The Benefits of Fiber, Revisited

By Paul E. Lemanski, MD, MS, FACP

Editor’s Note: This is the 124th in a series on optimal diet and lifestyle to help prevent and treat disease. Any planned change in diet, exercise or treatment should be discussed with and approved by your personal physician before implementation. The help of a registered dietitian in the implementation of dietary changes is strongly recommended.

Medicines are a mainstay of American life and the healthcare system not only because they are perceived to work by the individuals taking them, but also because their benefit may be shown by the objective assessment of scientific study. Clinical research trials have shown that some of the medicines of Western science may reduce the risk of Type 2 diabetes, heart attacks, strokes, cardiovascular death and even some cancers.

In the first 123 installments of the Non-Medicated Life, a healthy diet and lifestyle has been shown to accomplish naturally for the majority of individuals most of the benefits of medications in the prevention of the chronic medical conditions mentioned above. With respect to a healthy diet, certain foods, and food “components” may have a disproportionate benefit. This is certainly true for dietary fiber. Indeed, while optimal health may be compromised by a lack of fiber, actual disease processes may also result. Until recent times humans ate as much as 100 grams of dietary fiber per day. Today Americans may get as little as 10-15 grams a day. Why this is so, the benefits of proper consumption, and the consequences of under consumption of fiber are described below.

It is generally taken as established fact that fiber is good for us. For example, we are told it keeps our bowels regular. Many folks with constipation have learned through self-treatment of this benefit. Unfortunately, for most folks fiber means a highly processed product such as high fiber cereals, or fiber supplements like psyllium. In reality, fiber is a natural part of all plants. When we eat plants – vegetables, fruits, and unprocessed grain – we are eating fiber.

Many modern Western food products, unfortunately, have had fiber removed. For example, with respect to grains, processing has removed both the germ (fat) and fiber (an innocent bystander of fat removal) to avoid spoilage during transport and storage. While white flour may have been initially developed to increase shelf life, its incorporation into many cuisines further removed fiber from its natural place in the human diet.

Likewise, modernity, with its deemphasis of hunting in favor of animal husbandry and the industrial raising of animals, has increased the consumption of beef, pork, and chicken. Humans throughout our history have been omnivorous, but the amount of animals consumed had been determined in large part by scarcity. Although animals are sought because they are highly concentrated sources of protein and fat, they are also mobile and not as easily obtained for consumption as plants. Consequently, plants became the main source of food in the human diet, and human biochemistry became adapted to plants both as an energy source and also to their fiber, vitamins, and antioxidants.

The modern Western human diet favoring animals – and the consequent reduction in fiber, vitamins, and antioxidants – has had a significant impact on human health in a number of ways. I will outline some of the most important health benefits of optimal fiber intake, the consequences of insufficient fiber consumption, and suggest strategies to address the shortfall.

Fiber may be classified as soluble (able to be dissolved in water) fiber or insoluble fiber. The former in sufficient amounts will lower both blood glucose and LDL (the bad) cholesterol primarily by interfering with the absorption of glucose and dietary cholesterol from the gut. Soluble fiber is found in increased amounts in grains such as oats, barely, and psyllium, in fruits such as apples and citrus fruits, and in legumes such peas and beans. Insoluble fiber increases stool bulk and may be found in increased amounts in grains such as wheat bran and whole wheat flour, nuts and beans, and vegetables such as green beans, broccoli, and cauliflower.

It must be emphasized that all plants contain a mixture of soluble and insoluble fibers. An overall increase in the consumption of a variety of plants has been shown to: 1) reduce the risk of developing Type 2 diabetes, 2) help in lowering blood glucose in those with Type 2 diabetes, 3) reduce cholesterol and cardiovascular risk, and 4) help lower body weight. 

The presumed mechanism for weight loss is based in the relatively low calorie content of most high fiber foods; when consumed in sufficient amounts their bulk will activate stretch receptors in the stomach, contribute to a feeling of fullness and achieve satiety at a lower total caloric intake. Conversely, insufficient fiber intake has the opposite effect on the conditions mentioned. Additionally, insufficient fiber intake may also increase the risk for cancers of the breast and colon.

In one study, premenopausal women who ate six grams of soluble fiber a day had a 62% lower risk for developing breast cancer than those who consumed less than four grams. Looking at a number of other studies of fiber and breast cancer, it appears that every 20 grams of total fiber intake a day may be associated with a 15% lower risk of breast cancer. The mechanism is not clear, except that fiber appears to lower blood estrogen levels, and reduced estrogen stimulation of breast tissue appears to lower cancer risk. Unfortunately, American women on average eat less than 15 grams of fiber a day and this intake may be less than needed to see fiber’s full benefit. The Institute of Medicine recommendation for women is 25 grams of fiber a day and for men is 38 grams a day.

Insufficient fiber intake may also increase the risk of colon cancer. In a large, prospective, population-based screening trial total dietary fiber intake was associated with a reduced risk of distal colon adenoma, a precursor of colon cancer, as well as distal colon cancer. In this study fiber mediated protection was most strongly associated with fiber derived from cereals or fruit. Possible mechanisms may include the dilution of bowel carcinogens from increased fiber or fiber derived alterations in the gut bacterial species (microbiome). In the latter instance, certain types of fiber may serve as probiotic food sources for the growth of healthy gut bacteria.

Insufficient fiber can also increase constipation, which in turn causes an increase in intra-abdominal pressure from bearing down. This increase in intra-abdominal pressure may push the upper part of the stomach out of the abdominal cavity and into the chest resulting in a hiatal hernia and an increase in gastrointestinal reflux symptoms such as heart burn. The increased pressure may also result in hemorrhoids, varicose veins, and anal fissures. More concerning the increased intra-abdominal pressure can cause diverticulosis, a condition in which small pockets form in the wall of the bowel. These pockets are the necessary anatomical prerequisite for an infection in the wall of the bowel called diverticulitis, which can be life threatening.

Increasing fiber in the diet may be as simple as attempting to consume more fruits and vegetables to achieve the above defined fiber targets. But how many fruits and vegetables are enough? In research published in the journal Circulation in 2021, two servings of fruit and three servings of vegetables, seemed the most effective combination. When assessing products and products containing grains, minimally processed, whole grains are best. For example, hulled barley, containing both germ and fiber has the lowest hypoglycemic index of any grain and will raise blood glucose gram for gram the least. When in doubt about a product, examining the nutrition label, and following Dr. Michael Greger’s suggestion to eat only those products having a ratio of carbohydrate to fiber of 5 to 1 or less is helpful.

In summary, dietary fiber in Western society has been largely removed in processed food to the detriment of health. The result may be increased risk for diabetes, high cholesterol, heart attack risk, increased body weight, and diabetes, as well as an increased risk of certain cancers. Decreased dietary whole food derived fiber can also result in constipation and conditions related to a resultant increase in intra-abdominal pressure. This decreased fiber may result in hemorrhoids, varicose veins, anal fissures, gastroesophageal reflux, hiatal hernia, and diverticulosis and diverticulitis. Returning to minimally processed whole grains, fruits, and vegetables will increase fiber naturally, feed the microbiome, and improve health with minimal effort, potentially making the non-medicated life more achievable.

Paul E. Lemanski, MD, MS, FACP (plemanski3@gmail.com) is a board-certified internist practicing internal medicine and lifestyle medicine in Albany. Paul has a master’s degree in human nutrition, he’s an assistant clinical professor of medicine at Albany Medical College, and a fellow of the American College of Physicians.