Soy is a legume, but nutritionally it does not look exactly like lentils, chickpeas, or beans. It provides more protein and considerably more fat, which helps explain both its higher energy density and the wide range of foods made from it.
Cooked soybeans are the more useful reference
Comparing 100 g of dry soybeans with 100 g of cooked legumes creates an artificial gap. Soybeans absorb water during cooking, gain weight, and become less concentrated per 100 g.
| Nutrient | Cooked mature soybeans, 100 g | 180 g serving |
|---|---|---|
| Calories | ~172 | ~310 |
| Protein | ~18.2 g | ~32.8 g |
| Fat | ~9.0 g | ~16.2 g |
| Fiber | ~6.0 g | ~10.8 g |
| Carbohydrate | ~8.4 g | ~15.1 g |
A generous serving can therefore provide more than 30 g of protein, which is unusually high for a cooked legume.
The serving column also helps translate food-composition data into an actual meal. A bowl of cooked soybeans is not nutritionally equivalent to looking at the dry-bean number before cooking.
Why dry soybeans look twice as high in protein
Dry soybeans are around 36 g protein per 100 g. Once cooked, absorbed water increases the total weight, spreading the same protein across a larger mass.
Cooking has not destroyed half the protein. The main explanation is water dilution.
The same principle applies to calories and minerals: the cooked number per 100 g looks lower largely because more of that 100 g is water. Always compare foods in the same preparation state.
Fat is a major difference from other legumes
Cooked lentils and many beans contain less than 1 g fat per 100 g. Cooked soybeans are closer to 9 g. That helps explain why soybeans provide around 172 calories per 100 g while many other cooked legumes are lower.
Much of soy fat is unsaturated, with a substantial polyunsaturated component. Soybeans also provide ALA, a plant omega-3.
That makes soy unusual among common legumes: it behaves more like a higher-fat legume rather than a very lean pulse. The extra fat is not automatically a disadvantage; it changes calorie density and fatty-acid profile.
High-quality protein for a plant food
Soy contains all nine essential amino acids and has relatively high protein quality compared with many plant foods. That does not mean other legumes are nutritionally useless or that every meal must contain soy.
It does mean tofu, tempeh, cooked soybeans, and soy milk can be especially useful protein foods in vegetarian and vegan diets.
Quantity still varies by product. Soy milk can provide much less protein per serving than tofu or cooked soybeans, so the word “soy” alone does not tell you how much protein a meal contains.
Fiber, iron, and other micronutrients
Whole soybeans retain fiber that is much lower in some refined soy products. They also provide iron, magnesium, potassium, phosphorus, and folate, although phytates can reduce absorption of some minerals.
Pairing plant iron with vitamin C-rich foods can improve nonheme-iron absorption. Soy contributes useful minerals, but the rest of the meal still affects how much is absorbed.
Soybeans, edamame, tofu, and soy milk are not interchangeable
Edamame is immature soybean and contains more water. Tofu is made from coagulated soy milk and varies widely by firmness and coagulant. Tempeh retains the whole fermented bean and is more concentrated. Soy milk is much more diluted.
So “soy” cannot be represented by one nutrient number across every soy food.
Calcium can differ dramatically too. Tofu set with calcium sulfate may provide a lot, while soy milk depends on fortification. For processed soy foods, the Nutrition Facts label is often more useful than memorizing one generic number.
The useful takeaway
Cooked mature soybeans combine something unusual among legumes: high protein, fiber, and a meaningful amount of unsaturated fat. Always compare foods in the same state instead of matching dry soybeans against cooked lentils or beans.
As a whole food, soybeans can provide substantial protein in a realistic serving while still contributing fiber and micronutrients. Their derivatives should be judged individually because water content and processing change concentration considerably.
Sources
- USDA FoodData Central — Composition of dry and cooked mature soybeans.
- FAO — Dietary protein quality evaluation — Context on protein quality and essential amino acids.