Ask the Nutritionist: How Much Fiber Do I Really Need?

Q: I know fiber helps keep people “regular.” I’ve also heard it helps flush fat from the body. Is that true?

A: Yes, fiber can help rid the body of some dietary fat. Because the human body is unable to break down fiber, it leaves the body pretty much in the same form it entered. As fiber winds its way through your digestive system, it grabs fats and carries them all the way through to the colon, where a fat-and-fiber package is bundled, water is extracted, and the bundle eliminated from the body. Keep in mind, though, that it’s all a matter of proportion. If you eat half an apple and a whole pizza, the amount of fiber in the apple is not up to the task of scrubbing out all the fat from the pizza.

Q: How much fiber do I need?

A: The amount you need is tied to the number of calories you eat in a day. For healthy adults, the USDA recommends 14 grams of fiber for every 1,000 calories consumed. So a person eating 2,000 calories a day should aim to get 28 grams of fiber daily. In “food” terms, you could hit (and even exceed) that target by eating the following foods over the course of a day: ½ cup oatmeal (3 grams fiber), 1 small banana (3 grams), ½ cup cooked red or black beans (7 grams), 1 small apple (5 grams), ½ cup lentils (8 grams), and ½ cup blueberries (3 grams).

Q: What’s the difference between soluble and insoluble fiber? How do I know if I’m getting enough of each?

A: Soluble and insoluble fiber can be found in different foods, and also in different parts of the same food. Insoluble fiber tends to be found in the peels and husks of plant foods, and soluble fiber is found in the fleshy interior.

For example, grape peels are mostly insoluble fiber, while the inside of a grape is mostly soluble fiber. White rice has mostly soluble fiber, while brown rice has insoluble fiber on the outside and soluble fiber on the inside. Soluble fiber, also called pectin, is what gives apricot nectar its full texture and applesauce its natural thickness. In your body, soluble fibers can help manage glucose levels by slowing the absorption of carbohydrates so that sugar is released gradually into the bloodstream. Carrots, legumes, cabbage, citrus fruits, and green beans are rich sources of soluble fiber. So are the interiors of fresh pears, apples, peaches, and apricots. In addition, some soluble fibers, including those in beans and oats, have been shown to reduce the risk of atherosclerosis and heart disease.

Insoluble fiber, also known as cellulose, is in the exterior of all fruits, vegetables, beans, and whole grains. It’s the peel of an apple, the membrane around the juicy part of an orange, the transparent cover over beans, the strings in a stalk of celery, and the nearly transparent slipcover on each kernel of corn. Because it’s indigestible, it speeds the transit time of food through the body. Insoluble fiber helps to prevent constipation, diverticulitis, and hemorrhoids, and may help decrease the risk of colon cancer.

Unfortunately, food labels don’t distinguish between insoluble and soluble varieties; they simply list total amount of “dietary fiber” in a food. To ensure that you’re getting both types, focus on eating plenty of whole foods, such as an apple rather than apple juice or brown rice instead of white rice.

Q: What is the connection between fiber and weight loss?

A: To lose weight you need to take in fewer calories than you burn. In general, the higher a food’s fiber content, the lower it tends to be in available calories. Here’s why: There are many types of carbohydrates, but they’re not all created equal. Sugar and starch are both carbohydrates. So is fiber. But while your body can easily digest and use calories from sugar and starch, it can’t break down fiber, so you can’t get appreciable numbers of calories from eating the fiber. That’s why you can eat a very large serving of salad, which is almost all fiber, and still eat very few calories.

Q: I sometimes see the term “net carbs” on food labels. Does this have anything to do with fiber?

A: Yes. “Net carbs” refers to the carbohydrate content of a food after the indigestible carbohydrates from fiber have been subtracted. For example, an orange has 60 grams of total carbs from sugar, starch, and fiber; 5 of those carb grams come from fiber. Subtract those 5 grams of fiber carbs from the total carb count, and you have 55 grams of net carbs, or carbs from which you can actually get calories.

Q: Should I use a fiber supplement to make sure I’m getting enough?

A: There are many types of “extracted” fiber products on the market, including tablets and powders that can be mixed with water and taken as a drink. Most health care professionals would advise a healthy adult to eat a pear or a handful of raisins instead of turning to a supplement. Satisfying your daily fiber needs with food is the best way to get a healthful balance of soluble and insoluble fiber. It’s also a great way to improve the overall quality of your diet, since fiber-rich foods tend to be rich in vitamins, minerals, and disease-fighting phytochemicals.

Nutritionist Nancy D. Berkoff, RD, EdD, enjoys cruising for her carbs at local farmers’ markets.