Walk into any gym and you’ll spot someone shaking up a brightly coloured drink between sets. Chances are it’s a BCAA supplement. These three amino acids have become almost synonymous with fitness culture, but the actual science behind them is more nuanced than the marketing suggests. I’ve spent time going through the clinical research, and the picture that emerges is genuinely mixed.

BCAAs account for roughly 35% of the essential amino acids in muscle protein and about 50% of the essential amino acids we get from food. That’s a substantial proportion, which explains why they’ve attracted so much attention from researchers and athletes alike. 1

What are BCAAs?

BCAA stands for branched-chain amino acid. The name refers to their molecular structure, which features an aliphatic side chain that branches off. The three BCAAs are:

Leucine - Often considered the most important for muscle protein synthesis
Isoleucine - Plays a role in glucose uptake by muscles
Valine - Involved in energy production and muscle metabolism

What makes BCAAs unusual is how they’re metabolised. Unlike most amino acids, which are broken down primarily in the liver, BCAAs bypass this step. The enzyme responsible for their initial breakdown (branched-chain amino acid aminotransferase, or BCAT) has relatively low activity in the liver. This means that after you eat protein, BCAAs enter your bloodstream quickly and can be taken up directly by your muscles, brain, kidneys, and other tissues.

This rapid availability is why BCAA supplements have gained popularity for exercise. The thinking goes: if you can get these amino acids to your muscles faster, perhaps you can speed up recovery or reduce muscle breakdown during training.

What the research actually shows

The evidence for BCAAs is genuinely complicated. Some studies show clear benefits, others show nothing. Here’s what we know with reasonable confidence.

1. Reducing delayed-onset muscle soreness

You know that deep muscle ache that hits 24 to 72 hours after a hard workout? That’s delayed-onset muscle soreness (DOMS), and it can genuinely interfere with training if you’re trying to maintain consistency.

A 2019 meta-analysis pooling data from five randomised controlled trials with 93 participants found that BCAA supplementation did reduce DOMS compared to placebo. The effect was consistent regardless of whether participants were male or female, and whether they were trained or untrained. 1

The proposed mechanism relates to how BCAAs influence muscle protein synthesis and reduce protein breakdown during recovery. Muscles repair faster, so they hurt less. That’s the theory, anyway.

My take: this is probably the best-supported benefit of BCAA supplements. If you struggle with soreness after training, they might be worth trying.

2. Post-surgical recovery in liver cancer patients

This one surprised me. BCAAs appear to help patients recovering from liver cancer surgery, particularly those with underlying cirrhosis.

Cirrhosis commonly causes malnutrition because it impairs protein synthesis and reduces appetite. When these patients undergo surgery for hepatocellular carcinoma (the most common type of primary liver cancer), their nutritional status affects recovery.

A systematic review and meta-analysis looking at 13 randomised controlled trials found that BCAA supplementation during cancer surgery reduced post-operative infection rates compared to controls. There were also benefits for ascites (fluid accumulation in the abdomen), weight maintenance, and length of hospital stay. 2

The review didn’t find differences in mortality, cancer recurrence, or other complications like liver failure. So BCAAs aren’t a miracle intervention, but they do seem to support recovery in this specific population.

For readers dealing with liver conditions, you might also find our article on fatty liver and health supplements relevant.

3. Tardive dyskinesia symptoms

Tardive dyskinesia is a movement disorder characterised by involuntary, repetitive movements of the face, tongue, trunk, and limbs. It typically develops in people who’ve taken antipsychotic medications long-term, affecting somewhere between 15% and 30% of long-term users.

The exact cause isn’t fully understood, but theories include dopamine receptor hypersensitivity, depletion of the neurotransmitter GABA, and oxidative stress.

A small randomised placebo-controlled trial with 36 men found that BCAA supplementation over three weeks improved tardive dyskinesia symptoms. The improvement wasn’t related to changes in antipsychotic medications or blood glucose levels. 3

This is a genuinely interesting finding, but I need to flag the small sample size. Thirty-six participants is not enough to draw firm conclusions. More research is needed before BCAAs could be recommended for this use.

4. Muscle mass and strength

Here’s where the marketing and the science diverge most sharply.

BCAAs are essential for muscle protein synthesis. That’s not in dispute. But there’s a big difference between “necessary for muscle building” and “supplementing extra will build more muscle.”

Current research doesn’t show that BCAA supplements produce better muscle gains than simply eating adequate protein. In fact, whey protein combined with training produces better results than isolated amino acid supplements. 4

This makes sense when you think about it. Whey protein contains all nine essential amino acids (including the three BCAAs) plus other compounds that support muscle protein synthesis. A BCAA supplement only gives you three.

If you’re already eating sufficient protein from food, adding BCAA supplements probably won’t make much difference to your muscle gains. The people most likely to benefit are those who aren’t getting enough total protein, or who train in a fasted state.

For more on muscle-supporting supplements, see our articles on creatine and L-glutamine.

5. Fatigue during exercise

Several studies suggest that taking BCAAs during exercise can reduce perceived fatigue compared to placebo. The mechanism may relate to reduced muscle damage and inflammation. 5-6

One controlled study with 13 participants doing a cycling stress test found that BCAA supplementation reduced fatigue and improved performance by 12%. 7

Again, small sample sizes limit what we can conclude. But if you’re doing long training sessions and notice significant fatigue, BCAAs might help. The effect seems more pronounced during endurance exercise than short, intense efforts.

6. Traumatic brain injury recovery

Traumatic brain injury (TBI) ranges from mild concussion to severe brain damage. Recovery depends on evolving processes including brain swelling, increased intracranial pressure, and reduced blood flow to affected areas.

A systematic review of 11 studies found that BCAAs may help improve cognitive function after severe traumatic brain injury. The evidence for mild TBI was insufficient to draw conclusions. 8

The proposed mechanism involves BCAAs’ role in ATP (energy) production and insulin regulation in the brain. Brain tissue has high energy demands, and providing additional substrates for energy production might support healing.

This is fascinating research, but it’s not at the stage where anyone should be self-treating head injuries with supplements. If you’ve had a TBI, work with your healthcare team.

Side effects

For most healthy adults, BCAAs are safe when taken at appropriate doses. Reported side effects include:

Nausea
Vomiting
Diarrhoea
Bloating
Fatigue
Loss of coordination during exercise

Most side effects seem related to either pre-existing health conditions or taking too much. Sticking to recommended doses and taking BCAAs with food can reduce gastrointestinal symptoms.

Who should avoid BCAAs (8 contraindications)

Not everyone should take BCAA supplements. Here are the groups who need to exercise caution or avoid them entirely:

1. Amyotrophic lateral sclerosis (ALS)

Also called motor neurone disease or Lou Gehrig’s disease. BCAAs may increase the risk of lung failure and mortality in people with ALS. This is a serious contraindication. 10

2. Maple syrup urine disease

This is a rare inherited metabolic disorder where the body cannot properly break down BCAAs. Taking additional BCAAs can cause serious complications including developmental delays, seizures, and coma. 11

3. Diabetes or blood sugar medications

BCAAs can lower blood glucose levels. If you’re taking medications for diabetes, this could cause hypoglycaemia (dangerously low blood sugar). Relevant medications include:

Glimepiride
Glyburide
Insulin
Pioglitazone
Rosiglitazone
Chlorpropamide
Glipizide
Tolbutamide

If you have diabetic bladder dysfunction or other diabetes-related conditions, speak with your doctor before taking BCAAs.

4. Pregnancy and breastfeeding

There isn’t enough safety data to recommend BCAAs during pregnancy or while breastfeeding. The precautionary principle applies here.

5. Poor liver or kidney function

Since the liver and kidneys process amino acids, people with impaired organ function should avoid BCAA supplements unless specifically advised by their healthcare provider.

6. Infants

Safety in infants hasn’t been established. Don’t give BCAA supplements to babies or young children.

7. Parkinson’s disease medications (Levodopa)

BCAAs may interfere with the effectiveness of levodopa, a common Parkinson’s medication. The amino acids compete for the same transport mechanisms in the brain.

8. Other medication interactions

BCAAs may interact with:

Diazoxide (used for hypoglycaemia)
Thyroid hormones
Corticosteroids

These medications can affect how your body metabolises BCAAs.

Pre-surgery caution: Stop taking BCAAs at least two weeks before any scheduled surgery because of their potential effects on blood glucose levels.

How much to take

Most studies use doses between 5 and 20 grams per day. Common ratios are 2:1:1 (leucine:isoleucine:valine), though some products use higher leucine ratios like 4:1:1 or 8:1:1.

The timing varies by purpose:

For exercise performance and fatigue: Take during your workout
For muscle soreness: Take before or immediately after training
For general recovery: Split doses throughout the day

According to the Mayo Clinic’s review of amino acid supplementation, starting with lower doses and gradually increasing allows you to assess tolerance. Mayo Clinic

The bottom line

BCAAs have genuine uses, but they’re not the universal muscle-building solution that marketing often suggests. The strongest evidence supports their use for reducing muscle soreness after exercise. Benefits for surgical recovery, certain neurological conditions, and exercise fatigue are promising but need more research.

If you’re eating adequate protein from food or using a complete protein supplement like whey, you may not need additional BCAAs. However, they could be worthwhile if you:

Train fasted
Have trouble eating enough protein
Experience significant post-workout soreness
Do very long training sessions where fatigue is limiting

As with any supplement, quality matters. Look for products tested by third parties like NSF International or Informed Sport, particularly if you compete in sports with anti-doping regulations.

For broader information on supplements and bladder health, you might find our urinary tract infection guide helpful, as some supplements can affect urinary function.

References

Fedewa MV, et al. (2019). Effect of branched-chain amino acid supplementation on muscle soreness following exercise: a meta-analysis. Int J Vitam Nutr Res. PMID: 30938579
Zhu W, et al. (2021). Branched-chain amino acid supplementation during cancer surgery: a systematic review and meta-analysis. J Cachexia Sarcopenia Muscle. PMC7930658
Richardson MA, et al. (2003). Branched chain amino acids decrease tardive dyskinesia symptoms. Psychopharmacology. PMID: 12777270
Hulmi JJ, et al. (2010). Effect of protein/essential amino acids and resistance training on skeletal muscle hypertrophy. Nutr Metab. PMID: 20565767
Shimomura Y, et al. (2006). Nutraceutical effects of branched-chain amino acids on skeletal muscle. J Nutr. PMID: 20087302
Newsholme EA, et al. (1987). Branched-chain amino acids and central fatigue. J Nutr. PMID: 9475648
Aquilani R, et al. (2016). Branched-chain amino acids enhance the cognitive recovery of patients with severe traumatic brain injury. Arch Phys Med Rehabil. PMID: 28060208
Testa D, et al. (1989). High-dose L-valine in ALS: a double-blind, placebo-controlled trial. Neurology. PMID: 8255440
Chuang DT, Shih VE. (2001). Maple syrup urine disease (branched-chain ketoaciduria). The Metabolic and Molecular Bases of Inherited Disease. PMID: 7744549