Dietary fiber

Fiber is defined by what the body cannot do with it: it resists digestion in the small intestine and reaches the colon largely intact. Soluble fibers, such as the beta-glucan in oats and barley and the pectins in fruit, form viscous gels in the gut. Insoluble fibers, such as the cellulose in wheat bran and vegetable skins, add bulk to stool. Many foods contain both. Whole grains keep all three parts of the grain kernel (the bran, the germ and the starchy endosperm), whereas refined grains keep mainly the endosperm and lose most of the fiber.

Different sources show somewhat different associations. In a review of prospective studies, insoluble fiber and fiber from cereals and vegetables were associated with lower risk of both coronary heart disease and cardiovascular disease, while fruit fiber was associated with lower cardiovascular disease risk [2].

The largest body of evidence comes from a series of systematic reviews and meta-analyses (studies that pool the results of earlier studies) published in 2019. They included prospective studies published up to April 2017 and randomised trials (studies that assign participants to groups by chance) published up to February 2018, and excluded studies of people with chronic disease, weight-loss trials and trials of fiber supplements. Comparing the highest with the lowest fiber consumers, the prospective studies showed 15 to 30% lower all-cause and cardiovascular mortality and lower incidence of coronary heart disease, stroke incidence and mortality, type 2 diabetes and colorectal cancer. Risk reduction was greatest at 25 to 29 grams a day, and the dose-response curves suggested that higher intakes might give further protection against cardiovascular disease, type 2 diabetes and colorectal and breast cancer. The certainty of the evidence was graded moderate for dietary fiber [1].

An earlier meta-analysis of 22 prospective study publications estimated a relative risk (the risk in one group relative to another) of 0.91 for both cardiovascular disease and coronary heart disease for each additional 7 grams of fiber a day, meaning 9% lower risk, with moderate variation between studies [2].

Full deep dive: Dietary fiber.

Whole grains are the main source of cereal fiber. The 2019 reviews found associations for whole-grain intake similar to those for total fiber, although the certainty of evidence was graded low to moderate [1]. A meta-analysis of 45 prospective studies found that each additional 90 grams a day of whole grains (about three servings, such as two slices of whole-grain bread and a bowl of whole-grain cereal) was associated with 19% lower risk of coronary heart disease, 22% lower risk of cardiovascular disease, 15% lower cancer mortality and 17% lower all-cause mortality. Risk continued to fall up to about 210 to 225 grams a day. Whole-grain bread, whole-grain breakfast cereals and added bran were associated with lower risk, while refined grains and white rice showed little association [3]. The Global Burden of Disease Study ranked low whole-grain intake as one of the two leading dietary risks worldwide, attributing about 3 million deaths to it in 2017 [4].

The glycaemic index measures how quickly a carbohydrate-containing food raises blood glucose compared with pure glucose, and the glycaemic load combines this with the amount of carbohydrate eaten. Both have been proposed as markers of carbohydrate quality. In the 2019 reviews, diets with a low glycaemic index or load showed smaller or no reductions in disease risk compared with high-fiber diets, and the certainty of evidence for glycaemic index and load was graded low to very low [1]. Fiber content was therefore a stronger predictor of health outcomes than the speed at which a food raises blood glucose.

The 58 trials in the 2019 reviews, with 4,635 adult participants, compared higher with lower fiber intakes from food. Higher intakes led to significantly lower body weight, systolic blood pressure and total cholesterol [1]. These are risk factors for cardiovascular disease and type 2 diabetes, which supports a causal interpretation of the observational associations, although no trial has tested fiber intake against death or heart attack directly. Proposed mechanisms include the binding of bile acids by viscous soluble fiber, which lowers cholesterol absorption; slower absorption of glucose; greater fullness after meals; and fermentation by colonic bacteria into short-chain fatty acids. Links with blood sugar regulation are covered under metabolic health and type 2 diabetes.

Colorectal cancer was among the outcomes with lower incidence in high fiber consumers in the 2019 reviews [1]. Screening has a larger documented effect on deaths from the disease. In the National Polyp Study, 2,602 patients who had precancerous adenomas removed at colonoscopy were followed for a median of 15.8 years; 12 died of colorectal cancer, against 25.4 deaths expected in the general population, a 53% reduction in colorectal cancer mortality [5]. Fiber intake and screening act at different stages and are complementary. Colorectal cancer and screening are described under colon health.

The associations between fiber and disease come mainly from prospective studies, in which intake is estimated with food questionnaires and people who eat more fiber tend also to smoke less, exercise more and eat less processed food. Statistical adjustment reduces but cannot remove this confounding (distortion by other factors linked to both diet and disease). The 2019 reviews excluded people with chronic disease and did not assess fiber supplements, so their findings apply to fiber from food in generally healthy populations [1]. Fiber is also a marker of whole plant foods, which contain vitamins, minerals and other compounds, so the effect of fiber itself is difficult to separate from that of the foods that supply it. Its place within whole diets is described under dietary patterns.