Chilli for lunch, and by four o'clock your waistband is digging in. Hummus with drinks, and the evening is spent loosening your belt. Beans and lentils have a reputation for bloating, and that reputation rests on well-understood chemistry.
Sugars your gut cannot break down
Lentils, chickpeas and dried beans contain particular sugars called alpha-galacto-oligosaccharides. The main ones are raffinose, stachyose and verbascose. Breaking them down takes an enzyme, alpha-galactosidase, which the human gut does not make. So these sugars pass through the small intestine undigested and reach the colon intact. Bacteria ferment them there, and fermentation makes gas.
At Monash University in Melbourne, a team showed this by supplying the missing enzyme. Thirty-one people with irritable bowel syndrome ate meals rich in these sugars for three days, with the enzyme at full strength, half strength or a placebo, in random order. In 21 of them, symptoms clearly rose on these meals. With the full-strength enzyme, symptoms and bloating dropped sharply (Tuck et al., 2018). It is these sugars that cause the bloating, not the bean as a whole.
I covered fermenting sugars in general in the article on bloating. Beans and lentils are a special case, because part of these sugars dissolves in water.
Soaking and cooking do part of the work
In Montpellier, a team measured these sugars in five legumes, raw, soaked and then cooked. Soaking cut them by 10% in lentils and broad beans, and by 40% in chickpeas. Cooking cut them further (Njoumi et al., 2019).
The method follows from that. Soak overnight, pour away the soaking water that has taken up some of the sugars, rinse, then cook for a long time in fresh water, until the bean crushes against the roof of your mouth. Long cooking also neutralises much of the other compounds in legumes that irritate the gut.
Lentils lose little in soaking. All the more reason to start with small amounts.
A healthy gut usually gets used to them
At Arizona State University, Donna Winham went back over three trials in which adults ate half a cup of beans a day for eight or twelve weeks. In the first week, about half reported more wind with pinto beans, and only 19% with black-eyed peas. In the first two trials, that figure fell to 6% by the second week for pinto beans. In the third, it went from 45% to 30% in three weeks, then settled between 15 and 23% (Winham and Hutchins, 2011).
These volunteers had high cholesterol but a gut that worked well. They did not arrive with a stomach that swells every evening. For most of them, the discomfort faded within a few weeks. In people who are already bloated, beans and lentils often keep fermentation running too high, and waiting for it to pass is not enough.
Why I set them aside for a while
Legumes need a robust digestion. When someone's stomach already bloats every day, I take them out for thirty days, while digestion settles. Then they come back one at a time, soaked overnight, well cooked, in small amounts. You eat them on their own, with no sauce or mixing, then watch for 48 to 72 hours before adding anything else.
I have nothing against beans and lentils. Dal in India, feijão in Brazil, black beans in Mexico, whole cuisines are built on them, and those cuisines know how to prepare them, soaked and then simmered for a long time. The real question is whether your gut digests them right now.
If a bowl of lentils leaves your stomach tight every time, take it as information. Soak, cook for a long time, start small, and watch what happens over two or three days. If the pain is strong or keeps coming back, see your GP.
This article is for information only and does not replace medical advice.