PROFESSIONAL RESOURCES Defining the Digestive Health Diet: The Role of Fiber, Phytonutrients and Other Dietary Factors David Heber, MD, PhD, FACP, FACN UCLA Center for Human Nutrition Gastrointestinal diseases have been increasing dramatically in the United States over the past few decades and now bear a price tag of some $85.5 million per year in health care costs. From the most through the least common, they can be roughly grouped as follows: Irritable bowel syndrome, constipation, diarrhea Infection Cancer - colorectal, stomach, pancreatic, gallbladder Inflammation - Crohn's, ulcerative colitis, diverticulitis Intolerances - Celiac disease, sprue, lactose Short bowel syndrome Malabsorption - secretions or enzyme insufficiency, transport defects As with other diseases, an ounce of prevention is worth a pound of cure. Only 30% of how well we age is controlled by our genes; the other 70% is modifiable through proper nutrition and exercise. Humankind evolved on diets that are quite different from that which is now prevalent in the U.S. and other Western nations. Fifty thousand years ago our ancestors lived on fruits, nuts, seeds, roots, tubers, flowers, leaves, stalks and beans. Their diet contained half the fat of ours and two to three times the protein. There were no processed foods, no alcohol and no tobacco. Today our diet is high in refined carbohydrates and starches (rice, refined breads and cereals, pasta, potatoes), sugars, and fats, and many of our protein foods are prepared with fats or consumed with added fats. Today, we also consume significantly fewer servings of plant foods than ancient humans, and do not meet the recommended number of servings of fruits and vegetables. During the 30-year period between 1968 and 1998, introduction of new condiments, candies and snacks outpaced the introduction of new vegetable food products by about six to one and bakery products by about four to one. During the same time period, the prevalence of obesity in the U.S. more than doubled. An easy guide to improve dietary choices and reduce weight, one that I have been trying to teach for the past several years, is simply to choose more colorful foods. Colors indicate the presence of phytochemicals that may have specific benefits in health. For example, by choosing orange foods one may obtain beta-carotene, which as provitamin A preserves night vision and as an antioxidant may help protect against cancer. In green vegetables we find glucosinolates, implicated in cancer prevention and detoxification. Lutein, present in yellow and green vegetables, is thought to play a preventive role in age-related macular degeneration and perhaps cataracts, the principal vision problems among seniors. Lycopene, the red color in tomato products, watermelon and pink grapefruit, is linked to prevention of cancers at several sites including prostate, breast, lung and stomach. Purple foods such as plums and berries contain anthocyanidins, which have been shown to play a role in brain function and to have anti-infection properties. White vegetables in the onion family contain allyl sulfides, also linked to cancer prevention and reduced risk of hypertension. Flavonoids, a large family of compounds present in many fruits and vegetables, especially in yellow-orange varieties, are linked to cancer prevention. Not surprisingly, diets high in fruits and vegetables - naturally high in all these protective compounds, have consistently been found to be protective against cancers. Major classes of these protective phenolic compounds and examples of compounds and their food sources are shown in Table 1. Table 1. Classification of Dietary Phenolics Health-Impacting Compounds in California Dried Plums Dried plums contain large amounts of phenolic compounds (184 mg/100 g), mainly as neochlorogenic and chlorogenic acids. These phenolics have been shown in cell cultures to inhibit the oxidation of low-density lipoprotein (LDL) cholesterol, an important step in the development of cardiovascular disease. Dried plums have significant antioxidant activity as assessed by the Oxygen Radical Absorbing Capability (ORAC) test of the U.S. Department of Agriculture. Their ORAC score, about 5700 ORAC units per 100 grams, is one of the highest among fruits. In addition, dried plums have other properties that have health effects, both directly and through the effects of their phytochemicals. Dried plums contain about 6.1 grams of dietary fiber per 100 grams, or about 2.4 grams per 40 gram serving. Both soluble and insoluble fiber are present in about equal proportions. The glycemic index of dried plums is a low 29, probably due to their sugar profile, dietary fiber and sorbitol. Understanding the Gastrointestinal System The human gastrointestinal (GI) tract is over 22 feet long and houses over 100 trillion bacteria. Health of this system is intimately tied to health of the whole body. The GI tract is the largest immune organ and the largest endocrine organ in the body. With an absorptive surface twice the size of a tennis court, it obtains its nutriture both from the lumen and the blood stream, enabling it to adapt to dietary changes. Twenty to 40% of protein synthesis occurs in the GI tract. Fiber has multiple effects during passage through the GI tract. In the stomach fiber contributes to fullness and gastric emptying. It increases transit time of food through the intestines. In the colon, the final portion of the GI tract, undigested fiber is fermented by bacteria. The inner lining of the GI tract, the mucosa, consists of three layers: the epithelium, the lamina propria and the muscularis mucosa. The epithelial cells are separated from the lamina propria by a basement membrane. This lining is critical because it is here that important interactions of the epithelial cells, bacteria and food compounds occurs. Phytochemicals interact with these cells in unique ways, both synergistically with related and unrelated compounds, and through activation of metabolic enzymes. Their effects extend well beyond their antioxidant capabilities and influence cellular signaling pathways, gap junctions (another mechanism whereby cells communicate with each other), and metabolic enzymes that catalyze other reactions. In addition, some phytochemicals have an effect on the proliferation and apoptosis (cell creation and programmed cell death) of the intestinal cells. The Intestine as an Immune Organ The immune system encompasses a broad range of cells and tissues. In addition to the bone marrow and thymus (the primary sources of immune cells), the lymphoid system includes secondary organs and tissues spread throughout the body. The mucosa-associated lymphoid tissue includes lymphoid tissues in the epithelial areas of the respiratory tract, urogenital tract, oral cavity and gastrointestinal tract. The portion of this system related to the GI tract is the gut-associated lymphoid tissue or "GALT." Cells in this lymphoid tissue produce secretory immunoglobulin A (sIgA), an important antibody that prevents the colonization of the GI tract by pathogenic bacteria, as well as IgG and IgM. Immune cells may arise in the lumen, the epithelium and the lamina propria, where lymphoid tissues called Peyer’s Patches are responsible for the production of lymphocytes. The immune actions initiated in the GI tract encompass the expression and proliferation of T and B lymphocytes, natural killer cells (important for protection against viruses and tumors), monocytes, neutrophils, eosinophils, basophils and cytokines (chemical messengers such as interleukins and tumor necrosis factor that regulate cell-mediated and humoral immunity). The system is highly sophisticated and able to discriminate between the benign bacteria that colonize the gut and the pathogens or carcinogens that travel through it daily. It is able both to tolerate the former and mount a defense against the latter. Importantly, the type of reactions that are signaled, including effects on the types and numbers of cells produced, are influenced by the diet and the many compounds such as phytochemicals provided by foods. Directions for Future Research The future of nutrition research will be an exciting journey as we identify and understand more about these complex interactions. Some areas of specialty likely will be: nutrigenetics - the individuality of response to nutrients, based on genetics nutrigenomics - how nutrition affects gene expression and disease risk immune function and inflammation - how we can modify the response so as to down-regulate in cases of allergy and autoimmune disease but up-regulate vigilance against pathogens and carcinogens energetics and obesity - going beyond diets to understand how metabolism may be fine-tuned to fight obesity functional foods, botanicals and supplements - deepening our understanding of the effects of food components on health Indeed, at some future time humans may truly be able to say, "Let food be your medicine," with full understanding of all that this implies. « Back to list of Summaries Sitemap | Terms & Conditions/Privacy Policy | Contact Us