake for muscle protein synthesis (24).
- What would be your recommendations regarding the intake of protein in ageing populations?
This is right; in ageing populations, protein intake should be higher as it is important for maintaining muscle mass, strength, and overall health. As individuals age, there is a natural decline in muscle mass and an increased risk of developing sarcopenia. This increases the risk of falls and accidents as they lose muscle control and strength. Therefore, adequate protein intake can prevent or delay this.
A protein intake of around 1.0-1.2 g/kg/d is often suggested in this aged population (25). However, individual needs may vary based on activity level and health status. Aged adults with chronic disease may need to increase this intake to 1.2-1.5 g/kg/d (25). I would prioritise consuming high-quality protein sources as in any other age group.
To avoid muscle loss, I would strongly recommend resistance training in this population, a combination of a higher protein intake and an overall nutrient-dense diet with balanced vitamins, minerals, sugars, and fats.
To summarise, a balanced diet that includes all the essential amino acids is crucial for promoting muscle recovery and overall health. While protein supplements may have some benefits, they should not be used as a replacement for a well-balanced diet. We should get our protein from raw sources like lean meats, fish, nuts, and seeds. When deciding on the right amount to consume, type and source, it's important to consider each individual's needs.
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