Fiber: The Workhorse

Fiber is a form of indigestible carbohydrate found mainly in plant foods. Over the years, fiber has been hailed as a potential weapon against cancer, high cholesterol, and heart disease. Fiber’s vaunted health benefits were diminished slightly by findings that it doesn’t prevent colon cancer or colon polyps (precursors of colon cancer). Studies continue to be published, however, presenting a mixed picture. (See “Controlling colorectal cancer.”) What is known is that fiber slightly reduces LDL cholesterol, improves insulin resistance, and is linked to a lower rate of heart disease, type 2 diabetes, and obesity. There’s also some evidence that fiber might reduce the risk for duodenal ulcers, breast cancer, and ovarian cancer. It’s considered one of the most important health attributes of foods.Fiber slows digestion and therefore lowers a food’s glycemic load, which likely helps to prevent diabetes. By increasing the bulk of foods and creating a feeling of fullness, fiber may also help you avoid overeating and becoming overweight.

Studies such as the Nurses’ Health Study and the Health Professionals Follow-up Study have found that people with the lowest rates of heart disease and heart attack have the highest intakes of fiber. These studies formed the basis for the DRIs for fiber.The DRI for fiber is 38 grams for men up to age 50 and 25 grams for women in this age group. For pregnant women, the DRI is 28 grams, and for breast-feeding women, 29 grams. DRIs are lower for people over age 50: 30 grams for men, 21 grams for women. That’s because older people tend to eat less food. On average, Americans eat only about 15 grams of fiber a day.

You can probably identify some high-fiber foods, such as bran cereals and whole-grain bread (see Table 2). But not all foods billed as “high-fiber” really have much; read the labels on packaged foods to see the number of grams of fiber they contain. You can be sure of getting fiber if you eat fruits, vegetables, and whole-grain foods such as whole-wheat bread, brown rice, bran, or oats each day.

Table 2: Good sources of fiber
Food Fiber content in grams*
Split peas, cooked, 1 cup 16.3
Red kidney beans, boiled, 1 cup 13.1
Raspberries, raw, 1 cup 8.0
Whole-wheat spaghetti, 1 cup 6.3
Oat-bran muffin, medium 5.2
Pear, medium with skin 5.1
Broccoli, boiled, 1 cup 5.1
Apple, medium with skin 4.4
Oatmeal, quick, regular, or instant, cooked, 1 cup 4.0
Green beans, cooked, 1 cup 4.0
Brown rice, cooked, 1 cup 3.5
Popcorn, air-popped, 2 cups 2.3
Whole-wheat bread, one slice 1.9
*Fiber content can vary among brands and varieties.Source: USDA National Nutrient Database for Standard Reference, 2007.

Fiber can also take the form of over-the-counter supplements, which come as pills or powders. These provide some of the same benefits as fiber in foods. But foods naturally high in fiber have the edge, as they help you feel full on fewer calories, which can help keep your weight in check. Fiber pills don’t provide that feeling of fullness. If you decide to take fiber supplements, be sure to take them with plenty of water to maximize their benefits.

Here are some ways to make sure that your diet meets the DRI for fiber.

Eat whole-grain cereal for breakfast. Oats are an excellent choice, particularly steel-cut oats, which have the most fiber and the lowest glycemic index. If you prefer cold cereal, choose products that contain bran or list whole wheat, oats, barley, or another whole grain first on the list of ingredients.

Choose whole-grain breads. As with cereals, true whole-grain breads list a whole grain first in the ingredients. Whole-grain sliced bread, pita bread, and rolls are equally good.

Skip the French fries and baked potatoes. Instead of white potatoes, eat sweet potatoes (sometimes called yams). Instead of white rice, eat brown rice or another intact grain as a side dish. Good choices are buckwheat (kasha), bulgur, millet, quinoa, and barley.

Try whole-wheat pizza and pasta. Prepared pizzas made with whole-wheat crust are joining whole-wheat pastas on supermarket shelves. Many are now made with a variety of flour or flour blends that look white and cook up lighter, but are actually whole wheat. Traditional brown whole-wheat pasta is a great choice, too, but if it doesn’t appeal to you, another option is to mix whole-wheat pasta with regular white pasta.

Cook with whole-wheat flour. You can make breads, muffins, and other home-baked goods healthier if you mix whole-wheat flour with white flour. Many stores sell a multigrain pancake mix you can use for pancakes or waffles. Or instead of traditional whole-wheat flour, buy white whole-wheat flour, which has a finer grain than traditional whole-wheat flour and looks white. It can be substituted for regular white flour in many recipes. If you use traditional whole-wheat flour, a straight substitution won’t work for every recipe, because whole-wheat flour is heavier than white flour. Try starting with a ratio of one part whole wheat to three parts white to see if you like the results. If you think the dish could stand a heavier, grainier texture, try increasing the share of whole-wheat flour. You may need to increase the amount of liquid at the same time.

The soy storyFor a long time, soybean-based beverages and foods like soy milk and tofu were the royalty of health foods: vegetarian, rich in protein, maybe responsible for the lower rates of heart disease and cancer in China and Japan. That many Americans had to acquire a taste for soy made it seem even healthier in that eat-your-peas way.Early research suggested that soy protein was “heart healthy” because it could lower LDL (bad) cholesterol levels, but subsequent studies and judgments have dampened that enthusiasm. The results for soy protein have been so unimpressive that the American Heart Association has asked the FDA to rescind its 1999 decision that allowed food companies to label soy products as having heart disease–reducing benefits.There have also been worries that the estrogen-like chemicals in soy, called isoflavones (pronounced eye-so-FLAY-vones), might promote the growth of estrogen-sensitive cells and therefore increase the chance of breast cancer recurrence. Study results reported in 2009 in TheJournal of the American Medical Association (JAMA) allay that concern to some extent.

But how soy and its isoflavones behave in the body is complicated. In some parts — such as bone — isoflavones appear to mimic estrogen, occupying the same receptors and therefore having a similar, if weaker, effect. If soy’s isoflavones impersonate estrogen in bone, that’s a good thing, because estrogen protects against bone loss by inhibiting osteoclasts, cells that break down bone, and stimulating osteoblasts, cells that build it up. But in other parts of the body — the breast, for example — the estrogen-like effects of isoflavones might mean extra cell growth and division and an increase in the risk of a cancer developing.

The bottom line: soy has emerged from the thousands of studies a bit humbled. It’s an excellent source of protein, which is especially important to vegetarians and vegans who need plant-based protein. And, yes, there are many nutritionally valid reasons to steer toward a vegetarian diet. But current science suggests that although soy is a relatively healthy food choice, it is not a particularly important player in preventing disease.

 

Comments

  • About PEC

    The Patient Education Center provides multimedia access to reliable and relevant medical information at and beyond the point of care. Our content is developed exclusively by Harvard Health Publications, the media and publishing division of the Harvard Medical School of Harvard University, and distributed by Health Media Network.