1. The metabolism of dietary fiber in the gastrointestinal tract

DF (dietary fiber) is decomposed by bacterial fermentation in the intestine, among which anaerobic bacteria dominate. Initially, these huge polymers are hydrolyzed into glucose, galactose, xylose and uronic acid, etc., and continue to carry out sugar The final products of glycolysis are short-chain fatty acids, hydrogen, CO2 and methane. Short-chain fatty acids are the main anions in human feces with a concentration of 60-170 mmol/L. Acetic acid, propionic acid and butyric acid together account for more than 80% of the short-chain fatty acids produced. Normal people produce 200-700nmol/L short-chain fatty acids daily. Short-chain fatty acids are mainly metabolized in the colon mucosa and liver. Butyrate is the best oxidation substrate of colonic epithelium, accounting for 80% of the oxygen consumption of colon cells. Butyrate is the main respiratory fuel for colonic epithelium, and the production of butyrate is a necessary guarantee for maintaining a healthy local mucosa.

2. The effect of dietary fiber on gastrointestinal function

(1) Stomach 

Intake of large amounts of DF increases chewing, stimulates more saliva secretion, and prolongs the buffer time of gastric acid. It not only delays gastric emptying and changes the type of gastrointestinal motility, but also improves the patient's tolerance to glucose. 

(2) Small intestine

Soluble fiber prolongs the running time of the small intestine, while insoluble fiber accelerates the running time of the small intestine. The addition of lemon pectin in the test of extensive small bowel resection can significantly enhance the adaptability of the small intestinal mucosa, which is manifested as improvement in mucosal weight, DNA content, mucosal thickness and disaccharide enzyme activity. Because short-chain fatty acids can stimulate small intestinal mucosal crypt cells, the mechanism is that soluble fibers are fermented into short-chain fatty acids, which provide fuel energy for intestinal cells and exert nutritional effects on the small intestinal mucosa.

(3) Colon

DF can change the state of the intestines, especially increase the amount of fecal excretion, which is the focus of its physiological effects. Fibers such as soluble apple, carotene, and pectin can stimulate the growth of bacteria in the intestinal cavity after digestion, while undigested bran fiber increases fecal capacity and retains water, making feces soft, shortening intestinal running time and reducing pressure in the colon.

3. Dietary fiber and intestinal barrier function

There is a lot of evidence that starvation can cause the villi structure of the intestine to shrink. Similar changes were found when animals were treated with parenteral nutrition for a long time or when the human small intestine was surgically removed due to obesity. This change may affect the secretion of digestive tract hormones.

The morphological changes of the small intestine indicate adverse effects on the internal organs without enteral nutrition. Therefore, nutritional therapy should be provided by the intestine as much as possible, rather than outside the intestine. It has been confirmed that the morphological change of the intestine is related to the change of barrier function. Therefore, parenteral nutrition therapy not only has an adverse effect on the morphological changes of animal intestines, but also is related to the transfer of bacteria from the gastrointestinal tract to other sterile tissues such as mesenteric lymphoid tissue, liver, and spleen. It is well known that bacterial translocation is prone to occur during parenteral nutrition. After experimental animals are fed with enteral nutrition, the occurrence of bacterial translocation is significantly reduced.

When considering the concept of DF and enteral nutrition, it should be noted that DF and its degradation products have special effects on the visceral barrier function. After oral administration of plant fiber in experimental animals, the incidence of bacterial translocation was significantly reduced when using parenteral nutrition. Adding corn stamens to the DF-free diet instead of soybean fiber can also significantly reduce the incidence of bacterial translocation. The formula diet without glutamine and psyllium fiber, but only with DF can effectively prevent bacterial translocation caused by endotoxin, showing a good protective effect.

4. Dietary fiber and intestinal diseases

(1) Chronic constipation and dietary fiber

It is well known that DF is effective for the treatment of chronic constipation, but insoluble DF such as cellulose plays a major role. It can increase the amount of feces, and soften the feces due to water absorption, shorten the time to pass the large intestine, thereby improving constipation.

(2) Colorectal cancer and dietary fiber

The results of epidemiological survey confirmed that DF intake is related to chronic diseases, among which colorectal cancer is the main one. The incidence of colorectal cancer in Japan has increased year by year in recent years, which is believed to be related to changes in diet. As the intake of animal foods such as beef increases, the intake of DF decreases, which increases the risk factor. High-fat and low-fiber diets are the pathogenic factors of colorectal cancer. High-fat diets may be related to changes in the quality and quantity of bile salt and cholesterol metabolism in the intestinal lumen. Low-fiber diets delay the running time of intestinal contents and increase carcinogens. Contact and absorption with intestinal mucosa. Many epidemiological investigations have confirmed that a high-fiber diet can help reduce the risk of colorectal cancer. A well-known scientist summarized 37 groups of epidemiological investigations and 16 groups of case-control studies showing that DF has a protective effect and can reduce the occurrence of colon cancer, including the effect of short-chain fatty acids.

(3) Intestinal inflammatory diseases and dietary fiber

• Ulcerative colitis

A foreign scientist found that the colon cells of patients with ulcerative colitis could not make full use of short-chain fatty acids. He applied short-chain fatty acid solution for rectal lavage treatment. Of the 12 patients with ulcerative colitis, 10 completed the full course of treatment, resulting in 9 symptoms. The disease activity index decreased from 7.9±0.3 points to 1.8±0.6 points on average (P＜0.002), and the mucosal histopathology score also decreased from 7.7±0.7 points to 2.6±0.7 points (P＜0.002). It can be seen that the increased contact of the colonic mucosa of patients with ulcerative colitis with energy substrates contributes to the recovery of their disease, and short-chain fatty acids are the main energy source of colon cells.

• Crohn's disease

The occurrence of Crohn's disease is related to the intake of DF. In 1972, scientists gave 62 cases of intestinal diverticulosis patients with fiber-rich foods (wheat bran, methyl fiber, psyllium, etc.). After 22 months, the symptoms were significantly improved. After long-term trials, it was believed that the improvement in symptoms was due to fiber reduction. The result of pressure in the colon cavity.

(4) Colonic Diverticular Disease and Dietary Fiber

Diverticulosis is mostly secondary to years of constipation or irritable bowel disease caused by increased pressure in the colon, resulting in the appearance of small sacs in the intestine, and then the formation of diverticula. High DF intake has a certain effect on the prevention and treatment of the disease, because DF can increase the volume of feces, rather than simply increase the pressure of the colon, and make the feces smoothly discharged, thus playing a role in the prevention and treatment of diverticulosis.