Are You Getting Enough Fiber?
New research singles out fiber as an important ingredient in successful aging.
An important nutrient for reaching old age free of disease and disability might surprise you. According to a new Australian study, it's dietary fiber - a nutrient that, by definition, you don’t even digest. In its path through your body, however, fiber is associated with reduced risk of cardiovascular disease, type 2 diabetes and some cancers.
Despite the benefits of dietary fiber, most Americans don't consume nearly enough. The 2015-2020 Dietary Guidelines for Americans recommends getting 14 grams of fiber daily per 1,000 calories in your diet. For adults over 50, the guidelines call for at least 28 grams a day of fiber for men and 22 grams for women; younger adults need even more (see box). The Daily Value used to calculate percentages on Nutrition Facts labels is 25 grams, which will increase to 28 grams as labels are updated. In practice, though, US men get only about 18 to 20 grams of fiber daily, while women average 15 to 16 grams a day.
"It is important to keep in mind the recommendations are based on fiber naturally occurring in foods such as whole fruits and vegetables, nuts, beans and whole grains," says Alice H. Lichtenstein, DSc, director of Tufts' HNRCA Cardiovascular Nutrition Laboratory and executive editor of the Health & Nutrition Letter. "Is it the fiber itself or an eating pattern rich in these foods and associated lifestyle habits that confers the health benefits?
"We don't have the answer, so the best advice we can give is to substitute high-fiber foods for fruit juices and sugar-sweetened beverages and grain-based products made with white flour. Regrettably, sprinkling some bran on a hot fudge sundae or searching out products with added fiber not typically in the food is unlikely to confer benefits, even if it gets you over the target fiber intake."
COMPARING CARBS: The new Australian study, published in The Journal of Gerontology, used data on 1,609 initially healthy people, ages 49 and older, who participated in the Blue Mountain Eye Study. Instead of looking at diet and eye health, however, researchers at the Westmead Institute dug into the data to compare carbohydrate intake and health outcomes over 10 years. Specifically, Bamini Gopinath, PhD, and colleagues focused on “successful aging,” which they defined as reaching old age free of chronic disease and fully functional.
Only 249 participants achieved that goal - and their characteristic carbohydrate consumption surprised the scientists. "Out of all the variables we looked at," Gopinath says, "fiber intake - which is a type of carbohydrate that the body can’t digest - had the strongest influence. Essentially, we found that those who had the highest intake of fiber actually had an almost 80% greater likelihood of living a long and healthy life over a 10-year follow-up. That is, they were less likely to suffer from hypertension, diabetes, dementia, depression and functional disability."
Those in the highest group of total fiber intake averaged 37 grams per day, while those in the lowest group consumed an average of only 18 grams per day. The association between fiber intake and successful aging held even when the data were adjusted for factors including age, sex, marital status, living status, smoking and weight. Other variables that might have contributed but were not assessed include higher levels of physical activity and more unsaturated compared to saturated fat.
Among older adults, fiber from fruits and breads/cereals (primarily from rolled oats and whole-grain breads), but not from vegetables, independently predicted successful aging. On the other hand, total carbohydrate intake, glycemic index (a measure of how rapidly a food raises blood sugar) and glycemic load (which adjusts glycemic index for typical serving sizes) were not significantly associated with successful aging.
As an observational study, the results can’t prove cause and effect, and Gopinath cautions that further research is necessary to confirm her findings. Moreover, as in other population studies of fiber and health, it's possible that eating more dietary fiber is actually a marker for an overall healthy lifestyle. But the findings add to the mounting evidence that high-fiber foods are an important part of a healthy dietary pattern - and there are plenty of potential mechanisms by which fiber might affect your body even though it’s just passing through.
SOLUBLE VS. IN-: If you want to make sure you’re getting enough fiber for "successful aging," you’ll have to focus on eating plenty of plants. Fiber, which comes in various forms (see below), is the part of plants - grains, fruits, vegetables, legumes, nuts - that your body can't absorb or digest. Meat, dairy products and oils contain no natural fiber. (In refined grains most of the fiber is removed.) Unlike protein, fats or other carbohydrates such as sugars and starches, fiber passes through your stomach and small intestine into your colon almost unchanged. (This is also why when counting carbohydrates, most experts subtract the grams of fiber to calculate "net carbs.")
The health benefits of fiber are often discussed along with those of whole grains, because the processing required to make white flour and similar refined-grain products strips away the fiber-rich bran as well as the germ. If you substitute whole-grain for refined-grain products in your diet, besides getting the nutrients of the original grain, you’ll add dietary fiber in the bargain.
Not all whole grains are necessarily high in fiber, however - just higher than their processed counterparts. Whole-wheat bread typically contains a modest 2 grams of fiber per slice - still more than white bread with 0.8 grams of fiber. A cup of cooked long-grain brown rice has only 3.2 grams of fiber, but that’s more fiber than the 0.6 grams in white rice.
Dietary fiber is primarily differentiated as either soluble or insoluble, depending on whether it reacts with water; both types are beneficial, and many foods contain both kinds. Soluble fiber attracts water and turns to gel during digestion, which slows down the rate your stomach empties, giving it more time to extract nutrients from food and making you feel "full" longer. In your intestine, soluble fiber can also bind with bile acids, which are made of cholesterol, and help carry them out of your body. Soluble fiber is found predominantly in oat bran, barley, nuts, seeds, beans (legumes), peas, carrots, celery, apples, and some other fruits and vegetables.
Insoluble fiber adds bulk to the stool and appears to help food pass more quickly through the stomach and intestines. You can get insoluble fiber from foods such as wheat bran, vegetables and whole grains.