Running the numbers
Beans are the only common food that is simultaneously a protein source, a starch, and one of the densest fibre sources on the shelf. Every claim below is computed live from the composition table — change the bean and watch the arithmetic follow.
Black bean
Energy, and where it comes from
Atwater general factors: protein and available carbohydrate 4 kcal/g, fat 9, and fibre credited at 2 kcal/g for the short-chain fatty acids absorbed after colonic fermentation. Per 100 g cooked.
Macronutrients
Minerals, as % of an adult daily value
Per 100 g, cooked
Serving calculator
Daily values are the adult reference intakes used on nutrition labels (2,000 kcal basis). Dry-weight equivalent assumes the seed absorbs its stated multiple of water during soaking and cooking.
The one thing beans are short of
Protein quality is not how much protein a food has. It is whether the mix of amino acids matches what a human needs — so it is measured as a ratio against a reference pattern, and the lowest ratio is the one that counts. Beans are exceptionally rich in lysine and consistently short of the sulphur amino acids, methionine and cysteine.
Amino-acid score
WHO/FAO/UNU 2007 adult patternBeans and rice, arithmetically
Cereals are short of lysine and rich in methionine. Legumes are the exact mirror. This is not folklore — it is two vectors whose deficits do not overlap, and you can watch them cancel.
Protein blender
Rice and beans
Latin America, the Caribbean, the American South. Roughly half the protein from each gives a blend with no amino acid below requirement.
Dal and roti
The same arithmetic with lentil and wheat. Wheat is the more lysine-deficient of the two cereals, so it needs proportionally more legume.
Maize and beans
Mesoamerica, and the Three Sisters. Maize is short of both lysine and tryptophan; beans supply both, which is why the pairing outlasted every empire that grew it.
The iron you read is not the iron you get
A cup of cooked lentils lists about 6.6 mg of iron — more than a similar serving of beef. Almost none of that comparison survives contact with absorption data.
Bean iron is entirely non-haem iron, which enters through the DMT1 transporter and is at the mercy of everything else in the meal. Haem iron from meat has its own receptor and absorbs at 15–35% almost regardless of context. Non-haem iron from a phytate- and tannin-rich legume absorbs at roughly 2–10%.
- Vitamin C, 50 mg with the mealup to ×3–4
- Soaking, sprouting, fermenting×1.5–2
- Meat in the same meal×1.5–2
- Tea or coffee with the meal×0.4–0.6
- Calcium supplement with the meal×0.5–0.7
- Whole-seed phytate, untreated×0.3–0.5
Iron, per 100 g cooked
mgProtein against energy
Every legume in the reference, plotted per 100 g cooked. The cluster at the bottom left is what "a bean" means nutritionally; the outliers are the oilseeds, which are a different kind of food.