Digestion entails the breaking down and absorption of food, although it’s a little more complicated than it sounds. Many foods are combinations of carbohydrates, fat and protein, which all digest at different rates in your body. Most carbohydrates are quick to digest, while fatty foods take the longest amount of time. The actual amount of time a food takes to digest depends on many factors, especially your level of hydration, intestinal motility and stomach acidity.
Digestion in the Mouth
Chemical digestion of food begins in your mouth with the secretion of saliva. The first macronutrients to undergo chemical digestion are starch, which is a type of complex carbohydrate found in potatoes, corn and root veggies, and fats from animals and plants. The enzyme alpha-amylase reduces starch to a less complex sugar called maltose, and lingual lipase starts to prepare saturated and unsaturated fats for further digestion in the small intestine. In contrast, protein is not chemically changed in your mouth.
Digestion in the Stomach
Digestion continues in your stomach with the help of gastric juice, which contains hydrochloric acid and enzymes. Some gastric amylase in your stomach continues to break down carbohydrates into simpler sugars, whereas protein is reduced by pepsin into smaller units called peptide chains. In contrast, fats are not chemically changed very much while in the stomach. A lack of acidity in your stomach significantly reduces digestion, especially protein. Drinking lots of water with meals not only reduces stomach acidity, but it also dilutes the enzymes in your mouth and stomach and inhibits the digestive process.
Digestion in the Small Intestine
The majority of digestion and virtually all absorption occur in your small intestine. The intestinal wall and pancreas gland release enzymes that reduce carbs, protein and fats into their smallest constituents. Amylase, maltase, sucrase and lactase reduce larger sugars into glucose, which is quickly absorbed into your bloodstream and delivered to all cells with the help of the hormone insulin. Glucose is the main fuel source for your body. The partially digested protein is further reduced by pancreatic proteases into single amino acids, which are used to build and repair your muscles, skin and hair. Bile secreted by your gallbladder and pancreatic lipase break down fat into fatty acids and glycerol, which are absorbed through the small intestine and typically stored as chylomicrons.
Rate of Digestion
Almost all carbohydrates, with the exception of dietary fiber, are the quickest to digest and absorb. For example, carbohydrate-rich food such as fruits, veggies, baked goods, rice and pasta can spend as little as 30 minutes in your stomach and can take only two hours to fully digest and absorb, but it varies a great deal from person to person. On the other hand, dietary fiber is very poorly digested and most of it passes through your intestines intact.
Protein usually takes longer to digest than carbohydrates, although eggs are quicker to be broken down than complex carbs such as beans and whole grains. On average, lean meat spends about three hours in your stomach and at least 12 hours in your intestines, but these rates may be much slower for people who are aged, dehydrated, unhealthy or inactive. Fats take the most time to fully digest and absorb because they are the largest molecules and the least water “friendly.” Remember that almost no food is made of only one compound. Foods are typically combinations of nutrients, which makes it difficult to estimate digestion rates and times.
- Encyclopedia of Human Nutrition; Benjamin Caballero et al.
- Contemporary Nutrition: Functional Approach; Gordon M. Wardlaw et al.
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.