High-Fiber, Low-Carb: Getting Fiber on a Low Carb Diet

Collard greens cut into ribbons
sf_foodphoto/Getty Images

Is It True that Low-Carb Diets Are Low in Fiber?

It is a common misconception that reduced carbohydrate diets are automatically low in fiber. In fact, most non-starchy vegetables and low-sugar fruits are quite high in fiber, and a well-constructed low-carb diet emphasizes vegetables and other sources of fiber. Unfortunately, most people, at least in the US, don’t get close to the recommended amounts of daily fiber, no matter how much carbohydrate they are eating!

But there is no reason for you to be one of those people.

What is Fiber?

Fiber is that part of plant food that is indigestible by humans. It passes through our digestive systems without being broken down and absorbed into our bloodstreams as other food components are. Think of a cow eating grass – that cow needs extra stomachs, chews its cud, etc, to digest the grass. We can’t digest grass because we don’t have specialized digestive systems. That indigestible stuff is fiber.

How is Fiber Beneficial?

When people think of the benefits of fiber, they usually think of preventing constipation. It’s true that fiber bulks up our stool and tends to make people more “regular.” But fiber has other benefits as well, some of which are very pertinent to low-carb diets, including lowering the impact of sugars and starches on blood glucose. A high-fiber diet is associated with lower risk for heart disease, Type 2 diabetes, and diverticular disease.

What Are the Different Types of Fiber?

Fiber has three different properties that matter to human health. The property most nutritionists talk about is solubility, the ability to disperse in water. Soluble and insoluble fiber are the labels most commonly used to describe fiber. However, two other properties of fiber are turning out to be important: fermentability (how easily the fiber ferments in the colon), and viscosity (the ability to gel with water) of the fiber, which may be more important than solubility.

What is Insoluble Fiber?

Insoluble fiber is what we usually think of when we think “fiber” or “roughage”. Wheat bran and most vegetables are examples of sources of insoluble fiber. It is tough and doesn’t easily break down. Insoluble fiber tends to increase the “speed of transit” through our digestive systems and increases the regularity of bowel movements.

What is Soluble Fiber?

A lot of soluble fiber is viscous, allowing it to absorb and retain water, forming a gel. This type of soluble fiber actually slows digestion down. Because of this, it has a tendency to stabilize blood glucose and permit better absorption of nutrients. It tends to reduce blood cholesterol. It also increases satiety, so people aren’t inclined to eat as much. Sources of soluble fiber include flax, beans, peas, oatmeal, berries, apples, and some nuts and seeds.

What is Fermentable Fiber?

Some fiber will ferment in the colon, producing compounds that help support colon health, and possibly have other benefits. There is some evidence that it is this type of fiber that may reduce the risk of colon cancer. Most soluble fiber is highly fermentable. Pectins (found in apples and berries) and the fiber in oats are examples of fiber with a large fermentable component.

Inulin and oligofructose are also highly fermentable, as is resistant starch.

Does Fiber “Count” as a Carbohydrate?

Although most fiber sources are carbohydrates, fiber doesn’t raise blood glucose, so low-carb diets don’t “count” fiber. (Fiber can provide calories, but not as glucose, but as products of fermentation in the colon.) In fact, fiber helps to moderate the effect of “usable carbs” on the bloodstream, so it furthers the goals of low-carb diets. To the extent that it creates satiety, it may also help prevent weight gain, and aid in weight loss.

How Much Fiber Should a Person Eat?

Generally, recommendations for adults are between 25 and 40 grams per day, and that 20-30% of the total fiber intake be soluble fiber.

Most people have a much lower fiber intake than is recommended. Researchers who study the diets of our prehistoric ancestors say that they ate upwards of 100 grams of fiber per day, so we probably can handle very high amounts of fiber without difficulty.

Do I Have to Eat Fiber in Food? Can’t I Just Take Pills?

While fiber supplements (in some circumstances) can be helpful additions to a high-quality nutritious diet, they should never stand in for high-fiber foods, which are also rich in antioxidants and other nutrients essential to health. There is some evidence that simply taking pure fiber as a pill or sprinkling high fiber additions over your food doesn’t carry all the same benefits as when it is in food. Also, some high-fiber additives such as wheat bran contain compounds (phytates) which block the absorption of some nutrients, so large amounts of this should be avoided.

Guidelines for Consuming Fiber

  • If you are unused to eating a lot of fiber, increase amounts gradually to prevent intestinal distress.
  • Make sure you drink lots of water when taking fiber supplements or eating high-fiber foods, as all fiber absorbs at least some water. Fiber can, in rare cases, cause intestinal blockage or  if eaten with insufficient fluid.
  • Since large amounts of fiber can reduce absorption of some medications, it is best to take medication either an hour before or two hours after the fiber.
  • Chitin and chitosan come from the shells of crustaceans and should be avoided by people allergic to seafood.

Which Low-Carb Foods are High in Fiber?

As stated above, vegetables and fruits that are low in carbs tend to be also high in fiber. Flax is very high in fiber and very low in carbohydrate. Some bran cereals are good choices, such as All Bran. Possible supplements include those that are psyllium-based. Also see:

Continue Reading