The gastrointestinal system (GI system) processes food that is ingested by breaking it down into small molecules that can be absorbed into the body. It also eliminates undigested and unabsorbed substances, including fiber.
Performing these experiments will illustrate how fiber type affects the time it takes for food to pass through the GI system and how fiber in food affects bile salt reabsorption back into the body.
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The gastrointestinal system (GI system) includes GI tract organs and accessory digestive organs. The GI tract, or alimentary canal, is a tube that extends from the mouth to the anus. Functions of the digestive system include: ingestion; secretion of digestive juices; mixing and propulsion of food; digestion of food into small, absorbable molecules; absorption of digestive products across the GI tract wall and into the body; defecation.
Normal transit time (sometimes called bowel transit time) is extremely variable. Peristalsis, coordinated contraction and relaxation of muscle layers in the wall of GI tract organs, is responsible for movement of food through the GI tract. These muscle contractions are controlled by the enteric nervous system.
Diverse factors that can cause variances in transit time in an individual include: meal composition, hydration, activity level, medications, and psychological stress. Additional factors that contribute to variability among individuals are: age, gender, and reproductive status. All of these factors make it extremely difficult to estimate the exact length of ‘normal’ transit time. Although studies vary, the ‘average’ transit time appears to be between 25 — 36 hours. If food moves through the GI tract too quickly, i.e. less than 18 hours, nutrients do not have enough time to be absorbed into the intestines causing malabsorption, which can lead to malnutrition. If transit time is too long, i.e. longer than 48 hours, the substances will sit in the intestines and putrefy, allowing toxins to be absorbed into the body. This can lead to other degenerative diseases. Normal transit time is critical for a person to have good health.
The following are estimates of ‘average’ transit times for food to travel through GI tract organs:
Dietary fiber (sometimes called roughage) affects digestion, absorption, and elimination of food and plays a role in the prevention of chronic diseases. Fiber content is often listed on the Nutrition Facts label under "Total Carbohydrates". Because fiber is undigested, it is not absorbed and includes zero calories. Dietary fiber may be categorized as either soluble (dissolves in water) or insoluble (does not dissolve in water).
Soluble fibers, made up of sticky substances like pectins and gums, are found in fruits (particularly oranges, apples and bananas), oats, beans and other legumes, vegetables, and psyllium seed.
Insoluble fiber is found in whole grains, seeds, nuts, fruits and vegetables.
Most fiber-containing foods contain both types of fibers. Fiber provides bulk, making elimination easier, helping to prevent constipation and abrasion. It also fills the stomach, reducing appetite, and could decrease eating, leading to weight loss.
The liver converts cholesterol into the two main bile acids, cholic and chenodeoxycholic acids that are necessary for fat emulsification. The liver secretes bile acids into the gallbladder to be stored until fatty food reaches the duodenum, at which time the gallbladder contracts to release the bile acids into the duodenum. The bile acids attach to and emulsify fat to facilitate the ability of lipases to break down lipid molecules into smaller molecules that can be absorbed. The body reabsorbs approximately 95 percent of the bile acids through the lining of the ileum for transport back to the liver in the bloodstream. Bile acid molecules recirculate by this process and are used about 15 to 20 times before being excreted from the body in the feces. Increased fecal excretion of bile acids results in the liver removing cholesterol from the bloodstream to produce more bile acids, thus lowering serum cholesterol levels.
Radiopaque markers were used to measure transit time. Radiopaque markers are small capsules that are ingested and appear opaque on X rays as they pass through the digestive tract. Transit time will be estimated by measuring the amount of markers remaining in stomach and colon at various times after ingestion.
A spectrophotometer is used to measure the amount of bile acids in solution. Bile acids adhere (bind) to food particles. The greater the amount of bile acids adsorbed to food, the less bile acids in solution. The solution containing bile acids is mixed with chemicals that cause the solution to turn pinkish. The intensity of color is proportional to the amount of bile acids in solution.