Let's start with what food does for the body. Food gives your body energy. Sugars and most starches are broken down into glucose, which circulates in the blood to be used as energy, while fibre is the part of food that is not broken down. Thiamin (vitamin B1) helps the body's cells change carbohydrates into energy. Fats are nutrients the body uses to build cell membranes, nerve tissue and hormones, and fats not used as fuel or building blocks are stored in fat cells and act as a backup energy source.

Let's follow a bite of food. Food is broken into tiny pieces so the body can absorb the nutrients it needs. Each digestive organ moves food along, breaks it into smaller parts, or both: the mouth chews and adds saliva, the stomach adds acid and enzymes, and the pancreas, liver and small intestine add juices. Saliva starts to break down starches in food while the food is still in your mouth. After you swallow, muscles push food down the esophagus into the stomach, and the squeezing movement of muscles that pushes food through the digestive tract is called peristalsis.

Stomach muscles mix food with digestive juices. The stomach turns food into a thick liquid called chyme, and a muscular valve called the pylorus keeps chyme in the stomach until it is the right consistency to pass into the small intestine. The liver makes bile, which helps digest fats; the gallbladder stores bile between meals and squeezes it into the small intestine when you eat. The pancreas makes a digestive juice with enzymes that break down carbohydrates, fats and proteins, and delivers it to the small intestine through ducts.

The small intestine takes digested nutrients into the blood. Your digestive system breaks proteins into amino acids, fats into fatty acids and glycerol, and carbohydrates into simple sugars. The large intestine absorbs water and turns waste into stool, and bacteria in the large intestine help break down remaining nutrients and make vitamin K. The adult digestive tract is about 30 feet (9 meters) long, from mouth to anus.

Fibre cannot be broken into sugar molecules; it passes through the body undigested. Soluble and insoluble are the two forms of fibre, and most high-fibre foods contain both. Soluble fibre attracts water and turns to gel during digestion, which slows digestion. Some fibre is fermentable, acting as food for gut bacteria. Fibre is a part of plant foods that helps other food and waste move through the gut.

Starches are also called complex carbohydrates, and starchy foods are the main source of carbohydrate in our diet. Whole grains such as brown rice, oats, barley and quinoa are starchy foods. Whole grain kernels have three parts, the bran, germ and endosperm, and each holds different nutrients. In a grain kernel, the bran is the fibre-rich outer layer, the germ is the core of the seed where growth occurs, and the endosperm holds carbohydrates and protein.

Your body needs protein to repair cells and make new ones, and protein is found throughout the body, in muscle, bone, skin, hair and almost every other tissue. Foods from animals such as milk, eggs and fish, and from plants such as soy and beans, both supply the amino acids in protein. Nine amino acids are called essential because they must come from food, so essential amino acids cannot be made by the body.

There are 13 vitamins that your body needs. Vitamins A, D, E and K are fat-soluble: they dissolve in fat and are stored in the body, and some fat in the diet is needed to absorb them. Vitamin C and the B vitamins dissolve in water and are not stored, so the body needs a fresh supply each day, and the body loses unused water-soluble vitamins mostly in urine. Vitamin B12 is not found naturally in fruit, vegetables or grains; it comes from animal foods or fortified foods.

In 1897 Christiaan Eijkman concluded that a substance in the husk of rice protected against the symptoms he had seen in hens fed milled rice; it was later named vitamin B1. Around 1910 Frederick Hopkins found that young rats fed only pure nutrients stopped growing, but a small amount of milk started their growth again. Hopkins's rat experiments led to the idea that unknown substances, later called vitamins, are needed in small amounts for life processes. Eijkman and Hopkins shared the 1929 Nobel Prize in Physiology or Medicine for discovering vitamins.

In 1912 biochemist Casimir Funk was the first to coin the term vitamin in a research publication accepted by the medical community. The word vitamin comes from vita, meaning life, and amine, a nitrogenous substance essential for life. Chemists worked to identify a vitamin's chemical structure so it could be replicated, and researchers then determined the amounts needed. The discovery of all the vitamins occurred by 1948.

Vitamin C helps the body heal wounds, and it is needed to make collagen, a fibrous protein in connective tissue. Carrots have substances that your body turns into vitamin A, which helps you see in dim light. Vitamin A in food comes in two forms: preformed retinol from animal foods, and carotenoids from plants that the body converts to vitamin A. Folate helps tissues grow and cells work, and helps produce DNA.

About 99% of the body's calcium is stored in bones, and the rest is found in blood, muscle and other tissues. Calcium helps build strong bones and teeth, and it also helps with blood clotting, muscle contraction, nerve signals and a steady heartbeat. Vitamin D helps the body absorb calcium, which is needed for normal bone formation. Very few foods naturally contain vitamin D, so many foods are fortified, which means vitamins have been added. As of November 2026, most milk in the United States is fortified with 400 IU of vitamin D per quart.

Minerals are elements from soil and water that plants absorb and animals eat. Iron helps make red blood cells, which carry oxygen around the body. Foods rich in vitamin C, such as citrus, strawberries, tomatoes and potatoes, increase iron absorption, and tea can bind iron so the body cannot use it. Eating some meat, fish or poultry with beans or dark leafy greens can improve absorption of plant iron up to three times.

Minerals are used for keeping bones, muscles, heart and brain working, and for making enzymes and hormones. Macrominerals, needed in larger amounts, include calcium, phosphorus, magnesium, sodium, potassium, chloride and sulfur; trace minerals include iron, zinc and iodine. Phosphorus is needed to form bones and teeth and helps the body make ATP, a molecule that stores energy. Magnesium is needed for more than 300 biochemical reactions in the body, including helping make energy and protein. Sodium is a mineral the body uses to control blood volume and for muscles and nerves to work; table salt is sodium chloride. Iodized salt is table salt with iodine added, and it is the main food source of iodine. Potassium helps the heart muscle work and balances fluids in the body.

Thirst is the signal that your body is low on water. The body needs water for saliva, and it uses sweat to regulate body temperature. Water also lubricates joints and tissues, maintains healthy skin, and is necessary for proper digestion.

Fruits and vegetables give the body fibre as well as vitamins and minerals. Different fruits and vegetables contain different mixes of fibre, vitamins and minerals, so a variety gives the body more. Eating a mix of different foods is the best way to get the vitamins and minerals you need.

Words to know

• chyme: thick stomach liquid
• bile: fat-digesting juice
• glucose: blood sugar
• carotenoids: plant pigments
• fortified: vitamins added
• collagen: tissue protein