“Beans proteins are not good as animal proteins”

The evidence that matters most here comes from human feeding trials rather than test tubes. In one, nine healthy adults ate a cooked mash of dehulled faba beans containing 20 grams of protein while the contents of their gut were sampled directly through a tube; about 84 per cent of the protein and 88 per cent of the essential amino acids were absorbed [3].

That same trial rated faba bean protein between 0.67 and 0.77 on the standard protein quality scale, where 1.0 means a protein fully covers adult requirements for every essential amino acid. The range depends on which conversion method is used, and histidine and tryptophan were the two amino acids that ran out first [3].

The pattern is older than these methods. A review of dry beans found they hold 15 to 25 per cent protein by dry weight but are typically deficient in the sulphur-containing amino acids methionine and cysteine, and so rate below animal proteins on nutritional quality [1].

None of the 20 studies reviewed describes raw beans, and none of the shortfall is fixed. A review focused on older people concludes that plant proteins are less anabolic - they trigger less muscle building per gram - mainly because of lower digestibility and missing amino acids, but that fermentation, germination, food processing and blending several protein sources all narrow the gap [2].

Does the lower quality show up as less muscle? In the hours after a single meal, yes. In healthy older adults, a meal built around beef triggered about 47 per cent more muscle protein synthesis - the process by which muscle repairs and grows after eating - than a vegan meal with the same calories and protein [5]. The shortfall traces to the same two causes: less complete digestion and less leucine, the amino acid that switches on muscle repair [2].

Over weeks of training, the gap mostly disappears once total intake is high enough. In a 12-week trial, untrained young men on a high-protein vegan diet - about 1.6 grams of protein per kilogram of body weight a day, topped up with soy protein powder - gained the same muscle size and strength as men on a matched diet using whey [4]. That trial used soy, not beans alone, and no study has followed older adults eating only beans as their protein source long-term.

For most adults who already eat enough total protein, the practical effect is small. Beans pair naturally with grains, which supply the amino acids beans lack, and beans add fibre and micronutrients that meat, dairy and eggs do not.