When it comes to nutrients (as opposed to foods), rate of digestion isn't really a matter of opinion. Fat takes the longest to digest from among all of the possible food macronutrients. Many stomachs are able to process no more than approximately 10 grams of fat per hour. It would be common for a complete fast food meal to contain 50-60 grams of total fat — or even more. If a fast food dinner was eaten at 7:00pm, the fat in that meal might not be leaving the stomach until midnight or 1:00am. And it would only be leaving the stomach at that point, without bulk of the digestion process having even begun. After leaving the stomach, all of the fat in a fast food meal would need to emulsified (made more water-soluble) with the help of bile salts from the liver, metabolized by enzymes from the pancreas, and then further processed throughout the length of the intestine.
The time it takes to digest any type of food varies with the individual, the food, the person's state of physical and emotional health, medications they may be taking, etc. The length of time it takes food to digest is called transit time. Like many physiological events, transit time can become problematic if it's too fast or too slow. In the United States, the most common problem (by far) is too slow transit time, often diagnosable as chronic constipation. This overly slow digestive process has been associated in research studies with very low intake of dietary fiber, excessive intake of high-fat foods, and overconsumption of food in general. It is not uncommon for a very low-fiber, very high-fat, and very high-calorie meal to stay inside our digestive tract for more than two full days. This amount of time is too long, and it is associated with increased risk of chronic disease, including risk of colon cancer. It's worth mentioning here that overly slow digestion is especially common and problematic among older adults. Difficulties chewing that often lead to the selection of easier-to-chew low-fiber and descreased physical activity with loss of muscle tone around the digestive tract are particularly common issues for this part of the population.
In general, it would be healthier if our meals could fully digest within 24 hours or so, and under certain circumstances in even less time than 24 hours. The norm in this case would be for a person eating three meals a day to have about three bowel movements per day. In the United States, research shows that the majority of adults have far less than three bowel movements daily. While estimates of the exact number vary, it is clearly not uncommon for an adult in the U.S. to go an entire day without a bowel movement.
How long it takes a food to digest is not only a function of the food, however. It's also a function of our overall health, and especially our degree of physical activity and whole body exercise. It takes healthy muscle tone all around our abdomen for food to properly digest. We need the correct muscle tone to help move food through our digestive tract. Increasing exercise can improve our digestion, even when the kind of food we eat remains unchanged.
Excessive amounts of any food are hard on the digestive system and can also affect the rate of digestion. They call upon the body to expend too much energy and too many nutrients all at once. Processed foods are equally hard on the digestive system — the body has to supply energy and nutrients in order for these foods to digest, but it doesn't get enough nutrients back in exchange because they have been processed out of the food. High-fat, heavily processed foods would be the worst combination in terms of digestive difficulty. Finally, chewing thoroughly will help any food digest. We make digestion more difficult whenever we eat on the run or gulp down our food.
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