Chapter 3.5

Food and Energy

- The food we eat provides energy to do work in the body, which includes the growth and repair of cells -Carbohydrates are the primary fuel for the body - But if carbohydrate reserves are exhausted, fats and then proteins are used for energy.

What does a calorimeter do?

1) In the nutrition laboratory, foods are burned in a calorimeter to determine their energy value (kcal/g or kJ/g) - A sample of food is placed in a steel container called a calorimeter filled with oxygen. - A measured amount of water is added to fill the area surrounding the combustion chamber. - The food sample is burned, releasing heat that increases the temperature of the water. - From the known mass of the food and water as well as the measured temperature increase, the energy value for the food is calculated. - We assume that the energy absorbed by the calorimeter is negligible.

Composition and Energy Content for Some Foods

1) On packaged food, the energy content is listed on the Nutrition Facts label, usually in terms of the number of Calories or kilojoules for one serving. The composition and energy content for some foods are given in the table above. - The total energy in kilocalories for each food type was calculated using energy values in kilocalories. Total energy in kilojoules was calculated using energy values in kilojoules. The energy for each food type was rounded off to the tens place.

Energy Values

1) The energy values for food are the kilocalories or kilojoules obtained from burning 1 g of carbohydrate, fat, or protein, which are listed in the above table - Using these energy values, we can calculate the total energy for a food if the mass of each food type is known.

calorie (lower case "c") vs. Calorie (upper case "C")

1) The physical unit calorie (lower case "c) is the energy required to increase the temperature of one gram of water by 1 degree Celsius, and the dietary Calorie (upper case "C") is a kilocalorie = 1000 calories. - For many years in the field of nutrition, the energy in food was measured as Calories or kilocalories. The nutritional unit Calorie (Cal) is the same as 1000 cal, or 1 kcal. 2) The international unit, kilojoule (kJ), is becoming more prevalent. - i.e., a baked potato has an energy content of 100 Calories, which is 100 kcal or 440 kJ. A typical diet that provides 2100 Cal (kcal) is the same as an 8800 kJ diet. 1 Cal = 1kcal = 1000 cal 1 Cal = 4.184 kJ = 4184 J