Yesterday, I wrote that the basic ‘thermodynamic’ equation of weight loss (calories in – calories out = net calories) glosses over a lot of important information, mainly by obscuring the definitions of ‘calories’, ‘in’, and ‘out’.

So let’s clear things up a bit, starting today with calories themselves.

When we discuss “calories” in food, we actually mean “kilocalories”, as nutritional calories are based on 1000 thermodynamic calories. Outside of the nutritional world, a kilocalorie is a well-defined measure of energy: the amount of heat required to increase the temperature of one kilogram of water by one degree celsius at a pressure of one atmosphere (i.e., at ground level).

We test calories in food using what’s called a ‘bomb calorimeter’. Basically, it’s a device for blowing up food, and then seeing how hot the explosion makes surrounding water. To operate a bomb calorimeter, you take a small amount of food, and put it into a metal canister filled with pure oxygen, with a fuse that extends out to an electrical ignition. Then you submerge the metal canister, floating it in a kilogram of water. You carefully check the temperature of the water at the start. Then you hit the ignition, and the food explodes. As it does, the heat from the burning food begins to raise the water temperature. By tracking how much the temperature increases at its peak, how many degrees celsius the water temperature rises from the starting point, you’ve got the number of kilocalories in the food you just blew up.

Odds are, you aren’t a biologist. But you’re likely still aware that this isn’t really what happens inside of your body. You don’t walk around with a series of explosions detonating in your stomach all day long. (However, insert fart joke here.)

Tomorrow, when we look at ‘calories in’, we’ll try to get a better sense of what your body actually does to extract the energy from food. But before then, there are a few even more fundamental problems.

Towards the end of the 19th century, Wilbur Olin Atwater, a scientist at Wesleyan University, set out to understand the connection between heat calories (the kind you measure when blowing up food) and nutritional calories (the metabolisable energy your body derives from that food), through a series of experiments.

Atwater tested the heat calories in a wide variety of foods. And then he tested the heat calories in the feces of people who had eaten those same foods, to determine ‘apparent digestibility’, the percentage of the calories absorbed by the body rather than excreted.

The results of those experiments are the “Atwater factors”, the well-known idea that a gram of fat contains 9 calories, while a gram of protein or a gram of carbohydrate each contain 4 calories.

Most people don’t realize, however, that those numbers are averages. Not all fats, proteins and carbohydrates have the same amount of heat calories per gram. Similarly, not all fats, proteins and carbohydrates have the same apparent digestibility. So Atwater devised weighted values for the gross heat of each macronutrient, based on what he thought to be their prevalence in the standard diet at the time.

Similarly, Atwater first tested the apparent digestibility of individual foods. Then he started combining foods together, the way we do when we actually eat things. At that point, however, the numbers started to muddy up. Unfazed, Atwater simply came up with ‘adjustments’ that he hoped might account for the discrepancies in the mixed diet numbers.

When Atwater was doing his experiments, the best-selling Fannie Farmer cookbook was just introducing newfangled kitchen technology like the measuring cup and spoon. The cookbook featured perennial favorites of the time, like turnip croquettes, tomato cream toast, and Washington-style terrapin (turtle served in white sauce with its liver, small intestine, and “any turtle eggs”).

In other words, the ‘standard diet’ on which Atwater based his average computations is more than a bit different from what most of us eat today.

Similarly, Atwater’s ‘adjustments’ to account for mixing foods are difficult to justify in retrospect as anything other than liberal massages of the raw data to better align with his intended conclusions.

So even before we begin to look at how our bodies digest and assimilate foods, it’s clear that perhaps calorie math – which has the reassuring appearance of incontrovertible science – isn’t quite as objective and accurate as we might hope.

Indeed, calorie math doesn’t really take into account the difference between, say, a gram of carbohydrates from a strawberry and a gram of carbohydrates from a pear, or what happens when we eat them together, along with some sugar and cream. (Which, as an aside, sounds delicious.)

Further, it doesn’t even take into account the substantial differences from one strawberry to the next. A huge number of factors in the life of each strawberry can affect its nutritional content. At what time of the year was the strawberry planted and picked? How often was it watered, and how much? Was it fertilized? How much direct sunlight did it get, and how close was its nearest strawberry-plant neighbor? As a result of these and countless other questions, any given strawberry might have different amounts of fructose, glucose, sucrose, soluble and insoluble fiber, and micronutrients than the next.

The result of all of this is that we can’t even answer a very basic question, like “how many nutritional calories are in a cup of strawberries?” We have pretty much no idea.

So, calories. Not really what they’re cracked up to be.

But wait, it gets worse! Tune in for Part III, to learn why, even if we knew how many nutritional calories were actually in that specific cup of strawberries you’re holding, it still has only a small bearing on what happens once you actually put them into your mouth.