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There are numerous fiber products on the market today. Some contain a natural fiber, such as inulin (i.e., chicory root), psyllium (i.e., husk of blond psyllium seed), or β‐glucan (i.e., oat or barley; McRorie & Fahey, 2015). Others contain an artificially created product, such as polydextrose (synthetic polymer of glucose and sorbitol), wheat dextrin (heat/acid treated wheat starch), or methylcellulose (semisynthetic, chemically treated wood pulp; McRorie & Fahey, 2015). The Institute of Medicine distinguishes dietary fiber (the nondigestible carbohydrates and lignin that are intrinsic and intact in plants) from functional fiber (the isolated, nondigestible carbohydrates that have been shown to have beneficial physiological effects in humans; Institute of Medicine, 2002). To be considered a functional fiber, the isolated nondigestible carbohydrate found in a fiber supplement must have clinical evidence of a beneficial physiologic effect. While the term “fiber supplement” implies that the product can help make up for a shortfall in dietary fiber consumption from whole foods such as fruits, vegetables, and whole grains, it is important for nurse practitioners to understand which supplements actually have clinical evidence of a beneficial physiologic effect and qualify as functional fibers.Most of what we believe about the health benefits of high dietary fiber consumption from fruits, vegetables, and whole grains comes from population‐based (epidemiologic) studies. These studies compare subpopulations (e.g., those with high vs. low dietary fiber consumption) and look for statistical associations with decreased or increased incidence of disease. The adequate intake guidelines for dietary fiber are based on a significant association between a high‐fiber diet and a reduced risk for cardiovascular disease (Institute of Medicine, 2002). The Institute of Medicine recommends a fiber intake of 14 g/1000 kcal consumed, which translates to about 25 g/day for women and 38 g/day for men (adults aged 21–50). Older adults tend to consume fewer calories, so the recommendation for women and men over 50 is 21 and 30 g/day, respectively. Only about 5% of the U.S. population achieves the recommended level of dietary fiber consumption (U.S. Department of Agriculture, 2016). On average, adults consume only about 15 g of fiber per day, and those on a low carbohydrate diet consume less than 10 g per day.When considering the health benefits of dietary fiber (from whole foods), it is important to recognize that population‐based data lack the control necessary to establish causation. These studies can only establish statistical associations, so it is not possible to determine to what degree an observed physiologic effect is directly attributable to the fiber component of the diet, versus other health‐promoting components such as micronutrients, phytochemicals, or a reduction in fat/calorie intake. In contrast to whole foods, the physiologic effects of an isolated nondigestible carbohydrate (e.g., a fiber supplement) can be readily assessed for a direct effect in a placebo‐controlled clinical study. The purpose of this review is to provide nurse practitioners with an understanding of (a) the physical effects of isolated fibers in different regions of the gut that drive each specific health benefit, (b) which specific fibers possess the physical characteristics required to provide each specific health benefit, and (c) which specific fiber supplements are supported by rigorous evidence of a clinically meaningful health benefit.The small intestine is approximately 7 m long and the mucosa is studded with millions of villi, each of which is covered with approximately 1000 microvilli per 0.1 μm2 (i.e., brush border; McRorie & Fahey, 2015).