Dietary Fiber: Benefits, Sources, Recommended Intake

Fiber is a food substance found in plants that contains no nutrients or calories. It occurs in both a soluble and insoluble form, both of which help with digestion and neither of which can be digested or absorbed by your body. Soluble fibers bind to fats in foods and prevent their absorption, which helps lower cholesterol and blood sugar. Insoluble fibers help food move through your body and are excreted more quickly. Fiber is suspected to help prevent certain cancers.

Dietary Fiber Health Benefits

Fiber-rich carbohydrate foods benefit health in many ways. Foods such as whole grains, legumes, vegetables, and fruits supply valuable vitamins, minerals, and phytochemicals, along with abundant dietary fiber and little or no fat. The following are some of the health benefits of diets that emphasize a variety of these foods each day.

1. Prevent Heart Disease

Diets rich in whole grains, legumes, and vegetables, especially those rich in whole grains, may protect against heart disease and stroke by lowering blood pressure, improving blood lipids, and reducing inflammation.29 Such diets are generally low in saturated fat, trans fat, and cholesterol and high in dietary fibers, vegetable proteins, and phytochemicals—all factors associated with a lower risk of heart disease. 

Foods rich in soluble fibers (such as oat bran, barley, and legumes) lower blood cholesterol by binding cholesterol compounds and carrying them out of the body with the feces.30 High-fiber foods may also lower blood cholesterol indirectly by displacing fatty, cholesterol-raising foods from the diet. Even when dietary fat intake is low, research shows that high intakes of soluble fiber exert separate and significant blood cholesterol-lowering effects.

2. Prevent Diabetes

High-fiber foods—and especially whole grains—play a key role in reducing the risk of type 2 diabetes. The soluble fibers of foods such as oats and legumes can help regulate blood glucose following a carbohydrate-rich meal. Soluble fibers trap nutrients and delay their transit through the digestive tract, slowing glucose absorption and preventing the glucose surge and rebound often associated with diabetes onset.

High-fiber foods—and especially whole grains—play a key role in reducing the risk of type 2 diabetes. The soluble fibers of foods such as oats and legumes can help regulate blood glucose following a carbohydrate-rich meal. Soluble fibers trap nutrients and delay their transit through the digestive tract, slowing glucose absorption and preventing the glucose surge and rebound often associated with diabetes onset.

3. Improve Gut Health

Soluble and insoluble fibers, along with ample fluid intake, may enhance the health of the large intestine. The healthier the intestinal walls, the better they can block the absorption of unwanted constituents. Soluble fibers help to maintain normal colonic bacteria necessary for intestinal health.

Insoluble fibers that both enlarge and soften stools such as cellulose (in cereal brans, fruits, and vegetables) ease the elimination of the rectal muscles and thereby alleviate or prevent constipation and hemorrhoids. Some fibers (again, such as cereal bran) help keep the contents of the intestinal tract moving easily. This action helps prevent compaction of the intestinal contents, which could obstruct the appendix and permit bacteria to invade and infect it.

In addition, fibers stimulate the muscles of the gastrointestinal (GI) tract so that they retain their strength and resist bulging out in places, as occurs in diverticulosis.33 Recommendations typically suggest increasing fiber to protect against diverticular disease, although research findings are inconsistent.

4. Reduce The Risk of Cancer

Many studies show that, as people increase their dietary fiber intake, their risk for colon cancer declines. A recent meta-analysis using data from several studies exposed a strong, linear inverse association between dietary fiber and colon cancer.

People who ate the most fiber (24 grams per day) reduced their risk of colon and rectal cancer by almost 30 percent compared with those who ate the least (10 grams per day). Mid-range intakes (18 grams per day) reduced the risk by 20 percent. It is important to note that fiber from food but not from supplements demonstrates this association, possibly because fiber supplements lack the nutrients and phytochemicals of whole foods that may also help to protect against cancers.

All plant foods—vegetables, fruits, and whole-grain products—have attributes that may reduce the risks of colon and rectal cancers. Their fiber dilutes, binds, and rapidly removes potential cancer-causing agents from the colon. In addition, the colon’s bacteria ferment soluble fibers, forming small fat-like molecules that lower the pH. These small fat-like molecules activate cancer-killing enzymes and inhibit inflammation in the colon.

Other processes may also be at work. As research progresses, cancer experts recommend that fiber in the diet come from 21⁄2 to 41⁄2 cups of vegetables and fruit daily, along with generous portions of whole grains and legumes.

5. Promote Weight Loss

Fiber-rich foods tend to be low in fat and added sugars and therefore prevent weight gain and promote weight loss by delivering less energy per bite.38 In addition, fibers absorb water from the digestive juices; as they swell, they create feelings of fullness, delay hunger, and reduce food intake. Soluble fibers may be especially useful for appetite control. By whatever mechanism, as populations eat more refined low-fiber foods and concentrated sweets, body fat stores creep up. In contrast, people who eat three or more ounces of whole-grain foods each day tend to have lower body and abdominal fatness over time.

Commercial weight-loss products often contain bulk-inducing fibers such as methylcellulose, but pure fiber compounds are not advised. High-fiber foods not only add bulk to the diet but are economical, nutritious, and supply health-promoting phytochemicals—benefits that no purified fiber preparation can match.

Harmful Effects of Excessive Fiber Intake

Despite fiber’s benefits to health, when too much fiber is consumed, some minerals may bind to it and be excreted with it, without becoming available for the body to use. When mineral intake is adequate, however, a reasonable intake of high-fiber foods does not seem to compromise mineral balance.

People with marginal intakes who eat mostly high-fiber foods may not be able to take in enough food to meet energy or nutrient needs. The malnourished, the elderly, and young children adhering to all-plant (vegan) diets are especially vulnerable to this problem. Fibers also carry water out of the body and can cause dehydration. Advise clients to add an extra glass or two of water to go along with the fiber added to their diets. Athletes may want to avoid bulky, fiber-rich foods just prior to competition.

Food Sources of Soluble Fiber

  • Apples
  • Apricots
  • Bananas
  • Barley
  • Beans (kidney, lime, navy, and pinto)
  • Blackberries
  • Broccoli
  • Brussels sprouts
  • Cabbage
  • Carrots
  • Chickpeas
  • Flaxseed
  • Grapefruit
  • Nuts
  • Oat bran
  • Okra
  • Oranges
  • Pears
  • Prunes
  • Psyllium
  • Split peas
  • Sweet potatoes

Food Sources of Insoluble Fiber

  • Bananas
  • Beans
  • Broccoli
  • Brown rice
  • Brussels sprouts
  • Cauliflower
  • Celery
  • Corn
  • Crackers
  • Grains
  • High-fiber cereal
  • Lentils
  • Pasta
  • Potato (with skin)
  • Prunes
  • Spinach
  • Wheat bran
  • Whole wheat bread

Recommended Intakes of Fibers

The FDA set the Daily Value for fiber at 28 grams for a 2000-calorie intake. This is based on the DRI recommendation of 14 grams per 1000-kcalorie intake—roughly 25 to 35 grams of dietary fiber daily. These recommendations are almost two times higher than the usual intake in the United States.

As health care professionals, you can advise your clients that an effective way to add dietary fiber while lowering fat is to substitute plant sources of proteins (legumes) for some of the animal sources of protein (meats and cheeses) in the diet. Another way to add fiber is to encourage clients to consume the recommended amounts of fruits and vegetables each day.

People choosing high-fiber foods are wise to seek out a variety of fiber sources and to drink extra fluids to help the fiber do its job. Many foods provide fiber in varying amounts. As mentioned earlier, too much fiber is no better than too little. The World Health Organization recommends an upper limit of 40 grams of dietary fiber a day.

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