Amino acids are the building blocks of protein. Your body uses them to build and repair tissue, make enzymes and other working proteins, and produce signaling molecules such as hormones. When needed, it can also burn them for energy.[1][7] Nine amino acids are essential and must come from food. The current adult RDA for protein is 0.8 g per kilogram of body weight, and the 2025-2030 Dietary Guidelines set a higher target of 1.2 to 1.6 g/kg.[1][2][3]
Key Takeaways
- Nine amino acids are essential, meaning your body cannot make them. The rest can be made from other compounds.[1]
- Your body constantly breaks down and rebuilds protein. Between meals, muscle breakdown outpaces synthesis, and eating protein shifts the balance back.[3][5]
- Amino acids can be burned for energy. The liver turns the nitrogen they release into urea for removal.[1][8]
- Protein targets: RDA 0.8 g/kg; Dietary Guidelines 2025-2030: 1.2 to 1.6 g/kg; the ISSN advises 1.4 to 2.0 g/kg for most people who exercise.[2][3]
- Complete, leucine-rich proteins at 20 to 40 g per meal stimulate muscle protein synthesis. No human study has shown that BCAA supplements alone do the same.[3][5]
- In a meta-analysis of healthy adults, higher protein intakes did not change kidney filtration.[6]
Evidence grade: Strong. Amino acid roles and protein needs rest on established reference values, an expert position stand and meta-analyses of randomized trials in people. Evidence for isolated BCAA supplements is weak.
What Amino Acids Are
When you eat protein, digestion breaks it into amino acids. Your body then strings them back together into the proteins it needs.[1] MedlinePlus sorts the 20 amino acids in the table below into three groups.[1]
| Group | What it means | Amino acids |
|---|---|---|
| Essential | Cannot be made by the body; must come from food | Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine |
| Nonessential | Made by the body even if your diet lacks them | Alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, tyrosine |
| Conditionally essential | Usually made in enough amounts, but may need to come from food during illness or stress | Arginine, cysteine, glutamine, tyrosine, glycine, proline, serine |
Source: MedlinePlus.[1] You do not need every essential amino acid at every meal. What matters is getting a good balance across the whole day, so you do not need to pair foods like beans and rice at one sitting. A diet built on a single plant food, however, will not be adequate.[1]
Protein Turnover: The Body's Constant Rebuild
Body proteins are not permanent. They are continually broken down and rebuilt, and the amino acids released join a pool that supplies new protein. In the fasted state between meals, muscle protein breakdown runs ahead of synthesis. The essential amino acids needed to make new protein then have to come from breaking down existing protein.[5] Some essential amino acids are also always being burned, so the pool needs a steady supply from food.[5]
Eating protein reverses this. A dose of 20 to 40 g of protein, supplying about 10 to 12 g of essential amino acids and 1 to 3 g of leucine, stimulates muscle protein synthesis. After resistance exercise, muscle stays more responsive to protein for up to 24 hours.[3]
How Amino Acids Feed Energy Metabolism
Besides serving as building blocks, amino acids can also be used for fuel.[1] Before an amino acid can be burned, its nitrogen is removed, producing ammonia. Ammonia is toxic, so the liver converts it into urea in an energy-requiring process called the urea cycle, and the urea is then cleared from the body.[8] As your body uses protein, it produces this kind of waste, which the kidneys remove from the blood.[10]
Amino acids also help steer metabolism. Beyond building proteins, they act as signaling molecules and help regulate which genes are switched on. A group sometimes called functional amino acids, including arginine, cysteine, glutamine, leucine, proline and tryptophan, helps regulate key metabolic pathways.[7] For how hormones shape the way you use nutrients, see hormonal effects on nutrient metabolism.
Building Blocks for More Than Muscle
Amino acids are raw materials for many small molecules your body depends on. These include nitric oxide, glutathione, taurine, polyamines, thyroid hormones and the neurotransmitter serotonin. Balance matters, though. A normal supply supports these functions, while abnormally high levels of some amino acid products, such as ammonia and homocysteine, are harmful.[7] Digestion is the first step in getting amino acids from food; see why digestive enzymes matter.
How Much Protein Do You Need?
Several reference points exist, and they answer different questions. The RDA of 0.8 g/kg a day is the long-standing baseline for adults.[3] The Dietary Guidelines for Americans, 2025-2030 set a higher target of 1.2 to 1.6 g/kg a day, drawn from both animal and plant sources.[2] The International Society of Sports Nutrition advises 1.4 to 2.0 g/kg for most exercising people.[3] In a meta-analysis of 49 trials of resistance training, extra protein stopped adding fat-free mass above about 1.6 g/kg a day.[4]
| Reference | Daily protein | Example: 70 kg (154 lb) adult |
|---|---|---|
| RDA[3] | 0.8 g/kg | About 56 g |
| Dietary Guidelines 2025-2030[2] | 1.2 to 1.6 g/kg | About 84 to 112 g |
| ISSN, exercising adults[3] | 1.4 to 2.0 g/kg | About 98 to 140 g |
| Point where extra protein stopped adding muscle in training trials[4] | About 1.6 g/kg | About 112 g |
The ISSN suggests spreading intake as 20 to 40 g doses (about 0.25 g/kg) every 3 to 4 hours.[3] If you eat a plant-based diet, see essential nutrients often missing in vegan diets.
Leucine and the BCAA Question
Leucine, isoleucine and valine are the branched-chain amino acids (BCAAs).[5] Leucine has drawn the most attention because it helps switch on muscle protein synthesis. The ISSN recommends that each protein dose contain about 700 to 3,000 mg of leucine as part of a full set of essential amino acids. Rapidly digested proteins rich in essential amino acids and leucine, such as whey, are the most effective, and whey taken around exercise increases mTOR signaling, a key growth pathway in muscle.[3]
BCAAs on their own are a different story. A 2017 review found no human studies that measured muscle protein synthesis after oral BCAAs alone. Two intravenous studies found that BCAAs lowered both muscle protein synthesis and breakdown. The author concluded that the claim that BCAAs alone build muscle is unwarranted, because the other essential amino acids are still needed.[5] He has received funding from food and nutrition companies, which readers may weigh. By contrast, a meta-analysis of 49 randomized trials with 1,863 adults found that whole-protein supplements added on average 0.30 kg of fat-free mass and 2.49 kg of one-rep-max strength during resistance training. The benefit was smaller in older adults.[4]
Food Sources of Protein and Leucine
| Food (per 100 g) | Protein | Leucine |
|---|---|---|
| Chicken breast, roasted | 31.0 g | 2.33 g |
| Atlantic salmon, farmed, cooked | 22.1 g | 1.80 g |
| Almonds | 21.2 g | 1.47 g |
| Tofu, firm (calcium sulfate) | 17.3 g | 1.39 g |
| Egg, hard-boiled | 12.6 g | 1.08 g |
| Lentils, boiled | 9.0 g | 0.65 g |
| Black beans, boiled | 8.9 g | 0.71 g |
| Whole milk | 3.2 g | 0.30 g |
Values from USDA FoodData Central (SR Legacy).[9] Values are per 100 g of the food as listed, not per typical serving.
Safety and Interactions
High-protein diets. A meta-analysis of 28 trials with 1,358 adults without kidney disease compared higher-protein diets (at least 1.5 g/kg, 20% of energy or 100 g a day) with normal or lower intakes. Kidney filtration did not change differently between the groups.[6] The ISSN likewise reports no controlled evidence of harm from higher protein in healthy people who exercise, though the longest studies are small.[3]
Kidney conditions. Protein breaks down into waste that the kidneys remove. Some people with chronic kidney disease are advised to eat moderate amounts of protein, while eating too little can lead to malnutrition. If you have a kidney condition, work with your doctor or a dietitian on your protein target.[10]
Supplements. Protein powders can contain other ingredients, some of which raise the risk of contamination. Third-party audited products help manage that risk but cannot guarantee safety.[11] Single amino acid and BCAA products are dietary supplements, not substitutes for adequate total protein.[5] Talk to your doctor or pharmacist before starting amino acid supplements if you take prescription medication, are pregnant or breastfeeding, or have a medical condition.
Frequently Asked Questions
What are the nine essential amino acids?
Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Your body cannot make them, so they must come from food.
Do amino acids give you energy?
They can. Amino acids can be burned for fuel after their nitrogen is removed and turned into urea by the liver. Their main job, though, is building and repairing body proteins.
How much protein do I need per day?
The RDA is 0.8 g per kilogram of body weight. The Dietary Guidelines for Americans, 2025-2030 set a target of 1.2 to 1.6 g/kg, and the ISSN advises 1.4 to 2.0 g/kg for most people who exercise. For a 70 kg adult, that is roughly 56 g, 84 to 112 g and 98 to 140 g a day.
Are BCAA supplements worth it?
The evidence is weak. A review found no human studies showing that oral BCAAs alone raise muscle protein synthesis, since the body also needs the other essential amino acids. Complete protein from food or whole-protein supplements has stronger support.
Can you get all the essential amino acids from plants?
Yes, if you eat a variety of plant proteins across the day. You do not need to combine them at each meal, but a diet relying on a single plant food will not be adequate.
Is a high-protein diet hard on your kidneys?
In healthy adults, a meta-analysis of 28 trials found that higher protein intakes did not change kidney filtration. People with an existing kidney condition should follow a protein target set with their doctor or dietitian.
Sources
- MedlinePlus Medical Encyclopedia. Amino acids. National Library of Medicine.
- U.S. Departments of Agriculture and Health and Human Services. Dietary Guidelines for Americans, 2025-2030 (realfood.gov).
- Jäger R, et al. International Society of Sports Nutrition position stand: protein and exercise. J Int Soc Sports Nutr. 2017.
- Morton RW, 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. Br J Sports Med. 2018.
- Wolfe RR. Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? J Int Soc Sports Nutr. 2017.
- Devries MC, et al. Changes in kidney function do not differ between healthy adults consuming higher- compared with lower- or normal-protein diets: a systematic review and meta-analysis. J Nutr. 2018.
- Wu G. Amino acids: metabolism, functions, and nutrition. Amino Acids. 2009.
- Barmore W, Azad F, Stone WL. Physiology, Urea Cycle. StatPearls. 2023.
- U.S. Department of Agriculture. FoodData Central, SR Legacy (FDC IDs 171477, 175168, 173424, 172475, 170567, 172421, 173735, 171265).
- National Institute of Diabetes and Digestive and Kidney Diseases. Eating Right for Chronic Kidney Disease.
- Maughan RJ, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Br J Sports Med. 2018.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
