The Nutrients in FoodA)Nutrients are the parts of food that are important for life and health. Nutrients are important for three reasons. First, some nutrients provide fuel for energy. Second, some nutrients build and repair body tissues. Third, some nutrients help control different processes of the body like the absorption of minerals and the clotting(凝结)of blood. Scientists think there are 40 to 50 nutrients. These nutrients are divided into five general groups: carbohydrates(碳水化合物), fats, proteins, minerals and vitamins.Carbohydrates and FatsB)The first group of nutrients is carbohydrates. There are two kinds of carbohydrates: starches(淀粉)and sugars. Bread, potatoes, and rice are starches. They have many carbohydrates. Candy, soft drinks, jelly, and other foods with sugar also have carbohydrates. Carbohydrates are important because they provide the body with heat and energy. Sugar, for instance, is 100 percent energy. It has no other food value. Sugar does not build body tissues or control body processes. If there are too many carbohydrates in the body, they are stored as body fat. The body stores fuel as fat.C)There are two types of fats: animal and vegetable. Butter, cream, and the fat in bacon(熏肉)are animal fats. Olive oil, corn oil, and peanut oil are vegetable fats. The body has fat under the skin and around some of the organs inside. The average adult has 10 to 11 kilograms(20 to 25 pounds)of body fat. If adults eat too many carbohydrates and fats, they can add another 45 kilograms(100 pounds)to their bodies. Fat is extra fuel. When the body needs energy, it changes the fat into carbohydrates. The carbohydrates are used for energy. Fat also keeps the body warm.ProteinsD)The third group of nutrients is proteins. The word "protein" comes from a Greek word that means " of first importance". Proteins are " of first importance" because they are necessary for life. Proteins are made of amino acids which build and repair body tissues. They are an important part of all the muscles, organs, skin, and hair. The body has 22 different amino acids. Nutritionists call eight of these amino acids essential because the body does not manufacture them. There are two kinds of proteins: complete proteins and incomplete proteins. Complete proteins, which the body needs for growth, have all the essential amino acids. Meat, fish, poultry(家禽), eggs, milk, and cheese have complete proteins. The body needs complete proteins every day. Incomplete proteins do not have all the essential amino acids. The proteins in vegetables and grains, for instance, are incomplete proteins. Two ways to form complete proteins from incomplete proteins are:(1)to mix vegetables and grains correctly, or(2)to add a small amount of meat or milk to a large amount of grains. The body can then use the complete proteins which result from the mixtures.E)Extra protein in the body can be changed to fat and stored as body fat. It can also be changed to carbohydrates and used for energy. If people do not eat enough carbohydrates and fats for the energy that they need, their body uses proteins for energy. Then the body does not have the proteins that it needs to build and repair tissues. A nutritious diet includes carbohydrates and fats for energy, and proteins for growth.MineralsF)The fourth group of nutrients is minerals. More than twenty different minerals are in the body. Three of the most important minerals are calcium(钙), phosphorus(磷), and iron. Calcium and phosphorus work together. The bones and the teeth have 99 percent of the calcium in the body. If people have enough calcium and phosphorus, their bones and teeth will be strong and hard. In addition, their muscles, nerves, and heart will work correctly. Milk and hard cheeses are the best sources of calcium. After the age of 19, people need 400 to 500 milligrams of calcium a day. People who do not drink three glasses of milk daily can eat 50 hamburgers or 56 apples to get the calcium they need.G)Iron is the mineral that makes blood look red. All lean meats have iron: liver is an especially good source of iron. Whole grains, nuts, some vegetables, and dried fruits also have iron. If there is not enough iron in their diets, people will get a disease that is commonly called anemia(贫血症). Anemia is found all over the world. People with anemia do not have enough iron in their blood. Because iron carries oxygen, people who do not have enough iron do not get enough oxygen for their normal activities. Their hearts beat faster so their bodies can get more oxygen. People who have anemia often get tired easily. Sometimes their skin looks white: it does not look pink and healthy.VitaminsH)Nutritionists think there are thirteen vitamins that humans need. Vitamins are important because they prevent diseases and help control body processes. Vitamin A is important for healthy skin and eyes. People who do not have enough vitamin A may have night blindness. Some automobile accidents happen in the evening because people who lack vitamin A do not see the road well after they look at the bright headlights of a car. Vitamin A in the diet comes from deep yellow fruits and vegetables, dark green leafy vegetables, and whole milk.I)When people have enough B vitamins, their appetite is good and their nerves are calm. B vitamins in the diet come from some meats and vegetables, milk, cottage cheese, and whole grains. When a grain is processed, it loses vitamins. For example, there is a big difference between brown and white rice. When rice is processed, the brown outside is lost. The brown outside of rice has an important B vitamin which white rice lacks. In short, brown rice has more B vitamins than processed rice.J)Vitamin C keeps the cells of the body together. It helps skin tissue recover from cuts and burns. Vitamin C in the diet comes from tomatoes, citrus(柑橘类植物)fruits like lemons and oranges, and some vegetables such as cabbage and green peppers. Vitamin D is called the " sunshine" vitamin. When people sit outside, ultraviolet(紫外线的)rays from the sun change a fat in their skin to vitamin D. Vitamin D is also in cod(鳕鱼)liver oil and the yellow of eggs. It is sometimes added to milk. Vitamin D helps the body absorb calcium. It helps build strong bones, and it prevents a disease in children that is called rickets(佝偻病). When children have this disease, their bones bend because they do not become hard. Rickets is seldom found in sunny, tropical countries. Rickets is more common in countries that have long winters with little sunshine, in cities that have pollution that keeps the sun out, and in towns surrounded by mountains that keep the sun out. There is no one food that is essential, but there are nutrients that are necessary for good health. .If people want to be healthy and active, they need to get all the essential nutrients. A healthy body needs carbohydrates, fats, proteins, minerals and vitamins.Vegetable protein sources are often lacking in essential amino acids, as opposed to animal proteins which are generally considered complete proteins.