The United States government publishes guidelines for appropriate nutrient intakes. These are known as the Recommended Dietary A

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问题     The United States government publishes guidelines for appropriate nutrient intakes. These are known as the Recommended Dietary Allowances (RDAs) and are updated regularly based on new research in nutrition. RDAs are suggested amounts of calories, protein, and some minerals and vitamins for an adequate diet. For other dietary substances, specific goals must await further research. However, for the U.S. population as a whole, increasing starch and fiber in one’s diet and reducing calories (primarily from fats, sugar, and alcohol) is sensible. These suggestions are especially appropriate for people who have other factors-for chronic diseases due to family history of obesity, premature heart disease, diabetes, high blood pressure, and high blood cholesterol, or for those who use tobacco.
    Snacks can furnish about one-fourth of the calorie requirements among teenagers. Those snacks should also provide much of the day’s allowances for protein, minerals, and vitamins. Sandwiches, fruit, and milk make good snacks for active teenagers. Food from the food pyramid may be part of any meal. A grilled cheese sandwich or a bowl of whole-grain cereal is just as nutritious in the morning as it is at noon. In addition, a good breakfast consists of any foods that supply about one-fourth of the necessary nutrients for the day.
Why are RDAs regularly updated?

选项 A、New discoveries in the science of nutrition are constantly being made.
B、Americans’ diets are constantly changing.
C、As people age, their nutritional needs change.
D、Very little is currently known about nutrition.

答案A

解析 从文章第二句(These are known as the Recommended Dietary Allowances(RDAs) and are updated regularly based on new research in nutrition.)得知RDAs会根据营养学上的新发现经常更新推荐的日摄食量。所以“推荐的日摄食量经常更新的原因是营养学上经常有新的发现”,选项A正确。
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