I am working hard to nail down The Number that represents how many calories of fuel energy it takes to make one calorie of food. My previous post about dinosaurs in your food describes how this Number is on average 10 calories of fuel to make one calorie of food.
So where does this number come from? Below is a narrative followed by references to the researchers mentioned.
I accept the 10:1 ratio because I have read various data sets and combed numerous papers and studies related to this issue. Further research on this issue is needed, something I am pursuing every day, and I will revisit this ratio if needed.
The ratio is based off of a complex number of variables: amount of energy used in the food system, definition of food system used, amount of calories per person per day to be considered, the role of import and exports, and what percentage of total U.S. energy is used by the food system.
Pimentel and, separately, Hall have estimated the average to be 10:1, while Heller/Keoleian has estimated 7.3:1. Heller/Keoleian say that the food system consumes 10.2 quadrillion Btu's (quads) of energy and provides 1.4 quads back out, based off a diet of 3,800 calories per person per day (because we produce more food than we need, eat too much of it, and then throw some away). If we use a 2,500 calorie diet, we would get an 11:1 ratio; a 2,000 calorie diet means a 13.8:1 ratio.
On the side, grain-fed beef requires thirty-five calories for every calorie of beef produced (Horrigan), and a can of diet soda that provides maybe 1 calorie of energy needs 2,200 calories to produce (70% tied up in the aluminum can)(Heller/Keoleian).
I am still dissecting Heller/Keoleian's comprehensive paper. Heller/Keoleian estimates food energy use at 10% of total U.S. energy. Hendrickson studied 8 different studies from the 1970's and found an average for food system energy use to be 15.6%. This suggests we are already reducing energy use in ag. The Earth Policy Institute (EPI) created some nice graphs showing total U.S. and farm energy use base off the more current numbers. In one graph EPI show the whole U.S. food system uses 10.25 quads of energy (10,250 quadrillion Btu's) and ag production accounts for 21% of energy use, or 2.125 quads. What
What I take from this is this: if Heller/Keoleian are correct that ag uses 10% of energy, this number would roughly align with the 1.691 quad number, but if we use the 21% figure that comes from Heller/Keoleian, the amount of embedded energy (and therefore the calorie ratio) is much higher than Heller/Keoleian report. I am trying to get hold of the Heller/Keoleian team to ask them about these discrepancies.
In the meantime I accept 10:1 to be an average ratio I can support. I will keep resarching this issue and updating as needed. Thanks to Cookson Beecher, a reporter with the Capitol Press (Olympia, WA), for asking me to source this fact.
Heller and Keoleian's article "Life Cycle-Based Sustainability Indicators for Assessment of the U.S. Food System" is very comprehensive.
One article by Pimentel and Giampietro is "The Tightening Conflict: Population, Energy Use, and the Ecology of Agriculture".
Sustainable Table has a good article "Fossil Fuel and Energy Use" with strong references at the bottom.
Hall, C. A. S., C. J. Cleveland, and R. Kaufmann, "Energy and Resource Quality" Wiley Interscience, New York: 1986.
Heller, Martin C., and Gregory A. Keoleian, "Life Cycle-Based Sustainability Indicators for Assessment of the U.S. Food System", Ann Arbor, MI: Center for Sustainable Systems, University of Michigan, 2000.
John Hendrickson, “Energy Use in the U.S. Food System: A Summary of Existing Research and Analysis” Sustainable Farming, Vol. 7, No 4, 1997
Horrigan, Leo, Robert S. Lawrence, and Polly Walker. "How Sustainable Agriculture Can Address the Environmental and Human Health Harms of Industrial Agriculture." Environmental Health Perspectives 110, no. 5 (May 5, 2002)
David Pimentel and Mary Pimentel, "Energy Use in Fruit, Vegetable, and Forage Production", in "Food, Energy, and Society", ed. D. Pimentel, and M. Pimentel, revised edition. University Press of Colorado, Niwot, CO, 1996,