The basics of protein digestibility

Protein, an indispensable part of our diet, has multiple roles in the body – particularly in aging for growth, repair and maintaining muscle mass. But dietary proteins are not all equal. Here are some nuts and bolts of this exquisitely complex nutrient.

Inside protein

Protein, carbohydrate and fat are all macronutrients formed by different ratios of carbon, hydrogen and oxygen. Additionally, protein contains nitrogen.

Proteins come in all shapes and sizes. They are created from twenty amino acids joined in fifty to tens of thousands of different combinations to make enzymes, antibodies and hormones, transport molecules and regulate the body’s acid-alkaline balance.

Nine amino acids are essential – the body cannot make them, so they must come from dietary sources. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Six amino acids are conditionally essential (needed in special circumstances like illness) and five are dispensable as the body can synthesise plenty of them.

Measuring protein quality

The biological activity and nutritional benefits of protein vary according to how the amino acids are configured and how the body digests them. Based on this, various methods have been devised to measure protein quality – availability of its amino acids – and digestibility – how the protein is best used.

The biological value (BV) scoring system assumes that protein is the only dietary source of nitrogen. It measures protein quality by calculating a ratio of how much nitrogen is absorbed versus the amount excreted then multiplied by 100 to give a percentage of nitrogen used by the body.

The protein digestibility-corrected amino-acid score (PDCAAS) ranks protein quality by comparing its amino acid profile with a reference score, corrected for faecal nitrogen digestibility. This score was backed jointly by the Food and Agricultural and World Health Organisations (FAO/WHO) in 1989 as the best way to assess protein quality.

But the PDAAS has limitations; for instance measuring amino acid synthesis in the small intestine would be a better assessment of their digestion than in the large bowel (colon). Further analysis has suggested that the PDCAAS tends to underestimate high-quality proteins and overestimate low-quality proteins.

The FAO now recommends the digestible indispensable amino acid score (DIAAS), which measures the digestibility of individual amino acids at the end of the small intestine and may therefore be more accurate.

Which foods are the best protein sources?

Despite some variation, the quality ratings generally agree on high protein food sources. Animal protein contains all the essential amino acids and is therefore complete. Eggs and dairy protein are ranked as high-quality proteins, followed by meat, chicken and fish. Eggs and dairy are easily digested and are good options for people with digestive disorders.

Vegetable protein typically lacks one or more amino acids. But the following combinations provide all essential amino acids and are therefore highly ranked: rice and peas; grains and legumes; grains and vegetables; grains, nuts and seeds; legumes, nuts and seeds. Importantly, the body doesn’t necessarily need all amino acids in one meal – they can be spread out over the day.

Some vegetable proteins do contain all essential amino acids. This includes hempseed, pea protein and quinoa.

This is good news in light of mounting research showing the superior health benefits of a plant-based diet. Plant foods provide fibre and nature’s medicinal cabinet of vitamins, minerals and phytonutrients. Concerns over ethics of animal farming and environmental sustainability are also driving a growing demand for diets higher in plant foods.


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