Are we over-counting the number of calories in food?

Q: How do you measure the number of calories in a food item?

A: You burn it in a calorimeter.

The calorie is a unit of energy, and the chemical energy in food can be measured by releasing this energy as heat during combustion in a calorimeter (Wikipedia):

"The current calorie-counting system was created in the late 1800s by Wilbur Atwater, a scientist at the Department of Agriculture, and has been modified somewhat over the past 100 years. Researchers place a portion of food in a device called a calorimeter and burn it to see how much energy it contains. The heat is absorbed by water; one calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius."

The calorimeter sets the food on fire and measures the heat that is released when it burns. Heat represents thermal energy, and a calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. Note that a food calorie is what a physicist would call a kilocalorie i.e. the amount of heat that would raise 1 kilogram of water by one degree.

The operation of a calorimeter is best described in perhaps the simplest type of calorimeter which is the bomb calorimeter (Figure 1):

"Basically, a bomb calorimeter consists of a small cup to contain the sample, oxygen, a stainless steel bomb, water, a stirrer, a thermometer, the dewar or insulating container (to prevent heat flow from the calorimeter to the surroundings) and ignition circuit connected to the bomb. By using stainless steel for the bomb, the reaction will occur with no volume change observed."

The "bomb" (reaction chamber) is submerged in water (Figure 1), and as the food is burned in a cup within the bomb it heats up the surrounding water. The increase in temperature multiplied by the volume of water (1 ml = 1 gram) gives you the calories. Dividing this number by 1000 results in the number of kilocalories or food calories. Note that the vessel containing the water and bomb are insulated to prevent heat exchange with the outside environment.

This energy in the food is absorbed by the body's digestive tract in the form of energy-containing basic nutrients such as glucose, amino acids, and fatty acids. These building blocks can be used to synthesize energy stores such as fat, protein, or glycogen, or used directly, in the case of glucose, as body fuel. One assumes that the body efficiently captures all (or nearly all) of the chemical energy in food, and for the most part this assumption is accurate.

However, the assumption is not completely true for all foods, and this discrepancy may give rise to inaccuracies counting the number of "absorbed calories" from a food item. A New York Times article highlights two factors that may contribute to the over-counting of calories:

"The system is most accurate when the foods are easily digested and all of their energy is made available to the body — as they are when consuming highly processed carbohydrates. But in the past few decades, scientists have begun to understand that a substantial number of calories are lost in the effort to digest food. For example, meat and nuts are harder to break down, and so the body expends energy trying to digest them. In the end, some foods are also not fully digested: significant portions are excreted, and so those calories should not be counted, either."

Thus, the calories expended or lost digesting a food product should be subtracted from the total calorie number to provide a net calorie figure (i.e. absorbed calories). First, the actual process of digestion consumes calories like any biological process that does work. Second, not all the food is digested and absorbed into the blood; some food breakdown products may be excreted, and so this chemical energy is lost. A third possible inefficiency is that some of the chemical compounds in food cannot be utilized by the body for energy purposes or energy storage.

Interestingly, the discrepancy is greatest for nuts which are hard to digest and so they consume more energy during the digestion process, and a greater amount is left undigested and hence excreted. As a result, the listed calorie value overestimates the absorbed energy after digestion by as much as 25%:

"Nuts are among the hardest to digest, and estimates of the calories they contain by the old method are the furthest off — the counts are about 25 percent too high, according to recent research by David Baer, a nutrition scientist at the Department of Agriculture."

Some have suggested implementing a revised system for calculating calories that takes into account the calories lost during the digestion process:

"An alternate and more accurate system of counting calories has been devised by Mr. Livesey and has been presented to the United Nations Food and Agriculture Organization, which provides recommendations to member nations. The new method has been discussed but not officially adopted. It counts not just how much energy (or calories) are available in a food, but what the body can actually use of that energy. Additional calculations are made to consider the energy expended by the body in digestion and the degree to which the food is processed."

However, others think that recalculating calorie values would be confusing to consumers while not making much of a difference (i.e. changing the current listed calorie values) for most foods. In addition, by lowering the calorie counts of some food, it may encourage people to eat more.

In summary, the listed calorie value on a food product represents the upper-bound on how many calories you will acquire by eating the food. However for most food products, this value provides a pretty accurate estimate. For some, like nuts it may over-represent the absorbed calories by as much 25%. But even this 25% is not a huge error especially compared to the errors made estimating food portions and content when trying to count calories. One area in which the absorbed calorie adjustment might make a difference is in substituting one food for another. Nuts are part of numerous healthy diets like the Mediterranean diet, and the the modified calorie number would make them even more attractive compared to junk food alternatives.

So yes we are probably slightly over-counting the number of calories (that we actually absorb) in some foods, but this overestimate is minor in the big scheme of things.

Figure 1. Bomb calorimeter. The "bomb" represents the reaction chamber in which the food sample is burned. It is surrounded by water whose temperature is measured to calculate the heat produced in the bomb.