This is the type people think of as “roughage.” It’s the tough matter found in whole grains, nuts, and fruits and veggies (specifically in the stalks, skins, and seeds) that doesn’t dissolve in water. Insoluble fiber isn’t broken down by the gut and absorbed into the bloodstream. It adds bulk to waste in the digestive system, which helps keep you regular and prevent constipation (as well as any related problems, like hemorrhoids).
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Soluble fiber is soft and sticky, and absorbs water to form a gel-like substance inside the digestive system. Top sources include beans, peas, oats, barley, fruits, and avocados. Soluble fiber helps soften stool so it can slide through the GI tract more easily. It also binds to substances like cholesterol and sugar, preventing or slowing their absorption into the blood. That’s why it’s known to help regulate blood sugar levels, and protect against heart disease by lowering blood cholesterol. What’s more, soluble fiber boosts the population of good bacteria in the gut, which is linked to improved immunity, anti-inflammatory effects, and even enhanced mood. But that’s not all: Soluble fiber also has middle-whittling benefits. For starters, it makes you feel full for longer, which helps with weight management. One study showed that for each additional 10 grams of daily soluble fiber eaten, participants had a 4% decrease in belly fat over a five-year period.
Why you need both
Both soluble and insoluble fiber are important for your health, which is why a lot of research has focused on total fiber intake. For example, a study published in the Archives of Internal Medicine found that over a nine-year period, consuming more dietary fiber lowered the risk of death from any cause. People who ate the most fiber (about 25 grams a day for women and 30 grams for men) were 22% less likely to die compared to those who consumed the least fiber (10 grams per day for women and 13 grams for men). The effect was even stronger when researchers looked at deaths from heart disease, infectious diseases, and respiratory diseases; people with high-fiber diets had as much as a 50% or greater reduction in risk.