Bioavailability of protein in veg foods
Measuring bioavailability: the digestible Indispensable Amino Acid Score
Protein quality is arguably best described using the Digestible Indispensable Amino Acid Score (DIAAS; Moughan, 2021). Its calculation requires information on the IAA contents of a food
and provides estimates of their true ileal digestibility. A food is given a score of 1 or higher if the absorbed IAA are all utilizable, whereas a lower score indicates that only a portion of the absorbed IAA are available for utilization.
For meat, DIAAS values fall in the range of 0.8–1.4, whereas values for most traditional plant proteins are markedly lower (Marinangeli and House, 2017). In general, values for legumes range between 0.4 and 1.1, which are like nuts (0.4–0.9) but generally higher than cereal foods (0.1–0.8). In plants, some IAAs are limiting, and digestibility is reduced due to complex plant cell structures and the presence of fiber and anti-nutritional factors (the effects of which can be partially attenuated through processing). Consequently, with some exceptions (e.g., certain soy-based foods), many protein-rich plant foods fail to reach the ‘good source of protein’ criterion (Marinangeli and House, 2017).
The bioavailability of veg proteins are minimally lower than animal proteins
3 reasons why lower bioavailability of protein is good for westerners
- We eat too much protein anyway, which has negative health effects. If you're not actively building muscle, it's probably good that you're not absorbing all the protein you eat.
- Humanity overconsumes protein: Berners-Lee, Watson and Hewitt 2018
- One of the reasons for lower bioavailability is that fibre in food blocks absorption. Unlike protein, most westerners don't eat enough fibre. So when you eat plants, you get less of a nutrient that you probably don't need more of (protein), but that's directly caused by you getting more of a nutrient that you do need more of (fibre)
- Westerns also typically want to reduce their calorie intake, and the good news is that if protein is not bio-availabile, that means you don't take in the calories. Each g of pure protein contains 4 calories when broken down by your body. If you shit that protein out, you don't absorb the calories. If you're trying to eat fewer calories, lower bioavailability of macronutrients (protein, sugar, carbs, fats) can be a good thing.
- For example: if you ate a food with 20g of protein in it, that protein has 80 calories in it (the food will probably have calories from other nutrients likes fats and carbs too, but let's focus on protein). If the protein has 100% bioavailability, your body gets all 20g of protein, but must pay the "cost" of 80 calories. If the food has a bio-availability of 50%, sure you only absorb 10g of protein, but you only absorb 40 calories. You poo the other 40 calories out.
Key questions
- How widely does bioavailability vary between meat and plants?
- Are there plant protein sources that have bioavailability similar to animals? Is it feasible to just eat plenty of these?
- The argument implies that plant nutrient levels are lower than that of animal products once weighted by bioavailability. So 100g of plants But plants also contain fewer calories too. If we re-portion the plants to have same calories as meat, do plants still provide less nutrition? If so
- is it feasible to overcome to reduce bioavailability by simply eating more of them?
- Are these bio availability claims resting on a good body of evidence or a few weak studies?
Related lit
- Schulze et al., 2023 finds weak evidence that animal protein is worse for Type 2 diabetes risk than plant protein
- Ellinger et al., 2023 finds getting people to eat more protein does not result in weight loss, and there is not enough evidence at present to conclude that animal protein is better or worse for weight than plant protein.