The Physics
An encyclopedia of scientific essays

Energy Consumption of a Human

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Bibliographic Entry Result
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Life: The Science of Biology. Ed. William K. Purves and David Sadava. 7th ed. New York: Freeman, 2004: 962. "The basal metabolic rate of a human is about 1,300-1,500 kcal/day for an adult female and 1,600-1,800 kcal/day for an adult male." 5.44 - 6.28 MJ (female)
6.69 - 7.53 MJ (male)
"Human Nutrition and Diet: Recommended Intakes of Nutrients." The New Encyclopedia Britannica. 15th ed. Vol. 13. 1983: 422. "The energy expended for maintenance at rest, known as the basal metabolism, amounts to about 1.25 kilocalories per minute for a man weighing 65 kilograms (one pound = 2.2 kilograms) and 0.90 kilocalorie per minute for a woman weighing 55 kilograms." 5.42 MJ (female)
7.53 MJ (male)
Heini, A. F. Divergent Trends in Obesity and Fat Intake Patterns: the American Paradox. American Journal of Medicine. 102 (1997): 259-264. "Average total daily calorie intake also tended to decrease, from 1,854 kcal to 1,785 kcal (−4%)." 7.47 MJ
Stigliani, William M., and Thomas G. Spiro. Environmental Issues in Chemical Perspective. Albany, NY: SUNY Press, 1980: 6. "Table 1.1 shows some of the quantities that have been discussed. It also shows that the total energy consumed by the burning of fossil fuels amounted to 6.0 × 1019 calories in 1975. This is about one-twelfth of the net primary productivity. The energy content of the food consumed by human beings in the same year was only 3.2 × 1018 calories. This is 2.2 million calories per person per day, which is close to the minimum estimated to be required for the daily energy needs of humans." 9.20 MJ
Heslin, Jo-Ann, and Annette B. Natow. "Understanding Calories." Introduction. The Calorie Counter. 4th ed. New York: Simon & Schuster, 2007: 5. "On average, we eat 300 more calories a day than we ate 35 years ago and we weigh 24 pounds more. Women eat 1880 calories a day; men eat 2620." 7.87 MJ (female)
11.0 MJ (male)

In order for humans to carry out ordinary life functions and processes they must obtain energy. This energy comes from consuming food and is measured in calories in the United States and joules everywhere else. A gram calorie is the amount of energy need to increase the temperature one degree for one gram of water. However in terms of nutrition and diet a calorie is used to represent a kilocalorie. Thus if an apple has 70 calories, its true energy content is 70 kilocalories. Since 1925 the unit joule has replaced the calorie as the unit of energy; 1 calorie is equivalent to 4.184 Joules. Energy from food can be measured using a calorimeter, which measures the heat released when food is oxidized.

Human energy consumption trends show a gradual increase in energy intake for the past century. This may be due to several causes. Many companies have been generating more products and humans consume them in larger quantities. Food also comes in larger portions and manufacturers have started to exchange their ingredients with less expensive, more energy-filled substitutes. Another possible factor of increasing energy consumption is the decrease in average physical activity.

According to one of the above sources, in 1975 humans consumed 2.51 × 1020 Joules of energy through fossil fuels. Compared to the 1.34 × 1019 Joules of energy consumed through food, humans ironically use roughly nineteen times more energy than they eat. If we limit the amount of energy we consume via fossil fuels to the amount of energy we consume through food, would we be able to scratch the energy issue off our chalkboard?

Vickie Wu -- 2009