Best Protein Sources Compared: Animal vs Plant, Cost, Bioavailability, and Practicality

The protein requirement debate, how much to eat, from what sources, in what combinations, generates enormous confusion partly because the question conflates several separate issues: how much total protein, what quality (amino acid completeness), how digestible, and at what cost. The UK recommended dietary allowance is 0.75g of protein per kilogram of body weight per day for sedentary adults, rising to 1.2 to 2.0g/kg/day for active adults and athletes (the range reflects training intensity; 1.6g/kg/day is widely cited as optimal for muscle protein synthesis in people doing resistance training). The most important protein quality metric is not just the gram count per serving but the quality and digestibility of that protein, which varies significantly across sources.

Understanding Protein Quality: PDCAAS and DIAAS

Two scoring systems are used to evaluate protein quality:

• PDCAAS (Protein Digestibility-Corrected Amino Acid Score): The older standard (FAO, 1993), scoring proteins relative to human amino acid requirements with a maximum score of 1.0. Widely used on supplement labelling and in research to 2013.
• DIAAS (Digestible Indispensable Amino Acid Score): The current gold standard (FAO, 2013), which measures true ileal digestibility (how much of each amino acid is actually absorbed) rather than fecal digestibility used in PDCAAS. DIAAS values over 100 indicate a protein source that exceeds human requirements for all essential amino acids; values under 75 indicate an inadequate or limiting protein source.

Animal Protein Sources

Whey Protein (DIAAS: 109 to 133)

Whey is the liquid remaining after milk is curdled and strained to make cheese. Whey protein concentrate (70% to 80% protein by weight) and whey protein isolate (90%+ protein, lower lactose) are the most commonly used protein supplements. Whey's DIAAS score exceeds 100, meaning it delivers more than adequate amounts of all essential amino acids per gram. Leucine content is particularly high (approximately 10% of amino acids), which directly stimulates muscle protein synthesis through the mTOR pathway. A 25g scoop of whey isolate provides approximately 23g of protein at a cost of £0.25 to £0.50 depending on brand. MyProtein Impact Whey Isolate, Bulk Whey Protein, and Optimum Nutrition Gold Standard are the most commonly recommended brands by price-to-quality ratio.

Eggs (DIAAS: 113)

The egg white is pure albumin protein, DIAAS-scored as the reference standard for protein quality. A whole egg (large, approximately 50g) provides 6 to 7g of complete protein at approximately £0.20 to £0.35 per egg in the UK. Eggs are the most convenient and cost-effective complete protein for most home cooks, with no preparation required and high culinary versatility. The egg yolk contains most of the fat-soluble vitamins (D, A, K, choline) that make whole eggs nutritionally superior to egg whites alone.

Chicken Breast (DIAAS: 108)

100g of cooked chicken breast provides approximately 31g of protein at approximately 165 kcal, making it one of the highest protein-to-calorie ratio foods available. At approximately £3 to £5 per kilogram of raw chicken breast, the protein cost is £0.10 to £0.16 per gram of protein, one of the lowest available in animal sources. The limitation: palatability fatigue from high chicken consumption is one of the most common complaints from high-protein dieters.

Cottage Cheese (DIAAS: 117)

A 200g serving of full-fat cottage cheese provides 24 to 26g of casein-dominant protein at approximately 180 to 200 kcal. Casein is a slow-digesting protein (takes 5 to 7 hours to fully digest) compared to whey (absorbed in 30 to 90 minutes), making cottage cheese a particularly useful pre-sleep protein source for maintaining muscle protein synthesis overnight. The Arla and Longley Farm cottage cheese brands are widely available in UK supermarkets at £1.50 to £2.00 for 300g.

Greek Yogurt (DIAAS: ~115)

A 170g serving of full-fat Greek yogurt provides 15 to 20g of protein (higher in strained varieties). Greek yogurt has a convenient, high-palatability format and the added benefit of live probiotic cultures. Fage Total 0% and 2% are the most protein-dense commercially available options (approximately 10g per 100g for Fage 0%).

Plant Protein Sources

Soy (DIAAS: 91 to 104)

Soy is the only plant protein with a DIAAS score approaching completeness. Soy protein isolate (90%+ protein) scores 104 on DIAAS, making it the only plant-based protein supplement that is genuinely comparable to whey for amino acid completeness. Edamame (young soybeans, 18g protein per 240ml serving), firm tofu (15g per 100g), and tempeh (19g per 100g) provide soy protein in whole food formats with good bioavailability.

Pea Protein (DIAAS: 64 to 82)

Pea protein isolate is the most common plant-based protein supplement due to its allergen-free profile and reasonable amino acid profile, but it is low in the essential amino acid methionine. Most commercial plant protein supplements combine pea protein with rice protein (which has a complementary methionine content) to produce a more complete amino acid profile. The combination DIAAS score approaches 100 when the ratio is approximately 70:30 pea:rice protein.

Lentils and Chickpeas (DIAAS: 60 to 75)

Cooked lentils provide 18g of protein per 240ml cup at extremely low cost (approximately £0.40 to £0.60 per kilogram dried). The DIAAS score is limited by low methionine and cysteine content. As the basis of a varied plant-based diet that includes complementary amino acid sources, lentils and legumes are excellent protein contributors; as a sole protein source, their amino acid profile is insufficient.

Cost Per Gram of Protein: A Practical Comparison

Related: Whey Protein Guide: Types, Benefits, and How to Use It | Dairy and Bone Health: Calcium, Osteoporosis, and What the Research Shows