Nutrition begins with food. Nutrition is the process by which the body nourishes itself by transforming food into energy and body tissues. The science of nutrition concerns everything the body does with food to carry on its functions. Food provides essential substances called nutrients. The body needs these nutrients to help it make energy; to grow, repair, and maintain its tissues; and to keep its different systems working smoothly. Nutrition is important for all organisms. However, this article will focus on nutrition as it applies to the human body.

The term nutrition can also refer to the quality of someone’s food choices, or diet. A balanced diet is one in which foods eaten on a regular basis provide all the nutrients needed in the right amounts. A balanced diet has many benefits. It can help people feel and look their best. It can also help them stay energetic and healthy, both in the short term and later in life.


The body’s most basic need is for energy. The energy in food is measured in units called kilocalories (commonly shortened to “calories”). One kilocalorie is the amount of heat energy needed to raise the temperature of 1 kilogram (2.2 pounds) of water 1° C (1.8° F). (More precisely, it is the energy required to raise that water from 14.5° C to 15.5° C at one atmosphere of pressure.)

Three major types of nutrients supply the body with energy, or calories: carbohydrates, proteins, and fats. One gram (0.035 ounce) of either carbohydrate or protein provides four calories. Fat is a more-concentrated source of energy, with each gram providing nine calories. Water, vitamins, and minerals supply no energy in and of themselves, though the body uses many of them in energy-releasing processes.

The body needs the energy in food to do everything from blinking an eye to running a race. It also needs energy to perform such essential functions as breathing, maintaining body temperature, growing new cells, and even digesting food. The total number of calories needed each day depends on many factors, including a person’s age, sex, weight, and especially level of activity. For example, a woman who weighs about 120 pounds (55 kilograms) might expend 1,850 calories on a day when she is fairly sedentary but may use more than 3,000 calories on a very active day.

If a person takes in more food than is required to meet the body’s needs, the excess calories are eventually converted to fat—a form of stored energy found mostly within adipose tissue. That causes weight gain. Eating too little causes weight loss over time, because the body must use stored fat for energy. One pound (0.5 kilogram) of adipose tissue is equal to about 3,500 calories.

Maintaining a healthy weight is a balancing act. Food provides energy, and physical activity uses up energy. If weight loss is advisable, experts recommend both reducing one’s daily calorie intake and getting more exercise.


The nutrients are divided into six major types: carbohydrates, proteins, fats, vitamins, minerals, and water. Each nutrient performs specific functions to keep the body healthy. All the nutrients work together to contribute to good health.


The body’s main source of energy is carbohydrates, which include starches, sugars, and dietary fiber. The body breaks down starches and sugars into the simple sugar glucose, the fuel used by red blood cells. Glucose is also the main energy source for the brain and nervous system and can be used by muscles and other body cells. Fiber does not provide energy.

Starches are complex carbohydrates. They are found in dry beans and peas, such as kidney beans, pinto beans, soybeans, chickpeas, lentils, and split peas; grains and grain products, such as breads and cereals; potatoes; and other vegetables. These foods can also be good sources of proteins, vitamins, minerals, and dietary fiber.

Sugars, or simple carbohydrates, also are a natural part of many foods. There are several types. Fructose, for example, is found in fruits, maltose in grain products, and lactose in milk. These sugars are part of foods that also provide other nutrients.

Refined sugars are sugars that are removed from plants and used as sweeteners, or added sugars. Sucrose, or table sugar, is produced commercially from sugarcane and sugar beets and is used to sweeten many foods, such as candy and desserts. Other sweeteners include high-fructose corn syrup, a refined sugar that is commonly added to soft drinks and packaged baked goods; honey; maple syrup; and molasses. All these sugars provide calories but little or no additional nutrients. In addition, eating large amounts of sweetened foods can lead to weight gain and tooth decay (see dentistry).

Dietary fiber likewise provides no essential nutrients. Fiber is the structural part of plants, and the human body cannot digest it. However, eating dietary fiber is beneficial to the body in many ways. Fiber aids digestive health and may protect against certain disorders and diseases.There are two types of fiber: insoluble (which does not dissolve in water) and soluble (which dissolves or swells in water). Insoluble fiber, or roughage, speeds the passage of food—and potentially harmful substances in food—through the intestines. This type of fiber is thought to provide protection against some gastrointestinal diseases. Good food sources include whole-grain breads and cereals, wheat bran, and vegetables. Soluble fiber can help lower the level of harmful cholesterol in the blood and limit the rise in blood sugar levels after eating. Soluble fiber is found in apples and other fruits, dry beans and peas, oats, and barley.


Proteins are made of amino acids, small units necessary for growth and tissue repair. About one-fifth of the body’s total weight is protein. Hair, skin, muscles, internal organs, and bones are made primarily of protein. Foods from animal sources—such as meat, poultry, fish, eggs, and dairy products—supply all the essential amino acids. These are complete proteins.

Foods from plant sources are incomplete proteins, because they are low in or lack one or more of the essential amino acids. However, one can obtain all the essential amino acids by eating a variety of different protein-containing plant foods. Good plant sources of protein are legumes (including soybeans, tofu, and other soy products), nuts, and seeds. Plant sources supply all or much of the protein in the diets of vegetarians, who eat no meat, poultry, or fish. In addition, plant foods, which are often less expensive and lower in fat than meat, are an important supplementary source of protein for many nonvegetarians around the world.


Fats are a concentrated source of energy (having more than twice as many calories per gram as carbohydrates and proteins do). Fats in the diet are needed for healthy skin and normal growth. Fats also carry certain vitamins to wherever they are needed in the body and provide a reserve supply of energy. Because fats move through the digestive system slowly, they also delay hunger pangs.

The different fats found in food are made up of fatty acids. There are four basic kinds of fatty acids: saturated, polyunsaturated, monounsaturated, and trans. Each has a different effect on blood cholesterol levels. In general, saturated fatty acids and trans-fatty acids tend to increase one’s risk of heart attack and other cardiovascular disease. Polyunsaturated and monounsaturated fatty acids are thought to lower those risks. Studies have shown that a buildup of fatty deposits in one’s arteries, a common factor in heart disease and stroke, can begin in adolescence or earlier.

Saturated fats (fats with a high percentage of saturated fatty acids) are usually solid at room temperature and come primarily from animals. For example, saturated fats are found in meat, poultry skin, lard, and non-skim dairy foods such as butter, cheese, and milk. Such plant fats as coconut and palm oils and cocoa butter (in chocolate) are also high in these fats. Saturated fats raise the level of a substance called low-density lipoprotein (LDL) cholesterol in the blood. Higher levels of this type of cholesterol are associated with a higher risk of cardiovascular disease.

Unsaturated fats are typically liquid at room temperature and come mainly from plants. Both polyunsaturated and monounsaturated fats tend to lower one’s levels of LDL cholesterol. Polyunsaturated fats may also lower levels of high-density lipoprotein (HDL) cholesterol. High levels of HDL cholesterol can help protect against heart disease. Monounsaturated fats tend not to lower HDL levels, and they might raise them. Many vegetable oils, including corn, sunflower, and safflower oils, are rich in polyunsaturated fats, as are fatty fish such as salmon. Monounsaturated fats are found in canola oil, avocados, olives and olive oil, and most nuts and nut oils. Both types of unsaturated fats are considered healthier choices than saturated fats or trans fats (fats with a high percentage of trans-fatty acids).


The discovery of vitamins began early in the 20th century. It is likely that some still are undiscovered. Although vitamins are needed in only small amounts, they are essential for good health. They help keep the body’s tissues healthy and its many systems working properly. Each vitamin has specific roles to play. Many reactions in the body require several vitamins, and the lack or excess of any one can interfere with the function of another.


Four vitamins—A, D, E, and K—are known as fat-soluble vitamins. They are digested and absorbed with the help of fats in the diet. Fat-soluble vitamins can be stored in the body, mostly in fatty tissue and the liver, for long periods. If one takes very large amounts of these vitamins in supplements, they can build up to toxic levels.

Vitamin A is needed for good vision, healthy skin, and proper functioning of the immune system. Dark-green leafy vegetables and many orange fruits and vegetables are excellent sources of beta-carotene, a substance that the body converts into vitamin A.

Vitamin D helps the body use calcium and phosphorus to build strong bones and teeth. With direct sunlight on the skin, the body can manufacture its own vitamin D. In the United States and Canada vitamin D is routinely added to milk during processing, and it is often added to cereals, margarine, and soy milk.

Vitamin E helps protect the body’s cells from damage by oxygen. It is found in vegetable oils, nuts, seeds, wheat germ, and whole grains.

Vitamin K is necessary for normal blood clotting. Food sources include green leafy vegetables, cabbage, cauliflower, eggs, and liver.


Many people rightly think of rocks when they hear the term minerals. Minerals are also found in soil, metals, and water. To one’s body, minerals are another group of essential nutrients, needed to regulate body processes and fluid balance. Minerals also give structure to bones and teeth.Minerals can be divided into two categories—major and trace—depending on how much the body needs. Major minerals, which are needed in larger amounts, include calcium, phosphorus, magnesium, sulfur, sodium chloride, and potassium. Trace minerals, or trace elements, include chromium, copper, fluoride, iodine, iron, manganese, molybdenum, selenium, zinc, and cobalt. Almost all foods contribute to a varied intake of essential minerals.


Water takes an active part in many chemical reactions in the body. It is also needed to carry other nutrients, to regulate body temperature, and to help eliminate wastes. About 50 to 60 percent of the body is water. Requirements for water intake can be met in many ways, such as drinking plain water, fruit juices, milk, and soups. Many fruits are about 90 percent water.


Government agencies and scientific bodies around the world publish a variety of nutritional recommendations to promote good public health. These guidelines have changed over time to incorporate new scientific findings. They also vary by culture. Diets in different countries have developed differently, on the basis of varying cultural traditions and preferences as well as access to different kinds of foods. For instance, on average, people in Pakistan get nearly 10 percent of their calories from dairy products, while people in China get less than 1 percent of their calories from dairy. Diets vary within countries too. The wealthy, for example, can afford foods that the poor cannot. A country’s guidelines are often tailored to combat specific types of nutritional problems commonly found there.

Some guidelines provide the amounts of specific nutrients that people need each day. For example, the Dietary Reference Intakes of the United States and Canada detail the daily amounts of vitamins, minerals, and other nutrients that men and women at various stages of life should consume.

Other guides, such as the United States government’s Dietary Guidelines for Americans, provide general advice for healthy eating. Some dietary plans recommend eating a certain number of servings from different food groups each day. Foods that provide similar nutrients are grouped together—for instance, into a bread-and-cereals group, a milk group, or a vegetable group.

To make the plans easier to understand and remember, they often feature a visual aid. The United States Department of Agriculture (USDA) published its first food pyramid in 1992. It arranged six food groups into a pyramid according to how many daily servings were recommended. Since then many countries have developed such aids, including food pyramids in Mexico and the Philippines, plates in Australia and the United Kingdom, a rainbow in Canada, a square in Zimbabwe, a pagoda in China, and a bean pot in Guatemala.

USDA's MyPyramid

Encyclopædia Britannica, Inc.

In 2005 the USDA introduced a revised food pyramid that included guidelines for healthy eating as well as for physical activity. Called MyPyramid, it provided a visual reminder to eat a variety of foods in moderate amounts and to be active every day. The pyramid featured five major food groups plus liquid oils. The amount one needed from each group depended on one’s age, sex, and activity level. The plan actually included 12 different pyramids, tailored to the needs of different groups. One could visit the guide’s Web site, enter information about the above factors, and receive the appropriate pyramid. The pyramids showed people how many calories they needed daily to maintain their weight and the number of servings they should get from each food group each day.

Although the specific guidelines of the plans differ, there are some common basic themes. Most advise eating a wide variety of nutrient-rich foods, including generous amounts of fresh fruits and vegetables. Another common recommendation is to eat whole grains (such as brown rice and whole-wheat bread), which provide more nutrients than refined grains (such as white rice and white bread). Most plans recommend limiting the amount of saturated and trans fats, sodium (salt), and added sugars, especially high-fructose corn syrup, in one’s diet. Exercising regularly and maintaining a healthy weight are also emphasized.

While government-based dietary guidelines are based on sound principles and can be followed safely by relatively healthy people, special diets for people with health problems should be prescribed by a physician. Individuals with food allergies, heart disease, hypertension (high blood pressure), or diabetes require careful meal planning. Many doctors refer such patients to a registered dietitian, who develops a customized diet plan.

In 2011 the USDA replaced MyPyramid with a new plan called MyPlate. The new plan uses the simple guideline of a place setting to illustrate the five food groups and how much of each should be served at mealtime. The visual guide has a plate divided into four sections—one each for fruits, vegetables, grains, and protein. The size of each section represents the relative amount of each food group that should be consumed. In addition, a circle at the edge of the plate shows the proportion of dairy products to include with the meal. Unlike MyPyramid and earlier plans, MyPlate does not include a section for fats and oils. The plan’s Web site includes interactive features that help visitors plan and track their food.

The MyPlate plan encourages people to be physically active, but it does not include specific guidelines on exercise. Instead, it is partnered with another U.S. government initiative called Let’s Move. Established by American first lady Michelle Obama in 2010, Let’s Move encourages individuals to engage in physical activity as part of an active and healthy lifestyle. The program encourages adults to be physically active for at least 30 minutes most days of the week. Children and teenagers should be physically active for 60 minutes every day or most days.


The grain group includes all foods made from wheat, rice, oats, cornmeal, barley, rye, and other grains. These include bread, pasta, oatmeal, breakfast cereals, tortillas, and pitas. Grains should make up a little more than one-quarter of the food on one’s plate, and at least half of all grains eaten should be whole grains, such as whole-wheat bread, brown rice, and whole-grain cereal. Whole grains provide B vitamins, iron and other minerals, carbohydrates, fiber, and some protein.