You've probably heard terms like "essential amino acids," "complete protein," and "BCAAs" thrown around in fitness and nutrition circles. But what do they actually mean? And does any of it matter for how you eat?
Here's the science, stripped of the marketing.
What Are Amino Acids?
Amino acids are the building blocks of protein. When you eat protein - whether from chicken, lentils, or eggs - your body breaks it down into individual amino acids during digestion. These amino acids are then absorbed into your bloodstream and used to build and repair tissues, create enzymes and hormones, and support countless biological functions.
There are 20 amino acids that combine in different sequences to form proteins. Think of them like letters in an alphabet - they combine in different arrangements to create different "words" (proteins) with different functions.
The 9 Essential Amino Acids
Of the 20 amino acids, your body can manufacture 11 on its own. The remaining 9 must come from food - these are called essential amino acids because it's essential you eat them.
Leucine
The most important for muscle protein synthesis. Triggers the mTOR pathway that signals your body to build muscle. Found abundantly in meat, dairy, eggs, and soy.
Isoleucine
Involved in muscle metabolism, immune function, and energy regulation. Helps produce haemoglobin. Found in meat, fish, eggs, cheese, and seeds.
Valine
Supports muscle growth and tissue repair. Works alongside leucine and isoleucine (together these three form the "branched-chain amino acids" or BCAAs).
Histidine
Produces histamine, a neurotransmitter vital for immune response, digestion, and sleep-wake cycles. Also important for maintaining the myelin sheath around nerve cells.
Lysine
Essential for protein synthesis, calcium absorption, and collagen production. Often the limiting amino acid in grain-based diets.
Methionine
Supports tissue growth and metabolism. Contains sulphur and helps with zinc and selenium absorption. Important for detoxification processes.
Phenylalanine
Precursor for tyrosine, which produces dopamine, adrenaline, and noradrenaline. Essential for brain function and mood regulation.
Threonine
Important for collagen and elastin (skin and connective tissue). Also plays a role in fat metabolism and immune function.
Tryptophan
Precursor to serotonin (mood regulation) and melatonin (sleep). Often associated with post-meal drowsiness, though the effect is overstated.
Complete vs Incomplete Proteins
A "complete" protein contains all 9 essential amino acids in adequate amounts. An "incomplete" protein is lower in one or more.
Complete Proteins
- Meat, poultry, fish
- Eggs
- Dairy products
- Soy (tofu, tempeh, edamame)
- Quinoa
- Buckwheat
Incomplete Proteins
- Most legumes (beans, lentils)
- Most grains (rice, wheat, oats)
- Nuts and seeds
- Most vegetables
But here's where it gets interesting - and where a lot of nutrition advice gets it wrong.
Myth: You Must Combine Incomplete Proteins at Every Meal
This idea was popularised by Frances Moore Lappé in her 1971 book Diet for a Small Planet. She later retracted it, writing: "In combating the myth that meat is the only way to get high-quality protein, I reinforced another myth."
The American Dietetic Association investigated and found no scientific basis for meal-by-meal protein combining. Your body maintains an amino acid pool that draws from everything you eat throughout the day.
The reality: Eat a variety of protein sources across your day and you'll get all essential amino acids. No complex meal planning required.
BCAAs: The Science Behind the Supplements
Branched-chain amino acids (BCAAs) - leucine, isoleucine, and valine - are heavily marketed as muscle-building supplements. Here's what the research actually shows:
What Leucine Actually Does
Leucine activates the mTOR pathway, which signals your body to start building muscle protein. It's not just a building block - it's a trigger. This is why leucine is considered the most "anabolic" amino acid.
However, a 2017 review in the Journal of the International Society of Sports Nutrition found a significant limitation:
- BCAAs alone can activate the muscle-building signal
- But without the other essential amino acids present, actual muscle protein synthesis is limited
- Your body can't build complete proteins with only 3 of the 9 essential amino acids
The researchers concluded: "There is no reason to consume BCAA supplements for enhanced stimulation of muscle protein synthesis... high-quality protein in foods or supplements should be consumed to provide sufficient BCAA and leucine, as well as the other EAA."
The Bottom Line on BCAAs
If you're eating adequate protein from real food (or complete protein supplements like whey), you're already getting plenty of BCAAs. Standalone BCAA supplements are unnecessary for most people and are less effective than complete protein sources.
How Much of Each Amino Acid Do You Need?
The World Health Organisation provides recommended daily intakes for essential amino acids. For a 70kg adult:
- Leucine: 2,730mg
- Isoleucine: 1,400mg
- Valine: 1,820mg
- Lysine: 2,100mg
- Methionine + Cysteine: 1,050mg
- Phenylalanine + Tyrosine: 1,750mg
- Threonine: 1,050mg
- Tryptophan: 280mg
- Histidine: 700mg
In practice, if you're hitting your overall protein targets (see our protein guide), you're almost certainly meeting these requirements - especially if you include at least some complete protein sources in your diet.
Practical Takeaways
- Eat protein at most meals - this ensures a steady supply of amino acids for your body to use
- Include variety - different protein sources have different amino acid profiles, so variety covers all bases
- Don't stress about combining - eat different proteins throughout the day, not necessarily at the same meal
- Skip the BCAA supplements - complete protein sources are more effective and usually cheaper
- Prioritise leucine-rich foods if building muscle - meat, dairy, eggs, and soy are excellent sources
References
- Wolfe RR. (2017). Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? Journal of the International Society of Sports Nutrition, 14:30.
- WHO/FAO/UNU. (2007). Protein and amino acid requirements in human nutrition. WHO Technical Report Series, 935.
- Cleveland Clinic. (2021). Amino Acids: Benefits & Food Sources. Health Essentials.
- Lappé FM. (1981). Diet for a Small Planet (Revised Edition). Ballantine Books.