The Digestive System As the white bread passes through your mouth, stomach, and small intestine, more amylase generated by your pancreas digests any residual starch molecules until only glucose remains. Amylase is an enzyme that breaks down starch into sugar. Your body can't store starch so it is always converted into sugar which can be used as energy. As soon as you eat bread, potatoes, or other starches, your body begins to digest them right away. The more rapidly you eat food, the faster your body will digest it and use the nutrients. When you eat slowly, your body has time to absorb the nutrients from your food rather than just using up its energy on digestion.
As mentioned, starch is a major component of wheat flour, which means that if you eat a lot of white bread, pasta, and other products made with flour you're going to end up eating a lot of starch. Starch is stored in the body as glucose, which is our blood's main fuel. So if you eat a lot of starch, then your body will have to make more insulin to pull some of this glucose out of your bloodstream and store it in your muscles and liver for later use. This explains why people who eat a lot of starch often feel tired and drained of energy- they are actually burning the energy they consume!
Amylase is found in both pancreatic fluid and saliva, so when you chew the bread, the amylase in your saliva interacts with the starch in the bread, breaking it down to form simple sugars - these simple sugars are what make the bread taste sweet. As your body absorbs these sugars, they are released into your bloodstream as glucose.
Since insulin is required to transport glucose from the blood into cells, this process has the effect of calming the pancreas down and reducing the release of additional insulin. As a result, some people may experience mild to moderate diabetes when they first start eating breads containing flour, because there isn't enough insulin to handle the amount of sugar being absorbed from the bread. However, over time the body becomes used to absorbing large amounts of sugar through the use of insulin, and so can cope better with subsequent doses.
The amount of sugar in most breads is very low compared with that found in most fruits and vegetables. Your body only needs a small amount of insulin to deal with this kind of intake, so there's no need for you to worry about developing diabetes as long as you don't eat an excessive amount of bread.
The way in which you eat food affects how much insulin you require.
Salivary amylase (a digestive enzyme) is secreted by the salivary glands (which produce saliva, which is a mixture of water, enzymes, and other chemicals) and begins the chemical breakdown of carbohydrates in bread, while lingual lipase (another digestive enzyme) begins the chemical breakdown of triglycerides...
Stomach acid breaks down any food particles or liquids that make its way into your stomach. The acidic environment of the stomach also destroys most bacteria, so antibiotics can be used to kill off harmful bacteria such as E. coli and salmonella.
Pancreatic juices contain both hydrochloric and phosphoric acids that are strong enough to break down most foods that enter the duodenum. The pancreas is located behind the stomach and above the small intestine. It is a large gland that produces insulin and other hormones.
The small intestine is the longest part of the gastrointestinal tract, measuring from 1-3 feet long. It consists of three regions: the duodenum, jejunum, and ileum. The duodenum is the first portion of the small intestine and it is where digestion of starch and sugar occurs. Bacteria in the large intestine help break down remaining nutrients before they are absorbed into the bloodstream. The byproduct of this process is carbon dioxide, which leaves the body through our lungs.
Food passes through the digestive system one piece at a time.