Our source of energy is the food we eat. Our bodies digest the food we eat by mixing it with fluids (acids and enzymes) in the stomach. When the stomach digests food, the carbohydrate (sugars and starches) in the food breaks down into another type of sugar, called glucose.
The stomach and small intestines absorb the glucose and then release it into the bloodstream. Once in the bloodstream, glucose can be used immediately for energy or stored in our bodies, to be used later.
However, our bodies need insulin to use or store glucose for energy. Without insulin, glucose stays in the bloodstream, keeping blood sugar levels high.
Insulin is a hormone made by beta cells in the pancreas. Beta cells are very sensitive to the amount of glucose in the bloodstream. Normally beta cells check the blood’s glucose level every few seconds and sense when they need to speed up or slow down the amount of insulin they’re making and releasing. When someone eats something high in carbohydrates, like a piece of bread, the glucose level in the blood rises and the beta cells trigger the pancreas to release more insulin into the bloodstream. When insulin is released from the pancreas, it travels through the bloodstream to the body’s cells and tells the cell doors to open up to let the glucose in. Once inside, the cells convert glucose into energy to use right then or store it to use later.
As glucose moves from the bloodstream into the cells, blood sugar levels start to drop. The beta cells in the pancreas can tell this is happening, so they slow down the amount of insulin they’re making. At the same time, the pancreas slows down the amount of insulin that it’s releasing into the bloodstream. When this happens, the amount of glucose going into the cells also slows down. The rise and fall in insulin and blood sugar happens many times during the day and night. The amount of glucose and insulin in our bloodstream depends on when we eat and how much. When the body is working as it should, it can keep blood sugar at a normal level, which is between 70 and 120 milligrams per deciliter. However, even in people without diabetes, blood sugar levels can go up as high as 180 during or right after a meal. Within two hours after eating, blood sugar levels should drop to under 140. After several hours without eating, blood sugar can drop as low as 70.
Using glucose for energy and keeping it balanced with just the right amount of insulin — not too much and not too little — is the way our bodies maintain the energy needed to stay alive, work, play, and function even as we sleep.
The calorie generating nutrients in the food are carbohydrates, proteins, and fats. The vitamins, minerals, water, and fiber form the non-calorie generating nutrients of the food. The simpler units of the carbohydrates, proteins, and fats once absorbed into the bloodstream after the process of digestion are taken to the various cells of the body. In the cells, these nutrients are then subjected to certain biochemical reactions and in the end; calories are generated in the form of ATPs.
Cells require a constant supply of energy to generate and maintain the biological order that keeps them alive. The proteins, lipids, and polysaccharides that make up most of the food we eat must be broken down into smaller molecules before our cells can use them—either as a source of energy or as building blocks for other molecules. The breakdown processes must act on food taken in from outside. One gram of carbohydrates, when digested completely in the above-said process, provides 4 calories. Similarly, when one gram of protein provides 4 calories. But when one gram of fat is broken down it releases 9 calories.