Legumes and soy

Beans, lentils, chickpeas, peas and soy are among the better-supported everyday foods, but the effects are modest. In randomised trials a daily serving lowers LDL ("bad") cholesterol a little and improves blood-sugar control; in cohorts, the people who eat the most die slightly less often. The popular worries about lectins and soy hormones mostly fail on the evidence, provided the beans are properly cooked.

Eat a serving of legumes most days — at least 130 g cooked, half to two-thirds of a cup, the median dose in the cholesterol trials; a full cup gets you most of the way to the fibre target — as a regular protein in place of red and processed meat. Build up over two to three weeks to let your gut adjust, and never eat raw or undercooked kidney beans.

What counts

Legumes are the seeds of pod-bearing plants. The dried ones — beans, lentils, chickpeas and dried peas — are called pulses; soybeans and their foods (tofu, tempeh, edamame, soy milk) are legumes too, but soy has enough evidence of its own that it gets its own page: Soy. Natto, the fermented Japanese soybean, has another. Peanuts are botanically legumes but are studied with nuts.

Living longer: a small, consistent association

Across 32 cohorts of about 1.1 million people, those eating the most legumes had about 6% lower death rates from any cause than those eating the least, and about 9% fewer stroke deaths. Each extra 50 g a day went with about 6% lower mortality. Deaths from cardiovascular disease as a whole, from coronary heart disease and from cancer showed no clear link, and the authors rated the certainty of the evidence low to moderate.^[1]

A review done for European diabetes guidelines found a similar small association for getting the diseases rather than dying of them: about 8% less cardiovascular disease and 10% less coronary heart disease among the highest legume eaters, with no link to heart attack, stroke or diabetes. It graded the certainty "low" for cardiovascular disease and "very low" for everything else.^[2] A 2023 Nordic review of 31 cohorts found no clear link with cardiovascular disease, stroke or type 2 diabetes at all, though legume intakes in those populations were generally low.^[3]

These are observational studies, so legume eaters may differ in other ways, and much of the benefit probably comes from what legumes replace. Swapping red meat for legumes or nuts is covered under Red and processed meat.

The Blue Zones claim. Beans as a daily staple are a shared feature of the long-lived "Blue Zone" diets, and a study of 785 people aged 70 and over in Japan, Sweden, Greece and Australia is often quoted for it: each extra 20 g of legumes a day went with 7–8% lower mortality over up to seven years, and no other food group predicted survival consistently.^[4] That is a small study, and the case for eating like the Blue Zones rests on large studies run elsewhere; the record ages themselves are contested. See Blue Zones.

Cholesterol: a real but small drop

This is the strongest evidence for legumes, because it comes from randomised trials. Pooling 38 trials in 2,095 people, a median 130 g of whole pulses a day for about six weeks lowered low-density lipoprotein (LDL) cholesterol by 0.14 mmol/L and non-HDL cholesterol (everything except the high-density, "good" fraction — the cholesterol carried by artery-clogging particles) by 0.22 mmol/L, with the certainty rated moderate to high. Beans showed a dose-response up to about a cup a day, and the different pulses behaved alike.^[5] The Nordic review's feeding trials found a similar LDL fall of 0.19 mmol/L.^[6]

That is a few percent, but it comes free with a food you would eat anyway. Most of it is probably the soluble fibre, which binds bile acids in the gut so the liver uses up cholesterol to replace them. Soy protein does something similar on its own, about 3–4%; see Soy. How LDL is managed as a whole is on Lipid management.

Blood sugar: better control in trials, no protection against diabetes in cohorts

Legumes are digested slowly and raise blood sugar less than most starches. In 41 trials, pulses eaten on their own lowered fasting blood sugar and insulin, and as part of low-glycaemic-index or high-fibre diets they lowered HbA1c, the three-month average of blood sugar — with large, unexplained differences between trials.^[7] The broader case for slow-release carbohydrate is on Glycemic index.

The long-term studies don't follow. Pooling 15 cohorts of about 566,000 people, total legume intake was not linked to type 2 diabetes; only tofu, soy protein and soy isoflavones were linked to lower risk, on low-quality evidence.^[8] A larger analysis of 27 cohorts and about 808,000 adults found a weak positive link overall — about 2% more diabetes per extra 20 g a day — driven by studies from Germany, the UK and Sweden, with no link in the Americas, the Eastern Mediterranean or the Western Pacific, and none for pulses or soy. The authors raise the possibility that the foods legumes are eaten with in those countries explain it.^[9] Treat legumes as good for blood-sugar control, not as diabetes prevention.

Protein: good enough, especially in combination

Legumes are the main protein of most vegetarian diets, and the protein is good but not identical to animal protein. In measured protein isolates, soy and pea carry about 27–30% essential amino acids, against 32–43% for egg, casein, milk and whey, and both meet adult requirements; plant proteins typically carry less methionine, and blends of plant proteins close most of the gap.^[10] Legumes are relatively short of methionine (see Protein); eating them alongside grains over the day is the everyday version of a blend.

The practical difference for muscle looks small. In nine trials of 266 people doing resistance training, soy protein produced the same gains in strength and lean mass as whey or other animal protein.^[11] Older adults eating mostly plant protein need a little more of it per meal; the targets are on Protein and Leucine, and how a plant-based diet fares overall is on Vegetarian diet.

Soy: most of the fears don't hold up

Soy's isoflavones are weakly estrogen-like, which drives the scares. The human evidence, all on the Soy page:

Breast cancer: no higher risk in large cohorts, and about a quarter fewer recurrences in survivors who eat soy foods — observational, and about food, not isoflavone pills.
Men: across 41 studies, no effect on testosterone or estrogen. A lower prostate cancer risk is mostly a case-control finding; cohorts show about 10%.
Thyroid: thyroid hormones are unaffected in trials. With subclinical hypothyroidism, one trial saw more progression at an isoflavone dose a vegetarian diet can supply, so a high soy intake warrants a TSH recheck; levothyroxine is best taken apart from soy.
Hot flushes: isoflavone pills cut them modestly; the North American Menopause Society (now The Menopause Society) does not recommend soy for them.

Lectins and raw kidney beans: cook them properly

Lectins are proteins that bind sugars on the surface of cells, and raw legumes are the most concentrated food source. In animals, isolated lectins and raw bean flour damage the gut lining. But soaking and boiling destroy lectins, and trials of whole cooked beans in people don't show those effects; a review found no strong human evidence that cooked lectin-rich foods cause inflammation, a leaky gut or nutrient problems in the general population.^[12]

The exception is real. Red kidney beans are rich in a lectin called phytohaemagglutinin, and reports say four or five soaked raw beans can cause food poisoning. Commercially cooked canned beans have over 99% less of it; a whole can carries roughly a thousandth of the amount linked to poisoning.^[13] Of 50 suspected UK incidents, 9 were confirmed in the lab, with nausea, vomiting and diarrhoea 1–7 hours after eating, and 23 more were diagnosed on symptoms; the beans had usually been eaten raw or inadequately heated.^[14] In 2018, undercooked kidney beans in a chili con carne made about 200 of 1,700 people at a French military base ill.^[15]

High-lectin beans such as kidney beans and soybeans need a proper boil, because lower cooking temperatures don't significantly reduce their lectin; microwaving, which works for most legumes, did not deactivate the lectin in common beans such as kidney beans.^[16] In practice that means a slow cooker on low is not enough for dried kidney beans: soak them, discard the water, and bring them to a full rolling boil first.

Phytates in legumes and whole grains do reduce iron absorption from the same meal, which matters mainly if you are short of iron; see Iron and aging.

Gas: real, mostly temporary

In three feeding studies where adults added half a cup of beans a day, fewer than half reported more wind in the first week — and only 19% with black-eyed peas — while 3–11% reported more wind on the bean-free control diets too. Seventy percent or more of those affected found it had settled by the second or third week.^[17]

The gas comes from sugars in the raffinose family that only gut bacteria can break down. Soaking beans and cooking them in fresh water cut these by about a quarter to two-fifths in one study.^[18] Start with small portions, increase over two to three weeks, and try lentils, split peas or tofu if whole beans stay a problem. People with irritable bowel syndrome may need to cut back, at least during a low-FODMAP trial, since legumes are among the foods high in these fermentable carbohydrates (FODMAPs); see Fiber.

Canned or dried

Both are fine. Canned beans are already cooked, which removes the kidney-bean risk and most of the effort; dried are cheaper.^[19] Two things to watch with cans:

Salt. Many canned beans are packed in salted liquid. Choose "no salt added" versions where you can and drain the liquid; see Salt.
Can linings. In a US national survey of 7,669 people (data from 2003–2008), eating one canned food in the past day went with about 24% higher urinary bisphenol A (BPA), a hormone-disrupting chemical from can linings, and two or more with about 54% higher; canned beans were not analysed separately.^[20] Cooking beans from dry avoids this. What BPA exposure means for health is covered under Environmental toxins.

What to actually do

Eat legumes most days, at least 130 g cooked (half to two-thirds of a cup); up to a cup is better for fibre if you tolerate it.
Use them as a main protein, in place of red and processed meat, and pair them with grains across the day.
Rotate: lentils, chickpeas, white and black beans, split peas, tofu, tempeh, edamame.
Build up over two to three weeks if beans are new to you.
Boil dried kidney beans hard after soaking, never slow-cook them from raw, and never eat them raw.
Prefer no-salt-added canned beans, and drain them.