Amino Acids in Youth and Old Age

Amino Acids in the Body: What the Body Produces on Its Own and What Must Come from Food

Amino acids are the building blocks of proteins, and the body uses 20 different amino acids to assemble all its proteins. Contrary to common belief, the body cannot produce all of them on its own. Only about 11 amino acids, known as non-essential, are produced in the body in sufficient amounts. The remaining nine amino acids are called essential, and the body cannot produce them at all, so they must come from food.

The nine essential amino acids (those that must come from the diet) are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Animal protein sources, such as meat, fish, eggs, and dairy, provide all nine, and certain plant sources, like soy, also provide a complete profile.

Where non-essential amino acid production occurs: The liver is the main site for the synthesis and breakdown of amino acids in the body. Muscle mainly contributes to the production of two amino acids, alanine and glutamine, which are used to transport nitrogen and carbon between tissues. It is important to clarify: even the non-essential amino acids that the body produces on its own are ultimately built from nitrogen and carbon skeletons derived from dietary protein, so adequate protein supply is essential at any age.

How Age Affects Protein Utilization: Anabolic Resistance

One common myth is that the body "loses the ability to produce amino acids" with age. The accurate picture is different and more complex. Studies measuring the rate of muscle protein synthesis have found that in a fasting (basal) state, the rate of muscle protein production is mostly preserved and similar between young and old individuals. The main change is not in the basal rate, but in the response of the muscle to eating protein.

This phenomenon is called anabolic resistance. In a young body, eating a modest portion of protein (about 20 grams) sharply increases the rate of muscle protein building. In an older body, the same portion produces a blunted and weakened response. In other words, an older person needs a larger amount of protein in each meal to stimulate the same building response that a young person achieves from a smaller amount.

A central mechanism in anabolic resistance is the weakening of the cellular signaling pathway mTOR, which translates the presence of amino acids (especially leucine) into a command to build protein. When the pathway is less sensitive, a stronger stimulus (more protein, more activity) is needed to activate it.

Additional factors affecting protein utilization in old age:

Changes in digestion and absorption: Protein digestion and absorption in the gastrointestinal tract may be less efficient, and a larger portion of amino acids is captured and utilized in the liver and intestine before reaching the muscle.
Decreased physical activity: Inactivity worsens anabolic resistance. Exercise, especially resistance training, restores the muscle's sensitivity to protein.
Loss of muscle mass (sarcopenia): Less metabolically active muscle tissue means less of a "reservoir" for amino acids and protein metabolism.

It is important to note: there is no single numerical figure for the "percentage of decline" in amino acid production with age. The magnitude of the effect varies greatly from person to person and depends on the level of physical activity, the amount and quality of protein in the diet, and overall health status.

Why This Matters: The Consequences of Poor Protein Utilization

When the body struggles to translate dietary protein into tissue building, several problems can arise, primarily:

Loss of muscle mass and strength: Amino acids are essential for building and maintaining muscle. Poor utilization of them is a key component of sarcopenia, age-related muscle loss, which impairs strength, mobility, and independence.
Immune function: Certain amino acids serve as raw materials for immune system cells and antibodies, and insufficient supply may impair the immune response.
Recovery and tissue repair: Wound healing, post-exercise repair, and maintenance of connective tissues (collagen) all depend on an available supply of amino acids.

How to Improve Protein Utilization in Old Age

The good news: anabolic resistance is not a decree of fate, and it can be largely overcome through diet and activity.

Adequate protein in each meal: To surpass the "threshold" of the blunted response, it is recommended for older adults to distribute protein throughout the day and include a quality portion in each meal (typically recommended 25 to 40 grams of protein per meal, depending on body weight), rather than concentrating all daily protein in one meal.
High-quality protein rich in leucine: Complete protein sources, such as lean meat, fish, eggs, dairy, legumes, and soy, provide all nine essential amino acids. Leucine in particular stimulates the mTOR pathway for muscle building.
Physical activity, especially resistance training: Strength training "refreshes" the muscle's sensitivity to protein and eliminates a significant portion of anabolic resistance. Combining exercise with protein intake afterward is particularly effective.
Dietary supplements if necessary: When it is difficult to reach the protein target from food alone, protein supplements or essential amino acid supplements can be used, preferably in consultation with a doctor or dietitian.

Amino Acid Table: Essential vs. Non-Essential

Nine essential amino acids (must come from food, the body does not produce them):

Name in English	Name in English	Functions in the Body
Histidine	Histidine	* Histamine production: essential for producing histamine, a mediator of inflammation and immune response. * Protein production: an important component of many proteins, including hemoglobin.
Isoleucine	Isoleucine	* Branched-chain amino acid (BCAA): contributes to muscle building and tissue repair. * Energy production: serves as an energy source for muscle during exertion. * Protein production: an important component of many proteins.
Leucine	Leucine	* Stimulation of muscle building: a branched-chain amino acid (BCAA) that activates the mTOR pathway and promotes muscle protein synthesis. * Protein production: an important component of many proteins.
Lysine	Lysine	* Protein production: an important component of many proteins. * Collagen and carnitine production. * Immune system support: contributes to normal immune system function.
Methionine	Methionine	* Protein production: an important component of many proteins. * Methyl donor: produces S-adenosylmethionine, an important compound for many methylation processes in the body.
Phenylalanine	Phenylalanine	* Tyrosine production: serves as a raw material for tyrosine, and subsequently for dopamine and norepinephrine. * Protein production: an important component of many proteins.
Threonine	Threonine	* Protein production: an important component of many proteins. * Collagen and elastin production: contributes to connective tissues and elastic tissues.
Tryptophan	Tryptophan	* Serotonin production: serves as a raw material for serotonin, an important neurotransmitter. * Melatonin production: contributes to the production of the sleep hormone. * Protein production.
Valine	Valine	* Branched-chain amino acid (BCAA): contributes to muscle building and tissue repair. * Energy production for muscle. * Protein production: an important component of many proteins.

Non-essential amino acids (the body can produce them on its own, mainly in the liver):

Name in English	Name in English	Functions in the Body
Alanine	Alanine	* Energy and glucose source: can be converted to pyruvate, used for energy production and glucose production in the liver via gluconeogenesis. * Protein production: an important component of many proteins.
Arginine	Arginine	* Urea production: essential for neutralizing ammonia in the urea cycle. * Blood pressure regulation: serves as a raw material for nitric oxide (NO), which dilates blood vessels. * Protein production. (Considered semi-essential during periods of growth and illness.)
Asparagine	Asparagine	* Production of other amino acids: can be converted to aspartate. * Protein production: an important component of many proteins, and has a role in nervous system function.
Aspartic acid	Aspartic acid	* Urea cycle and nucleotide cycle: used in ammonia neutralization and in producing the building blocks of DNA and RNA. * Protein production: an important component of many proteins. (In humans, aspartate is not converted to lysine; this pathway exists only in bacteria and plants.)
Cysteine	Cysteine	* Glutathione production: essential for producing glutathione, a key antioxidant. * Protein production: forms disulfide bridges that stabilize protein structure. (Considered semi-essential, and is produced from methionine.)
Glutamic acid	Glutamic acid	* Production of other amino acids: can be converted to glutamine and proline. * Nerve signal transmission: acts as an excitatory neurotransmitter in the brain. * Protein production.
Glutamine	Glutamine	* Energy source: breaks down into glutamate and then into alpha-ketoglutarate, which feeds the Krebs cycle (TCA) for energy production. * Fuel for intestinal and immune system cells. * Protein production.
Glycine	Glycine	* Collagen production: a central component of collagen, an important protein in connective tissues. * Glutathione production: one of the three amino acids that compose it. * Protein production.
Proline	Proline	* Collagen production: essential for collagen structure and stability. * Protein production: an important component of many proteins.
Serine	Serine	* Phospholipid formation: contributes to building cell membranes. * Carbon metabolism: involved in producing components for DNA. * Protein production.
Tyrosine	Tyrosine	* Dopamine and norepinephrine production: serves as a raw material for neurotransmitters. * Thyroid hormone production. * Protein production. (Produced from phenylalanine, therefore semi-essential.)

Note: Some non-essential amino acids (such as arginine, cysteine, tyrosine, and glutamine) are called "semi-essential" because during periods of growth, illness, or physiological stress, the body may need supplementation from food.