# Does food really give our bodies the energies that we have determined by burning them in the lab?

You are likely to have come across numbers like sugars 17kJ/g (4kcal/g) as the energy supposedly available to our bodies after eating. Yet these values have been determined using very artificial means, typically lighting up food with a red hot iron in a steel chamber at high oxygen pressure.

Many historic energy values have been proven wrong in the past. For example, salad leaves had zero calories in many historic energy tables before we realized that bacteria in our gut can break them down and we in turn can break down bacterial products, gaining back part of the chemical energy of the plant.

Are you aware of any research that has determined the energy value of different foods in more natural conditions, ideally in humans? I expect some calorie values to change from those determined by the bomb calorimeter, don't you?

The device you refer to is a bomb calorimeter. Inside one of these, a sample of material is completely combusted. Typically, the heat from the combustion warms a measured mass of water and the rise in temperature is used to calculate the Calories/Joules in the material.

Of course our cells don't ignite food and burn it to completion. Even if they did, some of the heat from the process would escape collection. Cells use the process of cellular respiration to harvest the energy. The basic version of cellular respiration describes the utilization of glucose to create ATP, by far the most used substance to power the energy needs of the cell. Here's just one source that takes you through the calculations of how efficient this process is: http://www.tiem.utk.edu/~gross/bioed/webmodules/ATPEfficiency.htm

So, at least for the very common sugar glucose, only about 40% of the total Joules, as measured in the calorimeter, end up as ATP. Most other food substances we use for energy share much of the same chemical pathway as glucose. I haven't seen calculations of their efficiency but suspect it would be roughly similar.

One of the most basic and important lays of physics is that no transfer of energy is 100% efficient. (Read more here: https://en.wikipedia.org/wiki/Energy_conversion_efficiency ). The remaining energy in the glucose that was not converted to energy in ATP is "lost" as heat. By lost, it is meant that it's not avaible to do any useful work other than heat us up. Of course, being homeothermic ("warm blooded") animals, that's useful too.

• The U of Tennessee link you sent is great. They propose a very simple calculation that gives a good first approximation. Energy captured in ATP divided by theoretically available energy from glucose combustion ~ 50% after correcting for concentrations. Sep 30 '16 at 19:49
• But that's does not yet take into account the energy required to metabolize glucose. Most of us get our glucose from polymers, so we need to make enzymes to break them down. Then we need to import and ship it to the cells that do the chopping; more energy required. Then in the cell that gets that molecule of glucose, the glycolytic enzymes don't come for free, right? What would be left of the 50% efficiency estimate when all these expenses are taken into account? 25%, 10%,..? Sep 30 '16 at 19:49
• It's true there are a few steps in glycolysis that require energy but, in subsequent steps, this investment is returned. The calculations include this, they are a net value of the process. As to parsing out the energy required to produce the enzymes involved, and to manufacture mitochondria to govern most of this energy harvesting, I've never heard of anyone attempting to calculate this. All of that is generally included under the category of "what we need the energy for". Sep 30 '16 at 20:47