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.
FIBER FORMS: You may also see references to specific forms of soluble and insoluble fiber. The three main forms of soluble fiber naturally found in plants are:
– Pectin, familiar to home cooks from its use in jellies and jams, is found in most produce, but especially in apples and citrus fruit.
– Gum, often used as a food additive for thickening, is found in sticky substances exuded by plants.
– Mucilage, used as a stabilizer in some foods, is similar to gum.
The other forms of fiber are primarily insoluble:
– Cellulose forms the structure of cell walls in vegetables, fruits, legumes and whole grains; it’s the material used to make paper.
– Hemicellulose makes up the structural fiber of whole grains; some hemicelluloses are soluble fibers.
– Lignin, found in berry seeds and the woody parts of vegetables such as carrots, is the toughest fiber. (Don’t confuse lignin with lignan, a digestible phytochemical found in flaxseed and other plants.)
In unveiling updates to the Nutrition Facts panel (see August NewsBites), the US Food and Drug Administration also clarified what counts as “dietary fiber.” For labeling purposes, it “includes naturally occurring fibers and only fibers added to foods that show a physiological health benefit.” According to the FDA, adequate scientific evidence supports the claims that certain such fibers, including the amount added to foods, may have beneficial health effects for humans:
– Cellulose, which improves bowel function
– Guar gum, pectin, locust bean gum and hydroxypropylmethylcellulose, which improve cholesterol levels
– Psyllium husk, also called inulin, which may improve bowel health.
The FDA’s definition means that other so-called “functional fibers” not naturally present in food added to processed foods will not be able to be counted toward the dietary fiber reported on the Nutrition Facts label.
OTHER FINDINGS: The latest Australian study isn’t the only recent report of health benefits from eating foods rich in dietary fiber. Although most research on fiber and health is observational and subject to cautions about fiber possibly being a marker for an overall lifestyle, the evidence is nonetheless encouraging for fiber fans:
– A 2015 European study, published in Diabetologia, found that a diet rich in fiber – particularly cereal fiber from grains, followed by vegetable fiber – was associated with an 18% lower risk of developing type 2 diabetes. Those in the highest-fiber group averaged more than 26 grams a day of total fiber, compared to less than 19 grams a day in the lowest group. For each additional 10 grams of cereal-fiber intake, risk of developing type 2 diabetes fell by 25%. The study looked at data on more than 340,000 people over about 11 years. The benefit of dietary fiber seemed to be linked to its effects in lowering Body Mass Index (BMI).
– In 2013, researchers reported in the journal Stroke that increasing fiber consumption was associated with a modest decrease in the risk of suffering either of the two main types of stroke. While cautioning that fiber intake might just be a surrogate for other healthy choices, they found that each additional seven grams of daily fiber was associated with a 7% lower risk of suffering a stroke.
– It’s never too late to start eating more fiber – even after suffering a heart attack – according to a 2014 study published in BMJ. Increased consumption of fiber from cereals and grains following a heart attack was associated with a 31% lower risk of death from any cause and a 35% lower risk of cardiovascular mortality. The study used data from two large studies of health professionals, with an average follow-up of about nine years.
– A study published earlier this year in the journal Pediatrics suggests that eating more fiber earlier in life was associated with lower risk of developing breast cancer later. Using data on 44,263 women in the Nurses’ Health Study II, researchers found that those who recalled eating the most fiber as teenagers and young adults were 25% less likely to develop breast cancer than those who consumed the least. (Physical activity is also associated with lower risk of developing breast cancer, suggesting it may be healthier eating patterns and lifestyle habits that have beneficial effects later in life.)
An important limitation of the study was reliance on recollections of what participants ate as young women.
– Dietary fiber might is also associated with a lower risk of lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). A study published in the Annals of the American Thoracic Society earlier this year reported that adults with the highest fiber intake were less likely to suffer airway restrictions and more likely to have normal lung function, compared to those with the lowest intake. The study looked at 1,921 adults, ages 40 to 79. The beneficial association held even when data were controlled for known risk factors such as smoking and overweight.
Fiber from fruits and vegetables was more strongly associated with lung health than that from grains. This association was independent of antioxidant intake, which has previously associated with lung function.
MORE BENEFITS: Previous research has provided a wealth of other evidence for the importance of getting plenty of fiber as part of a healthy dietary pattern. Not surprisingly, many of these benefits concern your digestive system, where fiber has been shown to:
– Minimize constipation. This best-known benefit of dietary fiber is most strongly associated with fiber from oat and whole grains, rather than from fruits and vegetables. Experts recommend increasing fiber intake slowly rather than suddenly, and drinking plenty of fluids since soluble fiber absorbs water. Significantly upping your fiber intake all at once can invite embarrassing and uncomfortable consequences. By increasing your dietary fiber gradually, your body gets a chance to adjust and you minimize the effects of extra intestinal gas.
–Help prevent digestive disorders. A high-fiber diet can make you less prone to digestive disorders such as irritable bowel syndrome (IBS), which may affect as many as one in five Americans. For those already suffering from IBS, however, while fiber relieves constipation, it can make gas and cramping worse.
– Reduce your risk of diverticular disease. “Current studies have found that a high-fiber diet can reduce development of further diverticuli for individuals with diverticulitis,” says Joel Mason, MD, a gastroenterologist and professor at Tufts’ Friedman School and medical school. “Fiber may help to keep stools soft, allowing it to pass more easily and prevent buildup of pressure in the large intestine.” People experiencing active diverticulitis flareups, however, should avoid fiber. “Once symptoms improve, you should slowly add fiber back into your diet.”
– Work to prevent hemorrhoids. By forming softer, bulkier stools, fiber reduces the straining that can lead to hemorrhoids.
Surprisingly, however, the evidence that high intake of dietary fiber reduces the risk of colorectal cancer is mixed at best. Despite years of bran consumption being touted as a way to prevent colon cancer, studies have mostly failed to find a link between fiber intake and reduced risk of colon polyps or cancer. Research does, however, support a benefit from a high-fiber diet against the risk of small-bowel cancer. In an analysis of data on almost a half-million adults over seven years, those with the highest fiber intake from grains were 49% less likely to develop this relatively rare cancer.
HEART HEALTH: Other benefits associated with dietary fiber, however, relate to the heart rather than the gut. Several large observational studies have linked high fiber intake to lower risk of cardiovascular disease. In one study of more than 40,000 male health professionals, for example, those with the highest dietary fiber consumption were at 40% lower risk of coronary heart disease. The possibility that fiber is a marker for a heart healthy diet cannot be ruled out; however, there are some reasons why fiber can help your heart and arteries by:
– Improving blood-cholesterol levels. The FDA allows several foods high in soluble fiber, such as oats, psyllium and barley, to carry a health claim for cholesterol benefits. By lowering LDL cholesterol levels, these fiber-rich foods can reduce the risk of heart disease.
– Countering high blood pressure. One analysis of results from 25 clinical trials calculated that people who increased their fiber intake for at least eight weeks experienced significant reductions in blood pressure.
– Helping reach and maintain a healthy weight. By slowing chewing and digestion, high-fiber foods give your body time to feel the effects of the meal you’re eating. Fiber also makes you feel “full.” So the next time you are tempted to have a glass of apple juice, opt for the apple instead.
GETTING MORE: There’s no one best way to add fiber to your diet; simply eating a variety of different plant-based foods will insure that you’re getting enough of both soluble and insoluble fiber. “The best insurance that you will habitually eat a high-fiber diet is to choose foods you really enjoy,” says Tufts’ Lichtenstein. “Because these foods are also packed with important nutrients, they are a healthier way to boost fiber intake than taking fiber supplements. And there is no evidence that fiber supplements have the same benefits of fiber you can get from food.”
But keep in mind that, even though your body doesn’t digest fiber, it does absorb the calories that accompany the fiber. (No, it’s not true that some fibrous foods such as celery actually burn more calories in the effort to eat them than they contain.) The key is to substitute fiber-rich choices for foods already in your diet, rather than adding extra food to boost your fiber intake. When you do, be aware of the added sugars and unhealthy fats that often accompany some high-fiber packaged foods.
Switching to whole grains is a good step. Start the day with a whole-grain or bran breakfast cereal such as oatmeal (4 grams per cup) or bran flakes (7 grams per cup). Skip the sugary varieties—you can add a touch of sweetness, and still more fiber, by topping your cereal with fruit. A half-cup of berries adds about 4 grams of fiber to your morning.
Fruit juice contains less fiber than whole fruits, and fruit drinks have added sugar. Making smoothies in the blender is a tasty, convenient way to consume fiber-rich fruits (and even vegetables), but don’t go the “juicing” route, which leaves most of the fiber behind.
At lunchtime, change your sandwich bread to 100% whole wheat or bread that at least has a whole grain listed as the first ingredient; look for at least two grams of fiber per serving. Add some veggies to your sandwich, and have fruit for dessert. (If it’s an apple, eat the peel, too.)
Choose whole grains at dinnertime, too: brown rice instead of white, whole-wheat pasta, barley or farro rather than starchy side dishes. Allocate a greater portion of your plate to vegetables. When you do serve potatoes, eat the skin as well as the flesh. Substitute beans for some or all of the meat in some meals, and make lentils a regular part of your repertoire.
Snacks can also be an opportunity to go high in fiber. Crunch on baby carrots or pop some popcorn (from loose kernels, not microwavable bags – watch the salt and butter).
While filling up on fiber-rich foods, you’ll also enjoy all the vitamins, minerals and phytonutrients that these dietary choices deliver. And you’ll have less room for oversized meat portions, less-healthy snacks and sugary desserts.
Now that’s a recipe for “successful aging.”
TO LEARN MORE: Journal of Gerontology, online June 1, 2016 –