The time it takes for food to digest varies per person and is affected by how much food was eaten, the combination of foods, physical activity and metabolism. A typical, healthy adult digests food within a 24-to-72-hour time frame, beginning with a six-to-eight-hour window during which the food enters the stomach and passes through the small intestine, which is where most of the digestion and absorption of nutrients takes place to then turn food into fuel. It is impossible to state an exact amount of time for digesting nutrients, though it is known that some take longer than others.
The main source of fuel for your body is carbohydrates. Your body digests carbs and transforms them into glucose, or blood sugar, which it uses for energy. Carbs spend less time digesting in the stomach than proteins and fats. The body digests simple carbs more quickly than complex carbs. Examples of simple carbs include fructose from fruit, sucrose, or table sugar, and maltose, which is in beer and some vegetables. Simple carbs, or sugars, only require one step for digestion, which is why they digest faster. Fructose, glucose and maltose can be absorbed directly into the bloodstream through the small intestine. An enzyme in the lining of the small intestine transforms sucrose into glucose and fructose to then be absorbed in that one step.
Complex carbs, or starches, require more steps to digest, and therefore digest more slowly than simple carbs. Complex carbs include starchy vegetables, legumes, whole-grain breads and cereals. The enzymes in saliva break complex carb molecules into maltose, which is a smaller and simpler molecule. Next, an enzyme in the small intestine's lining splits maltose molecules into glucose molecules, which is then absorbed into the bloodstream. The blood transports glucose to the liver, where is is either used for energy or stored for later use.
Protein requires more digestion time in the stomach than carbs. Giant protein molecules are in foods like beans, eggs and meat. Because the molecules are so large, it takes a longer process to break them down before they can be used as fuel. An enzyme in the stomach begins to digest protein. Protein molecules then move into the small intestine, where several more enzymes break down the molecules into amino acids. The smaller amino acid molecules pass through the walls of the small intestine to get into the bloodstream. When you run out of the energy you got from glucose, which started as carbs, your body turns to protein or fats for energy. This process is known as gluconeogenesis. To make more glucose from protein, the body converts amino acids into glucose to use for fuel.
Your body can also use fat for fuel. Fats take more time to digest than carbs or proteins. Fats pass through the stomach and into the small intestine as other nutrients do. The body breaks fat molecules into fatty acids and glycerol, which the villi in the small intestine can absorb. The fatty acids and glycerol travel to areas of the body for storage in cells or for use as energy. Your body can only use about five percent of absorbed fat for fuel by converting it to glucose. Your liver absorbs the rest of the glycerol and uses it to assist in breaking down glucose for energy.