Dietary fibers are a portion of plants that are resistant to digestion. They are complex carbohydrates, classified based on their solubility in fluids. Soluble fibers (pectins, gums, mucilages) dissolve in fluids, and influence the absorption of glucose, lipids and other nutrients. Insoluble fibers (cellulose, hemicellulose, and lignin), which are not soluble in fluids, contribute more to an increased stool weight and reduced intestinal transit time.
According to the guidelines, an average American man should consume 30 to 38 grams of dietary fiber per day while a woman should consume between 21 to 25 grams of dietary fiber per day. However, the actual intake of dietary fiber is much lower than recommended. It is thought that women are consuming only 13 grams and men’s consumption is just 17 grams of dietary fiber per day.
Experts believe that soluble fibers form a gel in the stomach leading to sluggish gastric clearing and a higher rate of absorption of the nutrients. This increases satiety and leads to impaired food intake which is reflected in reduced weight gain. Additionally, fermentation of fiber in the large intestine leads to reactions which eventually reduce the blood cholesterol levels.
Studies have shown that greater fiber intake, especially water soluble fiber, is associated with a decrease in coronary heart disease risk. An analysis of ten studies detected a 12% decrease in the risk for coronary events and a 19% decrease in the risk of death for each 10 g increment in fiber per day.
The National Cholesterol Education Program recommends an ingestion of 5 to 10 g of soluble fiber daily to improve LDL-C (low density lipoprotein) levels. The American Heart Association (AHA) endorses a total dietary fiber intake of 25 to 30 g/d from foods, not supplements, to ensure nutrient balance and to maximize the cholesterol-lowering impact of a fat-modified diet.
Multiple studies have shown that a high fiber intake could reduce systemic blood pressure. One study showed that the effects of fiber intake were more pronounced in older than 40 years compared to younger patients. High fiber intake could also reduce blood cholesterol levels.
Fiber consumption could cause increased bloating. However, the severity of the side effects is limited when fiber intake is reduced. Fibers that are less fermented, such as psyllium, may also help in this regard. Drinking sufficient amounts of fluid and slowly increasing the amount of fiber intake could also limit bloating.
High fiber intake could interfere with the absorption of minerals, such as iron, zinc, magnesium, and calcium.