Dietary fiber alone can't make you gain weight; in fact, it’s more likely to support weight loss. High-fiber foods will rarely sabotage your weight, but too many refined foods -- which are usually low in fiber -- can cause you to put on pounds when your "calories in" are higher than "calories out". All fiber-containing foods are important sources of vitamins, minerals and beneficial plant-based antioxidants, so don’t cut them out of your diet even if you’re trying to restrict calories.
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Dietary Fiber and Calories
The two types of dietary fiber -- soluble and insoluble -- consist of carbohydrates that the human body can’t fully digest because it lacks the necessary enzymes. Because fiber can’t be broken down into small sugar molecules, it isn’t absorbed into the bloodstream. This means it doesn’t contribute the same number of calories as other carbs.
As soluble fiber travels through the digestive tract, it becomes food for microbes that naturally live in the large intestine. The bacteria are able to ferment soluble fiber, and during the process, gases and short-chain fatty acids are produced. These fatty acids -- butyrate, acetate and propionate -- may be used for energy, but they only supply about 1.5 to 2.5 calories for every gram of fiber fermented, according to the Institute of Medicine.
In theory, fatty acids from the fermentation of soluble fiber could contribute slightly to weight gain. For example, if you consumed the highest recommended intake of fiber daily and if all the fiber was soluble, fermentation might supply as much as 95 calories. But in reality, calories produced from fermentation of fiber aren’t likely to affect your weight. Fiber in foods consists of a combination of insoluble and soluble fibers, and most people only get 17 grams of total fiber daily, reported the Journal of the Academy of Nutrition and Dietetics in November 2015. As a result, the net caloric impact of soluble fiber in a typical diet is minimal.
Calories in Fiber-Containing Foods
High-fiber foods -- such as fruits, vegetables, whole grains and beans -- belong in a healthy diet because they’re packed with nutrients. Fiber-rich foods represent a range of calories, though, so be sure to watch portions of the higher-calorie kinds and include the calories as part of your daily tally.
Nonstarchy vegetables are low in calories, with about 25 per serving. So when you're trying to lose weight, these low-cal veggies are on the list of foods with the top fiber content -- brussels sprouts, cabbage, spinach and other leafy greens. A bowl of berries is a high-fiber treat that's low in calories, too, with raspberries and blackberries containing 8 grams of fiber per cup. Many of the best sources of fiber -- 100-percent wheat bran cereal, oats and beans -- are on the high end of calories, so limit your serving to 1/2 cup. You’ll also get fiber from nuts, but a 1-ounce serving has around 200 calories, so be careful with portions.
Fiber's Role in Weight Management
People who consume more dietary fiber gain less weight over time than people who consume little fiber, according to studies cited in the Journal of Nutrition in July 2012. Fiber helps you lose weight through several actions. When it absorbs water, it expands and stretches the stomach, which makes you feel full. It also promotes satiety that lasts a while because it slows down the movement of food through the stomach and small intestine.
Because they have lots of fiber and water and few calories, fruits and vegetables are considered low-energy-density foods. When foods have fewer calories per gram, they fill you up more quickly without ruining your daily calorie budget. In other words, dietary fiber makes it easier to stop eating, so instead of causing weight gain, it supports maintenance of a healthy weight and even contributes to weight loss
Fiber Intake Recommendations
The Institute of Medicine recommends that women consume 25 grams of fiber daily, while men should get 38 grams every day. After the age of 50, the adequate daily intake drops to 21 grams for women and 30 grams for men. Because the average amount of fiber consumed is roughly half of the recommended intake, most Americans need to increase fiber-containing foods in their diets.
Following the recommended intake for fruits, vegetables, grains and beans helps ensure you’ll get the right amount of fiber. Based on consuming 2,000 calories daily, the Dietary Guidelines for Americans 2015-2020 recommends 2 cups of fruits and 2 1/2 cups of veggies daily -- which includes beans and starchy veggies. Try to include a variety of colorful veggies each day. Aim for 6 servings of grains daily, with at least half coming from whole grains. One slice of bread, 1/2 cup of cooked grains and 1 cup of flaked whole grains cereal are all one serving.
Boost your fiber intake slowly to give your body time to adjust to the extra gas produced during fermentation, and be sure to drink eight glasses of low-calorie beverages daily to make up for the water absorbed by fiber.