Food scientists measure food energy in kilocalories or simply calories. Kilocalories are units of heat -- specifically, the amount of heat needed to raise the temperature of 1 kilogram of water 1 degree Celsius. Carbohydrates, fat and protein are the energy-yielding nutrients, meaning that the human body can use the energy they contain. Nutrient quantities are measured in gram weight. Carbohydrates and protein each contain 4 kilocalories per gram, whereas fat contains 9 kilocalories per gram of energy.
Simple Sugars
Six simple sugar molecules exist in human nutrition. The three single sugar molecules are the basis for carbohydrates -- glucose, fructose and galactose. During digestion, fructose and galactose are converted into glucose to be used for energy. Glucose is the most important sugar and is used for energy for most bodily functions. The other three sugar molecules are formed by two single sugar molecules. Lactose or milk sugar is made from glucose and galactose. Maltose is formed from two glucose molecules, and sucrose is made up of glucose and fructose.
Complex Carbohydrates
Along with sugar, glucose occurs in long stands of glucose units known as complex carbohydrates. Starch and fiber are the forms found in plants. Starch is easily digested after cooking. Fiber is mostly indigestible to humans. Carbohydrates -- especially from fiber-rich foods such as whole grains, fruits and vegetables -- are the preferred sources of glucose in the diet.
Food Labels and Carbohydrates
Food labels report grams of total carbohydrates, dietary fiber and sugars along with the percent of the daily value based on a 2,000-kilocalorie diet. The daily recommended intake for carbohydrates for adults and children greater than 4 years of age is 300 grams based on a 2,000-kilocalorie intake. The daily recommended allowance for fiber in adults and children 4 years and older is 25 grams of total fiber per day based on a 2,000-kilocalorie diet.
Example
Using the food label and calculator, convert the amount of kilocalories of carbohydrate from grams of carbohydrate. Look at a food label and find the total number of carbohydrate grams. One gram of carbohydrate is equal to 4 kilocalories. So, multiply grams of total carbohydrate by four. For example, if a food label reports total carbohydrates as 40 grams, multiply 40 grams by 4 kilocalories to yield 160 kilocalories of total carbohydrates.
References
- Food and Drug Administration: Appendix F: Calculate the Percent Daily Value for the Appropriate Nutrients
- Nutrition Concepts and Controversies; Frances Sizer and Ellie Whitney
Writer Bio
Heather Colleran is a registered dietitian and a board certified specialist in sports dietetics. She earned her Ph.D. in human nutrition from the University of North Carolina at Greensboro and is a certified strength-and-conditioning coach through the National Strength and Conditioning Association. Colleran has contributed to the “Journal of the Academy of Nutrition and Dietetics” and “Medicine and Science in Sports and Exercise.”