Not all proteins are equal. The difference isn't just about how many grams are in a food - it's about what amino acids are present and in what proportions.
Here's a practical, evidence-based breakdown.
What Makes a Protein "Complete"?
Proteins are chains of amino acids. The body uses 20 amino acids to build proteins. Eleven of these your body can manufacture itself. The remaining nine are "essential" - meaning the body can't make them in adequate amounts, so they must come from food.
A complete protein contains all nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Leucine in particular gets significant research attention - it's the primary trigger for muscle protein synthesis (the process of building and repairing muscle tissue). This is why protein intake quality matters alongside quantity, particularly for people focused on muscle maintenance or athletic performance.
An incomplete protein is missing one or more essential amino acids, or has them in amounts too low to support synthesis. This doesn't mean incomplete proteins are worthless - it means they're best combined with complementary sources.
Complete Animal Proteins
Eggs
One of the highest quality proteins available by any measurement. Eggs score extremely high on the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which adjusts for how well the protein is absorbed.
Protein per serving: ~6g per large egg, ~13g per 100g
All nine essential amino acids in well-balanced proportions. The yolk contributes fat and vitamins; the white is primarily protein. Whole eggs consistently outperform egg white in research on muscle protein synthesis - the fat and micronutrients in the yolk appear to enhance protein utilisation.
Chicken Breast
The standard lean complete protein. High in leucine, low in saturated fat, very high protein-to-calorie ratio.
Protein per serving: ~31g per 100g cooked
Turkey
Similar profile to chicken breast - complete protein, lean, high leucine content.
Protein per serving: ~29g per 100g cooked
Beef
Beef is a complete protein and also one of the most bioavailable sources of iron, zinc, and B12. Fattier cuts have a lower protein-to-calorie ratio but the protein quality remains high. Grass-fed beef has a slightly more favourable fatty acid profile.
Protein per serving: ~26g per 100g cooked (lean cuts)
Fish
All fish are complete proteins. Fatty fish (salmon, mackerel, sardines, trout) add omega-3 fatty acids to the complete protein profile. Leaner fish (cod, tuna, tilapia) have a higher protein-to-calorie ratio.
Protein per serving: salmon ~25g per 100g, tuna (tinned in water) ~26g per 100g
Dairy (Greek Yogurt, Cottage Cheese, Milk, Cheese)
All dairy products are complete proteins. Greek yogurt and cottage cheese stand out for protein density and versatility.
| Food | Protein per 100g |
|---|---|
| Greek yogurt (plain, full-fat) | ~10g |
| Greek yogurt (0% fat) | ~9g |
| Cottage cheese | ~11g |
| Hard cheese (cheddar) | ~25g |
| Milk (whole) | ~3.2g |
| Whey protein powder | ~80g |
Dairy proteins are particularly well-studied for muscle protein synthesis. Whey protein (derived from dairy) has the highest leucine content of any common protein source.
Shellfish
Shrimp, crab, lobster, scallops - all complete proteins, generally low in fat and calories.
Protein per serving: shrimp ~24g per 100g cooked
Complete Plant Proteins
The narrative that plant proteins are all incomplete is outdated. Several plant foods contain all nine essential amino acids in adequate proportions.
Soy (Edamame, Tofu, Tempeh, Soy Milk)
Soy is the most comprehensively studied plant protein. It's complete, high in leucine relative to other plant proteins, and has a PDCAAS comparable to meat and dairy.
| Form | Protein per 100g |
|---|---|
| Edamame (cooked) | ~11g |
| Firm tofu | ~8g |
| Tempeh | ~19g |
| Soy milk (unsweetened) | ~3g |
Tempeh is particularly useful - the fermentation process increases bioavailability and produces beneficial compounds for the gut microbiome.
Quinoa
Quinoa is a seed (not a grain, technically) that contains all nine essential amino acids. It's complete and also gluten-free.
Protein per serving: ~8g per 185g (1 cup cooked)
The protein density isn't high compared to meat, but quinoa's completeness makes it a useful anchor for plant-based meals.
Buckwheat
Another seed-grain often categorised as a grain. Complete protein profile, high in lysine (an amino acid most other grains are low in).
Protein per serving: ~6g per 170g (1 cup cooked)
Hemp Seeds
Hemp seeds are complete protein and one of the few plant foods with a favourable omega-3 to omega-6 ratio.
Protein per serving: ~10g per 30g (3 tablespoons)
Easy to add to yogurt, smoothies, or salads without significantly changing the flavour.
Chia Seeds
Complete protein with all nine essential amino acids. Lower in overall protein than hemp but high in fibre and omega-3 ALA.
Protein per serving: ~5g per 28g (2 tablespoons)
Sacha Inchi
A seed increasingly found in protein powders. Complete amino acid profile, particularly high in tryptophan and omega-3. Less common but worth knowing about.
Protein per serving: ~8g per 30g
Incomplete Plant Proteins - and How to Combine Them
Incomplete plant proteins are missing or low in one or more essential amino acids. The "limiting amino acid" concept: each plant protein typically has one amino acid that's present in the lowest relative amount, limiting the total protein utilisation.
Grains tend to be low in lysine. Legumes tend to be low in methionine. Combining them across meals (not necessarily within the same meal - the body maintains an amino acid pool for several hours) produces a complete amino acid profile.
Common complementary combinations:
| Combination | What It Covers |
|---|---|
| Rice + lentils | Grains (methionine) + legumes (lysine) |
| Beans + corn tortilla | Legumes + grain |
| Peanut butter + whole grain bread | Legume + grain |
| Hummus + pita bread | Legume + grain |
| Lentil soup + whole grain bread | Legume + grain |
You don't need to obsessively track combinations. A varied plant-based diet that includes legumes, grains, nuts, and seeds will cover essential amino acids without precise meal planning.
A Practical Comparison Table
| Source | Protein/100g | Complete? | Notes |
|---|---|---|---|
| Chicken breast | 31g | Yes | Lean, high leucine |
| Tuna (tinned) | 26g | Yes | Very convenient, affordable |
| Eggs | 13g | Yes | Highly bioavailable |
| Greek yogurt | 9-10g | Yes | Also provides calcium, probiotics |
| Tempeh | 19g | Yes | Best plant protein density |
| Quinoa | 4g* | Yes | *per 100g cooked |
| Hemp seeds | 33g | Yes | *per 100g dry weight |
| Lentils (cooked) | 9g | No | High lysine, low methionine |
| Chickpeas (cooked) | 9g | No | Pair with grains |
| Almonds | 21g | No | Also high in fat |
How Much Protein You Actually Need
Food quality matters - but quantity matters too. See the detailed breakdown in how much protein per day - the short version is 1.6-2.2g per kg of bodyweight for active adults trying to maintain or build muscle, and 1.2-1.6g/kg for general health maintenance.
Getting adequate total protein from a variety of complete and complementary sources covers both quantity and quality.