Soluble and Insoluble Fiber in Managing Diabetes

If you have diabetes, including plenty of fiber in your diet can be a smart move. This important nutrient can help you lose weight (if you need to), play a role in controlling your blood sugar levels, and more. Not all dietary fiber is the same, though: There are two types—soluble fiber and insoluble fiber—and each functions differently in the body.

To make the most of a higher-fiber diet, then, it can help to understand the differences between them in terms of how they can be most beneficial in helping you manage your diabetes, what the best sources of each are, how much daily fiber is ideal, and the smartest ways to meet that goal.

Dietary fiber is the part of whole plant foods the body can't break down and digest. Technically fiber is a carbohydrate, but unlike other carbs (starches and sugars), because it isn't absorbed it passes through the body without causing a rise in blood glucose.

In fact, research has found that for people with type 2 diabetes, eating more fiber can help with blood glucose control. In one study, for example, people who ate 50 grams of fiber each day had lower blood sugar levels after six weeks than those who ate 24 grams of fiber each day.

Similarly, another study found that making one simple dietary change—eating 30 grams or more fiber per day—was as effective as more complicated eating strategies for improving the body's response to insulin, as well as for weight loss and lowering blood pressure.

Both soluble and insoluble fiber contributes to these benefits, but they work differently in the body.

This type of fiber attracts water: It turns to a gel when eaten and slows the rate of digestion. Soluble fiber makes it harder for the body to convert carbohydrates into glucose that can be absorbed into the bloodstream. This can help prevent dramatic increases in blood sugar levels, which in turn helps insulin work better.

Soluble fiber also allows the body to more easily take in and use nutrients, and has been shown to lower blood cholesterol and block the absorption of fat—benefits known to reduce the risk of stroke, diabetes, gastrointestinal disorders, heart disease, and some cancers. And because soluble fiber is fermentable, it contributes to colon health.

Often referred to as "roughage," insoluble fiber comprises the cell walls of plants and is made of cellulose. As such, it's bulky and doesn't dissolve in water. It speeds the movement of food through the digestive system, functioning much like a scouring pad by "polishing" the intestines along the way. Insoluble fiber also adds bulk to stools and increases the regularity of bowel movements, helping to prevent constipation.

Only 5 percent of people in the United States get adequate fiber in their diets, according to the Academy of Nutrition and Dietetics (eatright.org). This is regardless of whether they have diabetes or not.

More specifically, the U.S. Department of Agriculture reports that the mean dietary fiber intake of all people 2 years and older is 16 grams per day, with men averaging 18 grams and women averaging 15 grams, which is far lower than current recommendations.

Since these guidelines are limited to what is regarded as adequate intake of fiber, how much more than is recommended would be ideal for getting the benefits of a high-fiber diet in terms of controlling blood sugar? This probably varies from person to person, and so ideally should be determined by a nutritionist who specializes in diabetes. (Note that more than 70 grams of fiber each day has been associated with negative effects.)

Nor do the guidelines specify what percentage of total fiber should come from soluble fiber sources and how much should come from insoluble fiber sources. However, given that soluble fiber is the type that's most associated with lowering blood glucose, it's safe to say that someone with type 2 diabetes (or prediabetes) might want to err on the side of getting more of that type.

Increasing the amount of fiber in your diet can lead to uncomfortable digestive symptoms such as bloating, gas, constipation, diarrhea, or cramps. Take it slow: Increase the fiber in your diet gradually, adding a bit more every few days. Spread your fiber intake throughout the day rather than cramming a lot of fiber into single meals or snacks, and drink plenty of water. Some simple ways to start: 

• Aim to eat 3 to 5 servings of non-starchy vegetables each day (a serving is 1/2 cup cooked or 1 cup raw)
• Consume two servings of high-fiber fruits such as berries, apples, or pears daily
• Include plenty of whole grains, such as whole-grain bread, oatmeal, and ancient (quinoa, bulgar, barley, farro, millet, freekeh)
• Snack on unsalted nuts—one serving is 1/4 cup or one handful
• Sprinkle ground flax, hemp, or chia seeds into your yogurt 
• Toss legumes, such as chickpeas, into your salad for a protein and fiber boost

When reading labels, note that any food containing 5 grams of fiber is considered an "excellent" source, according to the American Diabetes Association, and foods with 2.5 grams to 4.9 grams are "good" sources. In time, you'll become familiar with the amount of fiber in your favorite foods and getting more of these will become second nature.