A nutritionist discusses the nutritional properties of the main plant-based sources of protein and whether they are truly poorly digestible.
What are legumes?
Legumes (Fabaceae, or Leguminosae) are a large family of plants, comprising approximately 800 genera and 20,000 species [1]. Legumes include trees, shrubs, and herbs, easily recognizable by their fruit beans which contain seeds.
Legumes can be food (beans, chickpeas), fodder (alfalfa, clover), ornamental (wisteria, acacia) and woody (robinia, mimosa).
Common types of edible legumes include:
- beans (white, black, red);
- lentils;
- chickpeas;
- peas;
- mash;
- soybeans;
- peanut.
A bean is commonly known as a single kernel of a bean, chickpea, or pea, although botanically, such kernels are nothing more than seeds. A bean is the entire pod.
Legume seeds are nutritious foods with a high protein content, and are used to produce food products for people who do not eat meat:
- tofu and tofu pates;
- falafel;
- soy meat;
- hummus.
Nutritional composition of legumes
With a relatively low calorie content (about 115 kcal per 100 g), legumes are rich in protein, vitamins, minerals, fiber and antioxidants.
On average, half a cup of legumes contains:
- 1 g fat;
- 20 g carbohydrates;
- 7–9 g fiber;
- 8 g protein.
Protein
Many legumes (soybeans, lentils, chickpeas) contain 20–40% vegetable protein [3]. For example, 100 g of lentils contains 23.6 g of protein, and 100 g of red beans contains 21.3 g of protein, which covers approximately 30% of the daily requirement for a healthy adult.
However, plant protein is slightly less digestible than animal protein because it does not contain a complete set of amino acids. This is why vegans and vegetarians are advised to combine legumes with grains, nuts, and seeds.
Fiber
Legumes are rich in fiber, which is essential for digestion and overall health [3]. A cup (about 170 g) of cooked beans contains approximately 15 g of fiber. This amount covers almost 50% of the daily requirement.
Moreover, legumes contain both soluble and insoluble fiber:
- Soluble – slows down the absorption of carbohydrates and saturates for a long time.
- insoluble – helps food move through the intestines and prevents constipation.
Vitamins and minerals
Legumes are rich in B vitamins (especially B1, B6, and folate), which support nervous system function and are involved in metabolism. Legumes are also an excellent source of iron, necessary for the formation of hemoglobin, and magnesium, which strengthens muscles and the cardiovascular system.
Vitamins and minerals in legumes :
- calcium,
- iron,
- B vitamins,
- potassium,
- copper,
- magnesium,
- manganese,
- zinc,
- phosphorus.
Health benefits of legumes
Research shows that 50 grams (about ¼ cup) of legumes per day helps reduce all-cause mortality. This is because beans, chickpeas, soybeans, and lentils lower “bad” cholesterol levels in the blood and normalize digestive function.
Reduces the level of “bad” cholesterol in the blood
Legumes contain saponins natural compounds that lower levels of low-density lipoprotein (the so-called “bad” cholesterol) and, thus, reduce the risk of atherosclerosis and cardiovascular disease. For example, people who eat legumes four times a week have an average 14% lower risk of coronary heart disease than those who do not include beans in their diet.
Helps control weight
Despite their high nutritional value, legumes are relatively low in calories (approximately 110–130 kcal per 100 g of cooked beans, depending on the type) and contain virtually no fat. Therefore, they are suitable for those watching their weight.
Additionally, due to their high fiber content, legumes are very filling, meaning a person will snack less often throughout the day and be able to better control their overall calorie intake.
Supports intestinal microflora
Legumes contain fiber, which feeds beneficial bacteria in the gut. When digested, they release short-chain fatty acids, which strengthen the intestinal lining, improve digestion, support the immune system, and reduce overall inflammation in the body [7].
Reduce the risk of type 2 diabetes
Legumes generally have a low glycemic index , meaning they’re filling and don’t cause sharp spikes in blood sugar and insulin levels. Research shows that regular consumption of legumes reduces the risk of developing type 2 diabetes and helps manage the disease in people diagnosed with diabetes [8].
How many legumes should you eat per day?
The World Health Organization (WHO) recommends consuming at least 400 g of fruits and vegetables, including legumes, daily to provide the body with the necessary amount of fiber and nutrients [9].
The optimal amount of legumes in the diet is from 50 to 100 g per day (in dry form), which corresponds to approximately 150–250 g in cooked form.
In the United States, the recommended daily intake is approximately three cups (about 510–540 g of cooked legumes) per week [10]. The Russian Ministry of Health recommends eating 2–3 tablespoons of legumes per day (approximately 30–50 g dry) [11]. If you include them in your diet less frequently, you should increase your portion sizes.
Harmful effects of legumes
Most healthy adults tolerate this product well. However, sometimes it’s necessary to avoid legumes or significantly limit them in the diet.
Diseases and conditions for which legumes are contraindicated:
- gastric ulcer;
- exacerbation of gastritis and pancreatitis;
- inflammatory bowel diseases (ulcerative colitis, Crohn’s disease);
- intestinal infections;
- exacerbation of irritable bowel syndrome (IBS) ;
- condition after surgery on the digestive organs.
Furthermore, people with osteoporosis and other mineral deficiency-related conditions should limit their intake of legumes. Legumes contain phytic acid and lectins substances that can reduce the absorption of calcium, iron, zinc, and magnesium, and worsen the disease.
Why do legumes cause bloating?
Legumes contain oligosaccharides long sugar molecules that aren’t fully digested but serve as food for beneficial gut bacteria. During fermentation, these bacteria release gas bubbles, which can cause bloating.
To remove most of the oligosaccharides, legumes should be soaked in water for several hours and then boiled thoroughly.
Discomfort can be experienced not only by people with gastrointestinal disorders but also by healthy individuals. To alleviate these symptoms, it’s important to approach cooking wisely. Specifically:
- Before cooking, be sure to rinse the beans in cold water and soak them for several hours (no more than 12).
- Afterwards, drain the water and rinse again.
- Cook until done (until soft, depending on the type).
Don’t eat too much at once; start small and then gradually increase your serving size. It’s best to eat legumes with easily digestible foods: vegetables, white meat, and fish. I recommend trying different legumes; one type may be easier to tolerate than another.”
Allergy to legumes
Legumes contain potential allergens: lectins, albumins. Most often, allergies occur to soy and peanuts, and in rare cases, to chickpeas, lentils, and beans.
Symptoms of bean allergy:
- itching, rash, hives;
- nausea, abdominal pain, diarrhea;
- swelling of the mucous membrane of the lips, tongue, throat;
- In rare cases, anaphylaxis.
Cross-reactions are also possible, so people with allergies to soy and peanuts should exercise caution with other legumes, as similar proteins can trigger allergy symptoms. This is rare (5–10% of cases), so there’s no need to completely eliminate legumes from the diet, but it’s important to be aware of the risk and closely monitor your body’s reaction.
Canned and Processed Legumes: Are They Beneficial?
Legumes generally retain their nutritional properties regardless of how they are processed whether boiled, stewed, canned, or sprouted. However, heat treatment reduces the amount of nutrients.
Raw beans contain about 21 grams of protein, 2 grams of fat, and 60–61 grams of carbohydrates per 100 grams. They also provide 15–16 grams of fiber, 0.9 mg of vitamin B1, and 5–5.5 mg of iron.
When boiled, beans retain 7–9 grams of protein, 0.5–0.7 grams of fat, and 20–25 grams of carbohydrates per 100 grams. The fiber content drops slightly to 7–10 grams, while vitamin B1 decreases to 0.2 mg, and iron to 2.5–3.7 mg.
Canned beans have the lowest nutrient values, providing 5–7 grams of protein, 0.2–0.5 grams of fat, and 15–18 grams of carbohydrates per 100 grams. Their fiber content ranges from 4–6 grams, vitamin B1 from 0.06–0.1 mg, and iron from 1.5–2 mg.
Furthermore, the more a product is processed, the less healthy it is. For example, canned legumes are often high in salt, sugar, saturated fat, and other unhealthy additives.
To choose the healthiest options, it’s recommended to read the label and choose products with the fewest added ingredients. Ideally, the ingredients should only contain beans and water.