The process of digestion starts at ingestion and mastication and ends with defecation. Each step of the process in unique to the individual and there is no set time limit on digestion. However there are normal ranges for digestion and, in most healthy adults, the entire process of digestion takes anywhere from 24 to 72 hours. Fiber aids with elimination by providing roughage and stimulating peristalsis, muscle contractions, in the digestive tract . The more fiber in the diet, the faster the process.
Ingestion
The chewing process usually takes a few seconds as food is broken up in the mouth, moistened with saliva and formed into a bolus (or ball) for swallowing. During mastication saliva mixes with the food and the amylase in the saliva starts breaking down the starches and simple sugars. Once the food is adequately chewed, it travels down the esophagus to the stomach. The longer we chew, the more the food is broken down, enhancing the total rate of digestion.
The Stomach
In the stomach, the food is bathed in hydrochloric acid, which further breaks down starches and initiates protein digestion. The stomach also acts like a blender, churning the food into a substance called chyme. Once the food is adequately churned, it is released in small increments into the small intestine. The rate of release varies, depending on the amount of food matter already in the intestines. Often, if there is a lot of food in the stomach, this will initiate defecation in order to make room for more food. This is why people often have a bowel movement after a hearty meal. It takes anywhere from four to five hours for food to completely clear the stomach. This depends on the size of the stomach, the amount of food ingested, and the amount of material already in the intestines.
The Small Intestine
As the chyme enters the duodenum, the pancreas and gall bladder release digestive enzymes that break the food down into its chemical components. The small intestine is where the real process of digestion takes place, as the chemical elements from food are absorbed into the blood stream. The small intestine is eighteen to twenty-one feet long and it can take food anywhere from 3 to 6 hours to pass all the way through.
The Large Intestine
As the food clears the small intestine it migrates to the large intestine (or colon) where it is prepared for elimination. Water and minerals are removed, solidifying the chyme into fecal matter which then migrates to the rectum where it is eliminated from the body. The large intestine is approximately five to six feet long and it takes thirty to forty hours for food to pass through.
Abnormal Digestion
There are instances where digestion is abnormally slow or fast. In diets where fiber is lacking, fecal matter will stay in the large intestine longer because the fiber isn't there to stimulate the bowel muscles and assist with digestion. Dehydration can also slow digestion because there is not enough water to assist with the digestive process. Overuse of laxatives can disrupt normal bowel rhythms resulting in constipation when the laxative use is stopped. Illness can also affect digestion by causing diarrhea or constipation. Drugs can also affect bowel function, and nicotine and caffeine are both known to stimulate the bowel and speed defecation. Finally, a diet high in processed and high-fat foods also slows digestion because these components are harder to break down and take longer to absorb.
Different Foods, Different Rates.
Interestingly, all food does not move through the digestive tract at the same rate. Neither is food digested in the order ingested. It is possible for matter from dinner to pass through before matter from breakfast (or even matter from dinner the day before). This is because the digestive system digests different elements at different rates. Carbohydrates are the easiest to digest and clear the system first, meats and fats take significantly longer. So a hearty, meat-filled, breakfast will remain in the intestines longer, while a light salad from lunch will pass through.
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