A calorie is a unit of energy, not a measure of weight or nutrient density. The calories you see on nutrition labels,however,are actually kilocalories. The calories on food packaging always refer to kilocalories, even though it just says “calories”. One kilocalorie is the amount of energy required to heat 1 kilogram of water by 1 degree Celsius. Calories in our food all come from one of the three nutrients: fats, carbohydrates and proteins.
In 1990, the U. S. government passed the Nutrition Labeling and Education Act, which means that before any packaged food in the U. S. hits the shelves, food scientists have to measure its nutrients and calories. One way to do this is using a tool called bomb calorimeter(弹氧热卡计). This tool directly measures the amount of energy that a food contains. To use this tool, scientists place the food in question in a sealed(密封的)container surrounded by water and heat it until the food is completely burned off. Scientists then record the rise in water temperature to determine the number of calories in the product.
Food scientists also rely on a calculation developed by the 19th-century U. S. chemist Wilbur Olin Atwater to indirectly estimate the number of calories in food products. Calorimeters don’t take into account that humans lose some calories through heat and feces(类便). Atwater’s experiments found that proteins and carbohydrates each have about 4 calories per gram and fats have 9 calories per gram.
However, some experts say that the Atwater system is outdated and inaccurate. A 2012 study found that the energy content of certain foods, such as nuts, could not be accurately calculated by the Atwater system. Besides, it does not take into account the digestive process, but assumes the complete conversion of nutrients to energy.
【小题1】What can we learn about the unit of energy on nutrition labels?A.It actually means calories. |
B.It refers to kilocalories. |
C.It reads kilograms. |
D.It employs degrees. |
A.Heat the food in an open container until the food is completely burned off. |
B.Take the calories through heat and feces into consideration. |
C.Learn the calculation developed by Wilbur Olin Atwater. |
D.Calculate the amount of energy in food products directly. |
A.About 40. | B.About 45. |
C.About 85. | D.About 130. |
A.How Are Calorie Counts Calculated? |
B.Where do Calories Come from? |
C.Can Nutrients Be Converted to Energy Totally? |
D.Nutrients and Calories Listed on a Food Label |