High Fiber Diet Might Protect Against Range of Conditions
JAMA. Published online April 17, 2019. doi:10.1001/jama.2019.2539
The popularity of low-carb diets, such as Atkins and keto, have helped give the macronutrient a bad name, and in part, it’s deserved.
“Have carbs got a bad reputation? Certainly…for a very understandable reason,” said Jim Mann, MB, ChB, PhD, a professor in human nutrition and medicine at the University of Otago in New Zealand. “They are highly refined, and, in many countries, sugar intake is high.”
But just as there are “bad” fats and “good” fats, there are also bad carbs and good carbs. “We showed very clearly in our work in diabetes that the benefits of carbs came from the good carbs, and the good carbs are not high in sugar,” Mann said. “They are high in fiber.”
Mann, who spent years helping to hammer out the World Health Organization’s definition of fiber, recently coauthored a series of systematic reviews and meta-analyses that showed intake of fiber and whole grains—probably due to their high fiber content—but not foods with a low glycemic index was associated with significant reductions in mortality and chronic illnesses. (Breakfast cereals and other manufactured whole grain products are more highly processed today, so they provide less fiber and, likely, fewer health benefits, the authors noted.)
Observational data suggested a 15% to 30% decrease in all-cause and cardiovascular-related mortality and incidence of coronary heart disease, stroke, type 2 diabetes, and colorectal cancer in people who consumed the most dietary fiber relative to those who consumed the least, the authors found. In addition, clinical trials showed that compared with low intake, high dietary fiber intake lowers body weight, systolic blood pressure, and total cholesterol. Given that the findings from prospective observational studies and clinical trials were complementary and that there was evidence of a striking dose response, the relationship between a high-fiber diet and lower disease risk could be causal, the authors concluded.
Although high-fiber intake is associated with lower weight, which in turn reduces the risk of diabetes, cardiovascular disease, and many cancers, dietary fiber appears to have benefits independent of its effect on weight, Mann said.
These findings and others suggest that virtually anyone who wants to lower their risk of a variety of ailments—including cardiovascular disease and multiple types of cancer, such as ovarian, pancreatic, and liver as well as colorectal—should consume more, not fewer, carbs, as long as they’re good carbs, ie, fiber-rich whole grains, legumes, fruits, and vegetables.
However, in westernized countries, few people consume anywhere near the recommended amounts of dietary fiber. In the United States, the average dietary fiber intake by adult men and women is 18 g and 15 g, respectively, based on the 2013-2014 National Health and Nutrition Examination Survey (NHANES) data. And yet, the 2015-2020 Dietary Guidelines for Americans recommends a daily minimum intake of 33.6 g per day for men and 28 g for women 19 to 30 years of age, and only slightly decreasing amounts as people get older.
And what about people who swear by a low-carb diet and, as a result, consume relatively little dietary fiber?
“In the short-term, if a person has a very high body mass index, almost anything that would cause weight loss would be beneficial,” said Edward Giovannucci, MD, ScD, a professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health. “In the long-term, it’s a different story,” Giovannucci said, noting a lack of long-term studies of low-carb diets.
Figuring Out Fiber
An early theory for why dietary fiber was inversely related to colon cancer risk was mechanical: Fiber quickens the transit time of stool by bulking it up. There might be some truth to that explanation for why fiber intake and colon cancer risk are inversely related, said Giovannucci, who served on the American Institute of Cancer Research panel that produced a recent report on the relationship between diet, nutrition, and physical activity and colon cancer. “Certainly, you would think fiber would be more relevant to the diseases of the colon,” he said.
Indeed, “the lack of fiber is particularly pertinent to the remarkably high incidence rates of colon cancer in westernized societies,” Stephen O’Keefe, MBBS, MD, FRCP, wrote in a recent article. “In stark contrast, colon cancer is rarely seen in rural African communities consuming their traditional high-fiber (≈50 g/d), low-meat, and low-fat diets.”
In 2015, O’Keefe coauthored a widely cited study in which 20 African Americans and 20 rural Africans swapped their regular diets for 2 weeks. The researchers prepared high-fiber (50 g/d), low-fat African-style meals for the African Americans and high-fat, low-fiber meals for the rural Africans.
The brief diet swap was long enough for the scientists to observe reciprocal changes in participants’ gut microbiota and metabolome, which were associated with changes in mucosal biomarkers of cancer risk. The authors emphasized that changes in animal protein or digestible carbohydrate intake associated with changes in dietary fiber consumption could also have contributed to the observed mucosal alterations. They concluded that their results raise the hope that increasing fiber consumption, along with moderating fat intake, could reduce African Americans’ disproportionately high incidence of colon cancer.
Dietary fiber appears to be protective even after a diagnosis of colorectal cancer, Giovannucci and his coauthors suggested in a recent JAMA Oncology article. Using data from the Nurses’ Health Study and the Health Professionals Follow-up Study, they found that higher fiber intake after a diagnosis of nonmetastatic colorectal cancer was associated with a lower risk of death from that disease and all causes.
“Higher intake of fiber, especially cereal fiber, has been linked to improved insulin sensitivity, lipid profile, endothelial function, and reduced inflammation,” and evidence is just beginning to suggest that hyperinsulinemia and markers of insulin resistance and inflammation predict worse survival in patients with colorectal cancer, Giovannucci and his coauthors note.
In other words, the explanation for why a high-fiber diet appears to be protective goes far beyond speeding stool’s conveyance through the colon. And, it turns out, fiber’s health benefits extend beyond the colon as well.
For example, a recent study involving participants in the Prostate, Lung, Colorectal, and Ovarian (PLCO) cancer screening trial found an inverse association between dietary fiber intake and head and neck cancer, particularly oral cavity and pharyngeal tumors.
“We have found the inverse association between dietary fiber and head and neck cancer risk not only among smokers and drinkers but also among never smokers and never drinkers,” said coauthor Daisuke Kawakita, MD, PhD, of the Department of Otorhinolaryngology, Head and Neck Surgery, Nagoya City University Graduate School of Medical Sciences in Japan.
The late British surgeon Denis Burkitt, FRS,—aka the “Fiber Man”—popularized the notion a half-century ago that dietary fiber protected against a host of diseases, but not until recently have scientists begun to explain “the whole biochemistry of how fiber can have distal effects all around the body,” said O’Keefe, a University of Pittsburgh gastroenterologist.
O’Keefe and others posit that fiber’s effect on gut microbiota may explain its health benefits for the colon and beyond. “The metabolites of fiber, particularly butyrate, can prevent cancer in any part of the body,” he said.
Butyrate, a short chain fatty acid that serves as a fuel source for the cells that line the colon, facilitates apoptosis of colonic cancer cells and reduces gut inflammation, note the authors of a 2017 review article about butyrate in the colon and peripheral tissues. As the authors write, research has shown that increased fiber consumption or oral butyrate supplementation has been shown to decrease adiposity and improve insulin sensitivity.
But are the apparent health benefits from consuming foods naturally high in fiber due to fiber itself or to other nutrients that accompany it?
The correct answer is both, O’Keefe said. Dietary fiber is a marker for other nutrients in naturally fiber-rich foods, particularly phytochemicals, and at the same time it is also a biomolecule, he said. Plus, O’Keefe said, “people who eat fiber-rich foods are health conscious as well. Their lifestyle is better.”
Fiber in a Pill?
These days, it’s possible to find high-fiber brownies and high-fiber ice cream in many supermarkets. Of course, the only reason they’re high in fiber is because synthetic or isolated fiber was added to them.
Besides flour or milk, their labels list such ingredients as soluble corn fiber—“produced from corn through enzymatic hydrolysis of corn starch,” according to an industry website—or sugarcane fiber, also used to make plates and bowls.
“They do seem to be able to lower cholesterol a bit, but we do not know whether those synthetic or extracted fibers have the same long-term benefits,” said Mann, who, to illustrate this point in lectures, uses a slide of a high-fiber English muffin sold in the United States. A check of the ingredients of high-fiber English muffins is likely to reveal that their fiber comes from an additive such as cellulose, not whole wheat flour.
As Giovannucci said, “In most of the observational studies that are the basis of most of the data and recommendations, what we really studied are the whole foods.”
Still, when the US Food and Drug Administration (FDA) in 2016 finalized its Nutrition Facts and Supplement Facts Label, with the agency’s first-ever definition for dietary fiber, it included 7 isolated or synthetic nondigestible carbohydrates, such as cellulose and guar gum, as well as “intrinsic and intact” fibers in plants.
Based on small, short-term clinical trials, the agency said, the isolated or synthetic carbohydrates had “physiological effects that are beneficial to human health,” such as lowering blood glucose or cholesterol or increasing the frequency of bowel movements.
In June, the FDA released a scientific review of 8 more added nondigestible carbohydrates it plans to include in its definition of dietary fiber. However, none of the research cited by the FDA looked at the long-term impact, namely disease risk, of the 15 added nondigestible carbohydrates. In late March, the FDA announced plans to propose that manufacturers be allowed to identify a 16th nondigestible carbohydrate as dietary fiber on food labels.
Added fiber is better than no fiber, O’Keefe said, but “we’re designed to eat a balanced diet, and a balanced diet comes from what’s available naturally.”
Feasting on Fiber
Mann said he typically consumes at least 40 g of fiber a day, which might seem daunting to most people in westernized countries. “It does require a bit more preparation,” Mann noted. “If people are not well off, it’s cheaper to go and buy a Big Mac.”
Indeed, the NHANES data found that higher income was associated with higher fiber intake, although there are other explanations for that relationship besides the cost of high-fiber foods.
Increasing dietary fiber intake doesn’t have to be expensive. Mann, O’Keefe, and Giovannucci all pointed to legumes, which include beans, chickpeas, and lentils. A good source of protein as well as fiber, legumes are “a particularly underutilized food,” Giovannucci said.
Experts emphasize the need for public health messages about the value of consuming more dietary fiber and how to do it.
“We need a population push, just a bit like the lung cancer story,” O’Keefe said, referring to the worldwide decline in smoking rates. “The information’s got to get out to the population to understand the clear significance and strong relationship between fiber intake and various kinds of cancer and westernized diseases.”
