Dietary fiber is a group of food components which is resistant to digestive enzymes and found mainly in cereals, fruits and vegetables. Dietary fiber and whole grains contain a unique blend of bioactive components including resistant starches, vitamins, minerals, phytochemicals and antioxidants. Dietary fiber which indigestible in human small intestinal, on the other hand digested completely or partially fermented in the large intestine, is examined in two groups: water-soluble and water insoluble organic compounds. Dietary fiber can be separated into many different fractions. These fractions include arabinoxylan, inulin, pectin, bran, cellulose, β-glucan and resistant starch. Dietary fibers compose the major component of products with low energy value that have had an increasing importance in recent years. Dietary fibers also have technological and functional properties that can be used in the formulation of foods, as well as numerous beneficial effects on human health. Dietary fiber components organize functions of large intestine and have important physiological effects on glucose, lipid metabolism and mineral bioavailability. Today, dietary fibers are known to be protective effect against certain gastrointestinal diseases, constipation, hemorrhoids, colon cancer, gastroesophageal reflux disease, duodenal ulcer, diverticulitis, obesity, diabetes, stroke, hypertension and cardiovascular diseases. In this review the physicochemical and biological properties of dietary fibers and their important implications on human health will be investigated.
Alex’s Notes: Do you remember when your mom would tell you to eat your vegetables? I do, and although veggies contain numerous health boosting compounds, let’s focus on the one you notice most – fiber. Dietary fiber has countless health benefits ranging from cardioprotective effects to keeping you regular. Before looking at some of these benefits, however, it is important to understand what dietary fiber is.
At its simplest, dietary fiber is the indigestible carbohydrate component of food. All fibers resist digestion and absorption within the small intestine, and are classified as either insoluble or soluble in water. Soluble fibers dissolve in water, forming viscous gels, and are readily fermented by the bacteria of the large intestine. The most common forms include pectins, gums, and some beta-glucans. Conversely, insoluble fibers do not dissolve in water, do not form gels, and have a very limited fermentation capacity; insoluble fibers literally just move through you. Common examples are cellulose and chitin. Most vegetables contain about one-third soluble and two-thirds insoluble fiber.
All forms of dietary fiber act through three primary mechanisms: bulking, viscosity, and fermentation. Through these mechanisms, dietary fiber is able to interact with the digestive system to change how other nutrients and chemicals are absorbed, and even influence the composition of the GI tract itself. Although fiber is not formally considered an essential nutrient, it is nonetheless an important component of the diet that confers numerous health and physiological benefits. Furthermore, it is a diet high in a variety of soluble and insoluble fibers from whole foods that confer the greatest benefits.
So with that in mind, let’s look at some of the most common diseases plaguing Western civilization, starting with cardiovascular diseases (CHD). An estimated 82% of CHD is attributed to lifestyle practices such as diet, physical activity, and smoking, and 60% to dietary patterns, so whoever said you need medicine to reduce your risk never considered the importance of diet and exercise. Higher fiber intakes are associated with meaningfully lower prevalence rates for coronary heart disease, stroke, and peripheral vascular disease, and seven cohort studies presenting observations for over 158,000 individuals indicate that CHD prevalence is significantly lower (29%) in individuals with the highest intake of dietary fiber compared to those with the lowest intake, and that higher fiber intakes are associated with improved body composition, adiposity, insulin sensitivity, and reduced inflammation. More recent studies found interesting data illustrating that for every 10 g of additional fiber added to a diet the mortality risk of CHD decreased by 17–35%.
Moving on to the next epidemic, nearly two-thirds of U.S. adults are overweight or obese, putting these persons at a greatly increased risk for CHD, diabetes, and a host of other metabolic complications. So what is a simple solution to this problem? Eat less, exercise more? How about eating more of the right foods. Remember that vegetables are the best source of the various fibers, and thus aiming to increase dietary fiber consumption may decrease energy intake while still maintaining other important nutrients. It also wouldn’t feel like dieting. For instance, soluble fiber, when fermented in the gut, produces glucagon-like peptide (GLP-1) and peptide YY (PYY) – two gut hormones that play a huge role in inducing satiety. Also, people who tend to be obese have poor food choices to begin with, and as dietary fiber intake increases, the intake of simple carbohydrates and processed foods tends to decrease. Although, dietary fiber still contributes to the total caloric content of a diet, it is much more resistant to digestion by the small intestine and even somewhat resistant in the large intestine.
I wasn’t joking when I mentioned that obesity puts you at an increased risk of type-2 diabetes (T2D), yet another completely preventable health problem with diet and exercise. In fact, Hu et al. found that excess body fat was the single most important determinant of T2D in women, with poor nutrition playing a significant influential role. Given this, everything stated about obesity applies here as well. Additionally, short chain fatty acids, produced via the fermentation of soluble fiber, have been shown to reduce the glucose response of a meal and increase insulin sensitivity.
This brings us to fiber’s gastrointestinal effects. Insoluble fiber does not dissolve in water and acts mainly as a bulking agent, drawing water into the intestine and increasing the rate of passage of food through the digestive system. These effects make vegetable consumption renowned for keeping people regular (if you ever wondered why fiber is sometimes called roughage), and fiber supplementation is a proven treatment method of constipation. Soluble fiber also attracts water to it as it moves through the GI tract, but rather than becoming a bulking agent, soluble fiber dissolves in the water to form a viscous gel. This property works to slow digestion and the transit of food, leading to greater satiety and decreased absorption of other macronutrients within the meal. However, the fiber does not bind to vitamins and minerals and therefore does not restrict their absorption, but rather, some evidence shows that soluble fiber actually increases the bioavailability of micronutrients such as calcium, magnesium, and iron.
More importantly, soluble fibers act as prebiotics for our microbiota. Hippocrates once said that “all disease begins in the gut,” and given the intimate relationship between our microbiota and our health, he could not have been more correct. The human gut is home to ten times more bacteria than there are cells in our bodies, with over 400 known species. These bacteria work synergistically with our digestive system to reduce intestinal acidity, produce digestive enzymes and synthesize nutrients, enhance mineral absorption, produce antibacterial substances against pathogenic bacteria and suppress bacterial infections, enhance immune function, and remove carcinogens. Prebiotics, quite simply, stimulate the growth and maintenance of beneficial gut bacteria, and most of the benefits of soluble fiber come indirectly from the microbiota.
Fiber is beneficial for health. There is no denying this fact, and there is no excuse for not consuming adequate fiber in your diet. Be it from vegetables, oats, resistant starch, all should be a central component of our dietary practices.