Benefits of Fiber Revisited

By Paul E. Lemanski, MD, MS, FACP

Editor’s Note: This is the 114th 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 113 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. However, 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 underconsumption 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. Consequently, we may search out products either high fiber cereals, or fiber supplements like psyllium. But these are products. In reality, fiber is a natural part of all plants. When we eat plants – vegetables, fruits, and unprocessed grain – we are eating fiber.

It is mostly in Western culture that fiber is viewed as something separate that must be added back, because in some modern western cuisines, fiber has, indeed, been 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. While white flour may have been initially developed to increase shelf life, it has led to its increased incorporation in many cuisines further removing fiber from its natural place in the human diet.

Likewise, modernity, with its de-emphasis of hunting in favor of a process of industrialization with animal husbandry and slaughter has increased the availability of beef, pork, and chicken. Humans throughout our history have been omnivorous, but the number of animals consumed had been determined not only by hunting prowess, but also by scarcity. Animals have feet, plants do not. Although animals are highly concentrated sources of protein and fat, they are not always as easily available for consumption as plants. Consequently, human biochemistry became adapted to the consumption of large amounts of plants and their fiber, vitamins and antioxidants.

The result of the reduction in the consumption of plants in the modern Western human diet – and the consequent reduction in fiber, vitamins and antioxidants – has been significant to human health in a number of ways. I will outline below 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 (dissolvable 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 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 may: 1) reduce the risk of developing Type 2 diabetes, 2) may help in lowering blood glucose in those with Type 2 diabetes, 3) may reduce cholesterol and cardiovascular risk, and 4) may help lower body weight. The presumed mechanism for weight loss is based in the relative low-calorie content of most high fiber foods. When consumed in sufficient amounts, their bulk will activate stretch receptors in the stomach, contributing 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 above.

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 woman 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.

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 glycemic 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 five-to-one or less is helpful.

In summary, fiber in Western society has been largely removed in processed food. The result may be increased risk for diabetes, high cholesterol, cardiovascular risk, increased bodyweight, as well as an increased risk of certain cancers. Returning to minimally processed whole grains, fruits, and vegetables will help lower risk. Attempting to consume a minimum of two servings of fruit and three servings of vegetables a day rather than relying on a fiber supplement is more pleasant and supplies vitamins and antioxidants as well.

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.