How to Choose the Best Fiber Supplement for Your Low-Carb Diet

Low-carb and keto diets limit many foods like whole grains, beans and fruit that are rich in fiber. Since it can be hard to get the recommended daily amount of this vital nutrient while following these diet plans, you might consider taking a fiber supplement.

First, remember it's ideal to try to get the bulk of your nutrients, including fiber, from regular food, according to the Mayo Clinic. And before you add a supplement to your diet, check with your health care provider to make sure it's the best option for you.

If you get the green light, here's what you should know to help you choose the best supplement.

Fiber is the indigestible parts of plants. Unlike fats, proteins and carbohydrates — which are absorbed by the body and converted into fuel — fiber passes through the body's digestive system and out of the body along with waste products.

Read more: 19 High-Fiber Foods — Some May Surprise You!

Your diet generally contains two types of fiber: soluble (which dissolves in water) and insoluble (which does not). Soluble fiber helps lower cholesterol and glucose levels, while insoluble fiber helps food move through the digestive system, keeping you regular, per the Mayo Clinic.

Plant foods contain both soluble and insoluble fiber, but the amounts vary. To get the best health benefits, aim to eat a wide variety of foods, including those with both types of fiber, says Amy Goss, RD, assistant professor of nutrition sciences at the University of Alabama at Birmingham.

"A fiber supplement may not be necessary for everyone, because even on a low-carb diet you should still be eating foods high in fiber," says Goss. In fact, she says, some foods specifically marketed as keto-friendly or low in net carbs — like low-carb tortillas, for example — are much higher in fiber than their high-carb counterparts.

Plus, Goss adds, you should still be eating plenty of fiber-rich, non-starchy vegetables on a low-carb or keto diet. Kale, spinach, cauliflower, eggplant and zucchini all fit that bill.

If you're following a diet that starts out with a very low-carb "induction" phase, you might choose to take a supplement for the first few weeks. But as the diet progresses and your carbohydrate restrictions relax a bit, you may be able to stop taking the supplement and get enough fiber from foods like beans, peas, avocado and fruit, along with vegetables and whole grains.

Every gram of fiber in a food counts as a gram of carbohydrates. But even so, fiber is not inherently off-limits on a low-carb diet. In fact, low-carb diets usually recommend limiting the number of "net carbs" consumed every day — which is the total number of carbohydrate grams minus the number of fiber grams.

Because the body doesn't digest fiber, it doesn't affect blood sugar levels the way sugars and starches do, says Goss.

Read more: Are Supplements Safe? Here's What You Should Know

The problem, however, is that many high-fiber foods are also high in net carbs, which means they should be avoided or limited on a low-carb diet. During the first two weeks of the Atkins 20 diet, for example, nuts, seeds and berries are all off-limits.

Fiber aids in digestion, and when you don't get enough, it can increase the risk of constipation. That's one of the most common side effects of low-carb or ketogenic diets, and a reason why dieters might seek relief from a supplement, according to the Academy of Nutrition and Dietetics.

A wide variety of fiber supplements line store shelves, and they come in powders, chewables and capsules. The type of fiber used as the active ingredient varies, too. Some of the most common fiber supplements include psyllium, wheat dextrin, inulin and methylcellulose.

Of these, psyllium is a standout, with a few caveats. Also known as psyllium husk, psyllium comes from the husk of the Plantago ovata plant. It's the only fiber supplement shown to help lower cholesterol, control blood sugar levels, prevent constipation and diarrhea and promote satiety, according to a review published March 2015 in Nutrition Today. In a February 2019 clinical trial published in Clinical Nutrition ESPEN, researchers also found that psyllium helped participants lose weight.

Read more: Is Daily Use of Psyllium Husk as a Laxative Harmful?

Psyllium is sold under brand names such as Cilium, Alramucil, Genfiber, Maalox Daily Fiber Therapy and Hydrocil. Though it's available via capsules and chewable wafers, Franziska Spritzler, RD, author of The Low-Carb Dietitian's Guide to Health and Beauty, recommends the powdered form. It's easy to add to your food, she says, and can be mixed into things like yogurt or beverages. Experts recommend using psyllium only for about a week, because it can interact with medications and can even — rarely — block your intestines.

Other types of fiber supplements include methylcellulose (brand name Citrucel), polycarbophil (Fibercon, Fiber-Lax, Equalactin and Mitrolan), wheat dextrin (Benefiber), and the prebiotic inulin (Frutafiit and Frutalose). Like psyllium, these ingredients also absorb water, according to Michigan Medicine, which can help create a softer stool.

People respond differently to different types of supplements, Goss says, and some people may find that certain kinds have fewer side effects (like gas and bloating) than others. While psyllium has been shown to have the most benefits in clinical trials, according to the Nutrition Today review mentioned above, Goss recommends working with a doctor or nutritionist to find a supplement that works best for you.

Fiber absorbs water from your digestive system and expands, which makes it crucial that you drink enough H2O while taking fiber supplements. In addition to drinking plenty of fluids throughout the day, the Cleveland Clinic recommends drinking at least 8 ounces of water when you take your fiber supplement.

Suddenly starting a fiber supplement can have some digestive side effects, such as flatulence, bloating and abdominal discomfort. That's because many Americans eat less fiber than they should. The Dietary Guidelines for Americans recommend that men and women get 30 and 25 grams of fiber per day, respectively. But according to the U.S. Department of Agriculture, American women ages 20 to 39 get only 15 grams of fiber daily, while men in the same age group get only 18 grams. Fortunately, these side effects usually go away as your body adjusts.

Read more: Signs and Symptoms of Too Much Fiber

Always take any fiber supplement — and any other dietary supplements — as directed, and make sure to discuss them with your health care provider. (Be sure to contact your doctor if you experience any alarming side effects, as well.) That's especially important if you have an existing health condition, since fiber supplements can affect things like cholesterol and blood sugar.