Measuring Our Body's consumption of Calories
We have yet to answer a fundamental question: how do we really prove that our bodies get energy from food by burning it, like a sort of slow combustion? For that, we need to quantify how much energy we’re actually getting after we eat. A straightforward way is to build a human-sized calorimeter, but instead of setting food on fire, we let our bodies do their thing and then measure the heat that comes out. That is exactly what early scientists, including Atwater, did.
The earliest whole-body calorimeter was a large double-walled chamber. The space between the inner wall and outer wall was filled with ice. The research subject was confined inside the chamber and went about his business (it was always a him, it seems) as normally as possible. By measuring how much ice melted, scientists could work out how much heat he generated. The subject’s feces and urine were also collected so the energy content could be analyzed.
Mr. Atwater built one of these at Wesleyan University. His chamber not only measured heat but also measured the oxygen intake and carbon dioxide excretion, thus indirectly measuring energy produced from oxidation. That works because when you “burn” food in your body, you need oxygen for the reaction, and one of the products of that reaction is carbon dioxide. Here are the chemical equations of several different types of combustion:
Combustion of Coal (equation 1):
Combustion of cooking gas (equation 2):
Combustion of Glucose (equation 3):
Combustion of a typical fatty acid -- Palmitic acid (equation 4):
Equations 3 and 4 are not exactly how metabolism happens, more like approximations that skip a few intermediate steps. Despite the simplification, they still correctly indicate the ratio between calories and the volumes of oxygen consumed. This ratio is called the respiratory quotient (RQ).
Since Atwater’s days, portable respiration calorimeters have been developed. However, these devices still present limitations by impeding normal activities. In the 1960s, a less invasive and more accurate method to measure the RQ was developed. It is the doubly labeled water method.
Water molecules consist of two hydrogen atoms and one oxygen atom. Usually, hydrogen atoms have one proton and no neutron, while oxygen atoms have eight protons and eight neutrons. In the doubly labeled water variant, the hydrogen isotope has one proton and one neutron, and the oxygen isotope has eight protons and ten neutrons. At the beginning of the measurement, the subject drinks some doubly labeled water. The key insight here is that while the heavy hydrogen is expelled from the body exclusively through urine and perspiration, the heavy oxygen can leave both via excreted fluids and exhaled carbon dioxide. As the individual continues to consume regular water and food, the isotopes will be diluted at different rates. So with a few careful measurements of urine samples along the way and some sophisticated data processing, the amount of carbon dioxide exhaled during the measurement period can be deduced.
Incidentally, this answers the question: what do we actually lose when we lose weight? Mass leaves your body in the form of carbon dioxide and water. Although some water may come from your drinking, the carbon dioxide is exclusively derived from metabolized food. Unless you are breathing harder, you are not losing more weight. That explains why rapid weight loss is implausible without a loss of water weight: there is a limit to how hard you can breathe.
Various doubly labeled water measurements show that adult men typically burn more than 3,000 calories a day. A discrepancy arises from the self-reported intake of American adult males, which, according to the large-scale survey “What We Eat in America”—conducted jointly by the Centers for Disease Control and Prevention (CDC) and the United States Department of Agriculture (USDA)—averages 2,500 calories. This survey involved detailed and regular interviews of 8,500 individuals about what they eat.
If this upside-down result gives you pause about the first law of thermodynamics, consider the National Health and Nutrition Examination Survey, also administered by the CDC. The height of 5,000 Americans was measured, and then they were asked what their heights were. On average, men overestimated their height by half an inch. That is all you need to know about how much you can trust self-reported data.
On the other side of 3,000, the USDA adds up the foods produced in the United States, plus imports, and minus exports, sort of like a GDP for food. The per capita calorie availability number for Americans is 3,900. Notably, 12% of Americans say they regularly buy products that are labeled as having a small carbon footprint. If you truly care about the environmental impact of food, just eat less and waste less. It’s good for your body, your wallet, and your planet.