Fiber Barely Touches Your Metabolism. Its Calories Go Somewhere Else.

Keeping people regular was the job fiber was famous for. It belonged to bran cereal and a grandparent's advice at breakfast, a virtue so plain that nobody ever bragged about it.

Lately it has been promoted. Metabolism-boosting food lists and wellness blogs now credit fiber with making the body burn extra calories just to break it down, a free bonus for anyone already piling on beans and whole grains to stay full. You would want to know whether fiber increases metabolism before counting on it.

Does Fiber Increase Metabolism? Hours, a Day, Then Weeks

Psyllium is the natural first test, since it is the fiber people stir into water on purpose. A big dose taken in water just before breakfast left the burn over the next six hours the same as on a morning without it, in overweight and obese men. Their response to food may already have been muted, the researchers suggested, and it was one breakfast.

A single high-fiber meal leaned the wrong way for the theory. In healthy normal-weight men, the burn over six hours came out a little lower than after a low-fiber meal with the same calories, and the gap tracked how much the men enjoyed each meal, which makes it hard to pin on fiber alone.

You might reasonably say a few hours is too short to catch it. A whole day on a fermentable fiber left the total burn where it was, measured in a sealed room that tracks every breath, with healthy young adults getting about one in four calories from a cornstarch rich in resistant starch, in a test a chemicals company paid for. The body burned nearly a third more fat and less carbohydrate, which sounds like extra calories and is only a change of fuel with the total unchanged.

Stretching the test to weeks took a government-funded research ward where every bite was weighed, the burn was measured in a sealed room and every stool was collected. The healthy young volunteers' burn while asleep stayed level between a typical Western diet and one built to feed gut bacteria (more fiber and resistant starch, whole nuts in place of nut butter, less processed food), with calories matched, so fiber was one change among four. An extra 116 calories a day left in the stool on the gut-feeding diet, the body kept less of its food in 16 of the 17 people, and hunger and how much they chose to eat stayed the same.

Fiber slips past the small intestine undigested and reaches the far end of the gut, where bacteria ferment it. They salvage some energy from it and hand it back, so fermentable fiber is not calorie-free. A resistant starch measured in people delivered just under 3 calories a gram, about two-thirds to three-quarters of what ordinary starch gives, even though US food labels counted it as zero when that work was published.

The bacteria take a cut and multiply, and their own bodies leave in the stool. Bacterial bodies made up more than a quarter of stool energy on both diets, and on the gut-feeding diet that loss cancelled out the energy the bacteria handed back, by the researchers' model. The diet's share of the effect stayed impossible to separate from the bacteria's.

Whole-grain eating got its own metabolism headline years ago, and the numbers behind it point the same way. Swapping refined grains for whole grains for six weeks, with every meal provided, shifted people's daily energy balance by about 92 calories, which the researchers put mainly down to more energy leaving in the stool. A small rise in resting burn, measured after an overnight fast, could no longer be told apart from chance among the people who kept to the diet, in a trial co-funded by General Mills' research institute.

It is fair to weigh the strongest result pointing the opposite way. In a study paid for and mostly written by the probiotic's maker, a daily probiotic drink carrying 5 grams of inulin, a fiber, gave overweight adults in Japan a resting burn about 84 calories a day higher than a placebo after four weeks, a figure from the study's statistical model with a range from about 3 to 166. The higher reading came from 15 minutes in a chair, in volunteers chosen for the lowest resting metabolism among those eligible, and their weight, body fat and fat burning did not change. Nobody took the fiber on its own, so the result belongs to the drink.

Room for a modest effect in some people remains, because every test here was small and short, mostly in healthy adults and, in the single-meal tests, only in men.

Your fiber still pays its way, through the calories it carries out and, according to the trials on fiber and weight, through how much people end up eating over weeks. That second effect has its own story, told in what pooled trials found about fiber and fat loss. The fiber you added for fullness does its calorie work later, closer to the bathroom than the furnace.