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  3. Nutrients are the parts of food that are important for life and health. Nutrients are important for three reasons. First, some n

Nutrients are the parts of food that are important for life and health. Nutrients are important for three reasons. First, some n

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问题     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 of blood. Scientists think there are 40 to 50 nutrients. These nutrients are divided into five general groups: carbohydrates, fats, proteins, minerals, and vitamins.
    The first group of nutrients is carbohydrates. There are two kinds of carbohydrates: starches 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.
    There are two types of fats: animal and vegetable. Butter, cream, and the fat in bacon 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 carbohydrate 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.
    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 tissue. 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, 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.
    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.
One important source of fat is______.
选项 A、oil
B、grains
C、sugar
D、olive
答案A
解析 从第三段“Olive oil…are fats”可知，oil是fat的来源之一，故选A项。
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