Turning fuel into food

by Eric Garza

African BushmenIn the energy basis of food security and the energy cost of food I explore how energy is used throughout modern industrial food systems, noting in particular how incredibly energy intensive these systems are. A common criticism I receive regarding these analyses is that I’m comparing input and output energies that, despite both being measured in calories, are so different that combining them in a single ratio yields a meaningless number. I think it worthwhile to explore this critique in greater detail. 

All living organisms expend energy to get energy. They burn edible food calories within their bodies to obtain more edible food calories, with the goal of gaining more than they burn. Individual organisms that fail to achieve a positive energy return will starve, and if an entire species fails to achieve a positive return it eventually succumbs to extinction. Individuals and species that achieve positive energy returns will persevere and perhaps even prosper.

Humans are living organisms too. We labor in search of food, and to the extent that we can gather more food energy than we burn while searching for it we persist, as individuals and as a species. Anthropologists estimated the energy returns achieved by hunter-gatherer tribes in the range of 5-10 on the basis of food yield per labor energy input, meaning that for every calorie of labor energy these people invested searching for food, they acquired 5-10 edible food calories [1, 2]. Not too shabby, and this solid return illustrates why many in these groups only had to work a few hours each day for their subsistence.

How does modern man, Homo industrialis, compare? The energy return for the US food system is stunningly high when calculated solely with respect to human labor, topping out around 90 output food calories per input labor calorie over the last few years, even when losses due to waste and spoilage are accounted for [3, 4]. This efficiency blows those measured for hunter-gatherers out of the water. How can we be so efficient? Rather than using human muscle power to do work, we instead use machines. We use tractors to plow and harvest fields rather than laborers, and robots to slice and dice animals in our slaughterhouses, grind our grain and bottle our milk. By substituting vehicles, machinery and other technological infrastructure in place of labor, we radically reduce the labor-intensity of food production and enjoy substantial gains in metabolic energy return over those recorded for hunter-gatherers.

But just as human bodies must be fed, so too must machines. Their food is fuel – gasoline, diesel, natural gas and electricity, among others – and by accounting for all energy inputs rather than just those associated with labor we gain a more expansive understanding of our industrial quest to turn fuel into food. This broader scope yields the dramatically different picture of food system energy return articulated in the energy cost of food and similar essays wherein I note that it requires at least 15 calories of input energy to deliver a single calorie of food. Whereas the energy return relative to metabolic energy is positive and rising in industrial food systems as fewer people expend less labor to produce food, the overall energy return is negative and getting worse because of increasing reliance on machinery and the industrial fuels that power them.

It is indeed true that the calories of industrial energy used to fuel modern food systems are not the same as those of human labor energy. One is associated with human exertion, the other with an unprecedented energy subsidy derived largely from ancient sunlight. It’s also true that calculating a ratio with food energy output over total energy use (or the reciprocal) doesn’t quite compare apples to apples, as one of these energies is edible and the other distinctly not. When looking at all input energies, achieving an output over input ratio greater than one – a positive return – is not as important as when calculating a ratio that only includes food and labor. Nonetheless the total energy return ratio offers useful information by showing us how extreme our dependence on nonrenewable energy resources has become. Hopefully this will motivate us to pursue efficiency in a food system that drifts further from this ideal with each passing year, and who knows, maybe at some point we’ll achieve a food system that delivers a positive energy return, no matter how we calculate it.


  1. Richard Lee. (1969) Kung bushmen subsistence: an input-output analysis. In Environment and Cultural Behavior, Edited by Andrew Vayda.
  2. Marshal Sahlins. (1972) Stone Age Economics.
  3. Food Availability (Per Capita) Data System. United States Department of Agriculture’s Economic Research Service. Estimates of food availability corrected for waste and spoilage are called ‘loss-adjusted’ by the USDA, and are used as a proxy for food that’s eaten by a person.
  4. Hours Worked by Full-Time and Part-Time Employees By Industry. United States Bureau of Economic Analysis.