ENERGY

Energy is required for the functions of all biological systems. Dietary energy is commonly given in units of Calories or kilocalories, but the acceptable scientific terminology is joules. The primary sources of dietary energy are fats, carbohydrates and protein. Ethanol also contributes dietary energy in individuals consuming alcoholic beverages, but alcohol is not generally categorized as a nutrient. The energy for biological processes is obtained from these sources by intermediary metabolism. Our bodies are very efficient in storing energy for future use. Stored energy is primarily in the form of fat in adipose tissue, but a small amount of carbohydrate is stored in the liver and muscle as glycogen. Intermediary metabolism converts the energy consumed into the forms for storage or into forms which are readily available for biological processes such as compounds with high energy phosphate bonds.

Since energy cannot be destroyed or created, over a period of time the energy consumed must be expended or accounted for by gains or losses in body tissues. If energy intakes are less then expenditures, body weight loss will occur. When energy intakes exceed expenditures, body weight gain will occur. There is evidence of adaptive responses to fluctuations in energy intake which are dependent of genetic and environmental factors. While these adaptations may account for some of the variability between individuals, they do not prevent body weight changes in response to unbalanced energy equations. Energy expenditures are based on: Energy Needs = Basal metabolic rate (energy to keep heart beating, etc.) + physical activity + Thermic effect of feeding (energy for intermediary metabolism).

Diet recommendations: Recommended dietary allowances for energy are based on Median Heights and Weights set by the Food and Nutrition Board. The average energy allowance for infants (0.5-1.0 years, 9 kg or 20 lb, 71 cm or 28 in) is 98 kcal/kg or 850 kcal/day. The average energy allowance for children (4-6 years, 20 kg or 44 lb, 112 cm or 44 in) is 90 kcal/kg or 1,800 kcal/day. The average energy allowance for males (11-14 years, 45 kg or 99 lb, 157 cm or 62 in) is 55 kcal/Kg or 2,500 kcal/day. The average energy allowance for males (25-50 years, 79 kg or 174 lb, 176 cm or 70 in) is 37 kcal/kg or 2,900 kcal/day. The average energy allowance for females (11-14 years, 46 kg or 101 lb, 157 cm or 62 in) is 47 kcal/kg or 2,200 kcal/day. The average energy allowance for females (25-50 years, 63 kg or 138 lb, 163 cm or 64 in) is 36 kcal/kg or 2,200 kcal/day. There is wide variation in individuals' energy requirements, primarily because of differences in body size and physical activity. The energy allowances above are for individuals with median heights and weights, and light to moderate activity levels. The recommended allowances should be adjusted to account for increased physical activity and for larger or smaller body size.

Inadequate energy intakes: Starvation continues to be a major world problem. Fortunately, starvation because of inadequate food availability is almost unheard of in the US. However, self imposed starvation is recognized in a proportion of people in the US with anorexia nervosa.

Excessive energy intakes: Obesity is a major health problem in developed countries in the world. It has been estimated that 25-35% of the US adult population is obese. Obesity is associated with increased risk for cardiovascular disease, hypertension, stroke, diabetes and for certain cancers (endometrial, colon, ovarian and breast).

Recent Research: There is interest in identifying optimal intakes and expenditures of energy and optimal body weights for the prevention of chronic diseases associated with obesity. Factors which influence energy balance and energy needs, and account for some of the difference in energy utilization between individuals, including genetic differences, are actively being studied. Appetite regulation and the relationship of appetite to optimal weight maintenance are important research areas.

For further information:

Food and Nutrition Board, National Academy of Sciences. (1989) Recommended Dietary Allowances, 10th ed., National Academy Press, Washington, DC.

Scrimshaw, N. S., Waterlow, J. C. & Schurch, B, eds. (1996) Energy and protein requirements. Eur. J. Clin. Nutr. 50(S1): 1-197.

Flegal, K. M., Carroll, M. D., Kuczmarski, R. J. & Johnson, C. L. (1998) Overweight and obesity in the United States: prevalence and trends, 1960-1994. Internat. J. Obesity 22: 39-47.

Bouchard, C., Despres, J. P. & Tremblay, A. (1991) Genetics of obesity and human energy metabolism. Proc. Nutr. Soc. 50: 139-147.

Prepared By:

Diane F. Birt, Ph.D.
Professor and Chairman
Department of Food Science and
Human Nutrition
Iowa State University
2312 Food Science Bldg.
Ames, Iowa 5011-1061
Phone: 515-294-3011
FAX: 515-294-8181
Email: dbirt@iastate.edu

Nancy F. Butte, Ph.D.
Associate Professor
Baylor College of Medicine
Houston, Texas
Phone: 713-798-7179
FAX: 713-798-7187
Email: nbutte@bcm.tmc.edu