The energy content of food is calculated from the heat released by the total combustion of food in a calorimeter. This energy is expressed in kilocalories, or calories for short. The three major types of calories found in food are fat, protein and carbohydrate. The energy content of fat is more than twice that of carbohydrate or protein, equivalent to nine calories per gram, as compared to four calories per gram for either protein or carbohydrate. With few exceptions, fruits and vegetables derive the majority of their caloric energy from carbohydrates, while some contain greater concentrations of protein or fat.
Carbohydrates
The primary role of dietary carbohydrate is to provide energy. Carbohydrates are made of simple sugars. Glucose and fructose are the principal simple sugars. Both are found in most sweet fruit. Long chains of simple sugars are called starch. Starch is a type of complex carbohydrate that is found in abundance in vegetables, including carrots, potatoes, peas and squash. Many fruits and vegetables are predominately carbohydrate. Watermelon, for example, derives nearly 90 percent of its calories from carbohydrate, as do carrots.
Protein
Protein provides essential amino acids that function as the building blocks for important cellular messengers including enzymes and neurotransmitters, as well as cell membrane components. Protein can also serve as an energy source, as when carbohydrates are restricted. Fruits and vegetables are not commonly noted for their protein content, and indeed, most fruits are only 5 to 10 percent protein. Green vegetables, however, derive larger portions of their calories from protein. Spinach, for example, is 30 percent protein. Broccoli is 20 percent.
Fat
In addition to serving as structural components and precursors to cellular messengers such as prostaglandins, dietary fat is a concentrated form of energy, providing over twice the calories of protein or carbohydrates. Most fruits and vegetables are as low in fat as they are in protein. Exceptions exist, however. For instance, avocado and olives are much higher in fat than other fruits, deriving roughly 80 percent of their calories from fat.
Fiber
Chemically defined as a carbohydrate, fiber is the nondigestible portion of plants. Because your body is unable to digest fiber, it passes through the digestive tract essentially unchanged. Because it never makes its way into your blood, fiber is calorie-free. Its health benefits include stabilizing blood sugar levels and lowering cholesterol. Fiber-rich diets also decrease the risk for constipation, hemorrhoids and colon cancer. Fruits and vegetables are excellent sources of fiber. Topping the list are berries, artichokes and peas, according to MayoClinic.com.
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