The importance of amino acids
Do you have strong muscles? If the answer is yes, then you have a greater than average chance of living a healthy and happy life. Being physically strong helps us feel and look healthy, and carry out our everyday activities with ease. Muscle strength also helps reduce the chance of frailty and falls in later life.[1],[2],[3]
Muscle strength peaks when people are in their 20s and then begins a downhill slide, as depicted in the graph below.[4],[5],[6],[7]
Numerous studies show that individuals at the low end of the strength range have an inferior quality of life and a higher mortality rate from all causes.[8],[9],[10],[11]
Low muscle strength is associated with a higher risk of metabolic syndrome, heart disease, depression, and Alzheimer’s disease.
In fact, the effect of low muscle strength on mortality at any age is as great as the effect of established risk factors such as obesity or high blood pressure.[11] Low muscle strength is associated with a higher risk of metabolic syndrome,[12] heart disease,[13] depression,[14] and Alzheimer’s disease.[15]
If we can stay in the upper range of muscle strength throughout life, we have a good chance of avoiding many age-related disabilities. Let’s examine how exercise and nutrition can help.
Resistance training increases muscle strength at all ages
Regular physical activity has been proven to slow the age-related loss of muscle mass and strength.[16] According to the World Health Organization, the U.S. Department of Health and Human Services, and the American College of Sports Medicine, adults of all ages should do two kinds of regular exercise: (1) muscle-strengthening exercises involving the major muscle groups at least two days a week; and (2) at least 150 minutes (two and a half hours) of moderate-intensity aerobic activity throughout the week.[17],[18],[19],[20]
Resistance training (RT) in particular produces a significant increase in muscle strength in healthy adults of all ages.[18],[21],[22] In one study, even nursing home residents in their 90’s experienced an average 174% gain in muscle strength after eight weeks of RT.[23]
Adequate dietary protein is essential for strong muscles
Inadequate dietary protein intake is a major contributor to muscle atrophy.[24] Conversely, adequate protein nutrition – especially in combination with RT – helps maintain muscle strength throughout life.[2] Protein supplementation boosts the response of skeletal muscle to RT, and RT allows more of the ingested amino acids to be used by the muscles.[2],[21],[25],[26],[27],[28]
Muscle comprises about 30 to 40 percent of the body weight of adults.[29] About 20% of muscle is comprised of protein, which is made from 20 different amino acids. Nine of these amino acids — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine — are not synthesized by mammals and therefore must be obtained from the diet. For this reason, they are known as essential amino acids (EAAs).[30]
The other 11 amino acids are called “nonessential amino acids” (NEAAs) because they can be made within the body. Some or all NEAAs may be conditionally essential, however, as discussed below. Muscle protein synthesis is suboptimal if any of the EAAs are missing, whereas a shortage of NEAAs can sometimes be compensated by increased NEAA synthesis.[31]
How much protein is needed?
The American Council on Sports Medicine says: “When trying to build muscle, think quality, quantity and timing. Aim to eat 20 to 30 grams of high-quality protein foods within 2 hours of exercise to help muscle protein synthesis.”[32]
Older individuals may require larger amounts – between 30 and 40 grams of protein at each of three meals per day – as compared to younger adults.[28],[33],[34],[35] This is because aged muscles do not respond as well to EAAs, a phenomenon known as anabolic resistance.[3] Hypochlorhydria, a condition of having lower levels of stomach acid than one should, also is more common with increasing age and adversely affects the body’s ability to absorb protein and other nutrients.[36]
Individuals in their 70s with the highest protein intakes lost about 40% less muscle mass than those in the lowest protein bracket.
Increasing protein intake can overcome both of these problems.[3] A three-year study of individuals in their 70s showed that those with the highest protein intakes lost about 40% less muscle mass than those in the lowest protein bracket.[37] (High protein intake isn’t for everyone, though – individuals with some forms of chronic kidney disease are generally advised to eat a diet with lower amounts of protein, so check with your doctor before embarking on a high protein diet.)
High-quality (“complete”) protein foods, which are needed for optimal muscle protein synthesis, contain all nine EAAs needed to build muscle. Complete protein foods include dairy products, eggs, meats, poultry, and seafood. Except for soy,[38] plant-based protein sources are often missing important EAAs such as lysine, methionine, isoleucine, threonine, and/or tryptophan.[39],[40] The intakes of all dietary amino acids are nearly 50% lower in vegans than in meat-eaters.[41]
For reasons of dietary choice, appetite, and logistics, it may be difficult for individuals to consume large amounts of complete protein at every meal. Amino acid supplementation has been suggested to help avoid a shortage of EAAs.[42],[43],[44],[45]
Our muscles respond to essential amino acids!
Our muscles perk up at the sight of EAAs! Muscle protein synthesis (MPS) increases by 30 to 100% in response to a meal that contains EAAs.[46] EAAs stimulate MPS at all ages.[47] Supplementation with a large dose of EAAs (12 grams daily) was shown to improve handgrip strength and six-minute walking distance (a measure of physical function) in elderly individuals after only three months.[48]
The branched-chain amino acids (BCAAs) – leucine, isoleucine, and valine – are the most abundant EAAs.[49] It has been discovered that leucine directly stimulates MPS by activating key enzymes in the MPS pathway.[50],[51]
It is necessary to ingest all nine EAAs at the same time to boost muscle protein synthesis. Leucine or BCAA supplements alone are not effective unless they are added to a protein meal.
Whey protein is high in BCAAs and has superior muscle-building properties as compared to soy or casein.[52] However, studies have shown supplying the body with EAAs in their free format is even more effective than whey protein – a 3 gram dose of EAAs, providing 1.2 grams of leucine, produced the same MPS effect as 20 grams of whey protein.[53],[54] In another study, 1.5 grams of EAAs, which provided only 0.6 grams of leucine, was also effective.[54] However, it is necessary to ingest all nine EAAs at the same time to boost MPS; leucine or BCAA supplements alone do not appear to be effective unless they are added to a protein meal.[45],[46],[55],[56],[57]
Compared to intact proteins or hydrolyzed formulas, free amino acid supplements have the advantage of being hypoallergenic, because they do not contain peptides that can be recognized by immune cells.[58] Thus free amino acids may be helpful for individuals who react to dairy proteins and other proteins like those found in egg or soy.[59],[60],[61] Free amino acids are not bound in proteins that need to be broken down by gastric acids in the stomach and digestive enzymes from the pancreas, so for the many individuals with pancreatic insufficiency, hypochlorhydria, or on acid suppression therapies they can be absorbed much better.
Do we need “non-essential” amino acids?
The NEAAs are alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine. These amino acids are synthesized in the body, but not always in sufficient amounts.[62],[63] Taurine, an amino acid that is not used for MPS, nevertheless helps prevent the muscle damage seen with exercise.[64],[65] It also enhances bile acid production,[66] and helps protect against high blood pressure.[67],[68] Vegans, however, have only negligible intakes of taurine, as taurine is found only in animal foods.[69]
Among the other NEAAs, cysteine and glycine deficiencies can impede glutathione synthesis, especially in older individuals or in those on low-protein diets.[70],[71],[72] Glycine, arginine, and proline help support collagen biosynthesis under conditions of physical stress or injury;[73],[74] and glutamine plays an important role in intestinal barrier function.[75] Because of the body’s need for these and some of the other NEAAs at times of stress or illness they are often termed “conditionally essential amino acids.” As stated in a 2017 review, “Extensive studies indicate that animals and humans cannot adequately synthesize NEAAs to meet optimal metabolic and functional needs under either normal or stress conditions.”[62]
Summary
A combination of resistance training and adequate intake of high-quality dietary protein is recommended to forestall muscle atrophy and increasing disability with age.[76] If it’s not possible to obtain a sufficient amount of complete protein at every meal, free amino acid supplementation may help. Look for a supplement that contains all 9 EAAs, along with other key amino acids such as arginine, cysteine, glutamine, glycine, proline, and taurine.
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