How Your Body Uses Nutrients

Have you ever thought about how your digestive system—a complex network of hollow organs and long, twisting tubes—actually converts your meals into fuel to keep your body functioning well? Here's a quick survey of how your organs break down nutrients, including carbohydrates, protein, fats, and vitamins, to keep your body in motion.

How Your Digestive Tract Breaks Down Food

The mouth, esophagus, stomach, small intestine, large intestine (also called the colon), rectum, and anus make up the digestive tract. Because foods are not formed in a way that the body can use them as nourishment, your digestive tract will break down food into smaller molecules to separate its nutrients and convert it to beneficial energy for the body.

Your mouth, teeth, and saliva work together to soften and grind food so that it can pass through your throat, esophagus, and stomach. In the stomach, the food is mixed with digestive acids that are then pushed forward into the small intestine where the absorption of nutrients, including carbohydrates, protein, vitamins, and fats, take place. These nutrients are then added into the bloodstream and transported to the liver. The waste products of this process, which include undigested parts of the food, is then pushed into the colon (or large intestine) and expelled through a bowel movement.

How Your Body Uses These Nutrients

Carbohydrates: Your digestive system will break down carbohydrates into simple sugars that absorb glucose into the bloodstream. Any excess glucose is converted into glycogen, which is then stored around the body. The 2005 Dietary Guidelines for Americans recommends that 45 percent to 65 percent of your total daily calories should be comprised of carbohydrates, the primary source needed to fuel your brain and muscles. Foods that are rich in carbohydrates include:

  • Bread
  • Potatoes
  • Dried peas
  • Beans
  • Rice
  • Pasta
  • Fruits
  • Vegetables

Protein: Protein is broken down into amino acids, which is then absorbed in the villi—tiny, hair-like projections in the small intestine that absorb nutrients from foods. Your body uses protein to build and repair body tissue. These foods are the best sources of protein:

  • Lean meat
  • Fish
  • Poultry
  • Eggs
  • Beans
  • Tofu
  • Lentils and other legumes;
  • Grains, such as bread and pasta
  • Nuts and seeds

Fats: Fat molecules provide a rich source of energy for the body. Bile acids dissolve fat into tiny droplets and allow pancreatic and intestinal enzymes to break the large fat molecules into smaller ones. The bile acids combine with fatty acids and cholesterol to help these molecules move into the cells of the mucosa. In these cells, the small molecules are formed back into large ones, most of which pass into vessels called lymphatics near the intestine. These small vessels carry the reformed fat to the veins of the chest and the blood carries the fat to storage depots in different parts of the body.

Saturated fats, found in animal sources such as meat, cheese, and butter, and trans fats, found in prepared foods and baked goods, have been linked to health problems such as high cholesterol and heart disease, and should be limited to no more than 10 percent of your total calorie intake.

Unsaturated fats, such as monounsaturated and polyunsaturated fats, are typically found in plant food sources such as olive oil, avocados, fish, almonds, soybeans, and flaxseed. They all have health benefits including lowering cholesterol and reducing the risk of heart disease.

Vitamins: Vitamins are absorbed through the small intestine. There are two types of vitamins: water-soluble and fat-soluble.

Vitamins B and C are water-soluble vitamins that are absorbed through the intestinal wall. They help your digestive system process and absorb carbohydrates, proteins, and fats; they also help your body make red blood cells. Water-soluble vitamins promote healthy skin and support your immune and nervous systems. Excess amounts are flushed out in the urine.

Vitamins A, D, E, and K are fat-soluble and they get stored in the liver and fatty tissue of the body. These vitamins promote bone health, cell reproduction, and vision health.




National Digestive Diseases Information Clearinghouse (NDDIC)
Your Digestive System and How It Works

Water-Soluble vs. Fat-Soluble Vitamins

List of Water-Soluble Vitamins

Fat-Soluble Vitamins