A calorie is a unit of energy. Specifically, a calorie is the amount of energy required to heat up 1 gram of water by 1˚C. Technically speaking, each “Calorie” in food, denoted by the big “C” is equal to 1,000 little “c” calories, but that’s besides the point. The question we should ask is, why is a calorie defined as the amount of energy it takes to heat water?
It all has to do with how calories are measured in food. The technique used to measure the calories in, say, a banana, is called bomb calorimetry. Literally, you take the banana, place it in a chamber surrounded by water, and blow it up. KABOOM! Energy is released from the combustion into the surrounding water, and the number of ˚C the surrounding water is heated tells us how many calories are in the banana.
But can you apply this value to human biology? Well, not really. Our bodies don’t extract energy like bomb calorimeters. Once the nutrients in our foods make their way to our cells, energy is extracted in a more sophisticated manner than simple combustion. Food is methodically broken down by an orchestra of enzymes to generate cellular building blocks and liberate electrons that can then be systematically harvested for energy by mitochondria. Jargon aside, we are not bomb calorimeters. Not even close!
What’s more, for every you cell in your body, there is at least one bacterial cell residing in your gut’s microbial ecosystem that also has a role in extracting energy from the food you eat. Since we all have different “microbiomes,” we all extract energy from food differently. As an extreme example, if you were a gorilla with a gorilla’s microbiome, you could get 60% of your calories from fiber (1)! True, we aren’t gorillas, but the concept is the same. We each have unique microbiomes and each extract energy from food differently (2).
Let’s play devil’s advocate and assume that we can perfectly measure calories by blowing up bananas and apply those caloric values to human nutrition. Then would we be able to count calories? Still no. At least in the United States, the Food and Drug Administration’s Guidance for Industry allows for a 20% margin of error on nutrition labels (3). Even if food companies actually abided by this law, which many don’t, that would amount to a 400 Calorie error on a 2,000 Calorie diet.
So, let’s summarize. You can’t count calories eaten accurately because (i) you’re not a bomb calorimeter, (ii) you have a unique microbiome, and (iii) nutrition labels aren’t accurate.
That’s only half of the equation. Advocates of strict calorie counting provide the logic that calories in – calories out = weight gained. We’ve talked about calories in, which can’t be accurately measured, but what about calories out or burned? Here, things get even more messy. One could write an encyclopedia on the topic of why measuring calories burned in humans over an extended period of time is so complicated, but we’ll just mention a few highlights. To begin, physical exercise only accounts for a tiny fraction of total calories burned for most people. Much more energy is spent on the biological processes of temperature regulation, running your brain, and digestion.
Furthermore, “non-exercise activity thermogenesis,” or NEAT, also burns more calories in a day than exercise for most people. NEAT refers to those physical movements that are separate from exercise, like maintaining good posture or bouncing your leg or gesturing with your hands. NEAT activities amount to far more, in terms of calories than the 30-60 minutes a person might spend at the gym and is enormously variable among people. In some people, NEAT may account for 6-10% of total energy expenditure. In more active people, NEAT can burn up to 2,000 Calories per day and account for >50% of total energy expenditure (4)! Think about the people you know who bounce their legs? Are they overweight or skinny?
Perhaps most importantly, the human hormonal system governs how energy is distributed and used. Over the long term, this is enormously important. Your hormones determine if you expend energy or store it. And, if you store it, your hormones determine how you do so, as fat or lean muscle.
Let’s do like a sushi chef and wrap it up. Here are the key takeaways…
Take Home Messages
Calories in food are measured by bomb calorimetry, which doesn’t apply well to human biology because our cells use a very different process to extract energy.
We all have different microbiomes which uniquely influence how we extract energy from food.
The law allows for a 20% margin of error on nutrition labels.
Exercise contributes to a small proportion of calories burned. Far more energy is burned by temperature regulation, fueling your brain, digestion itself.
NEAT refers to non-exercise related calorie burning activities, like bouncing your leg. Variations in NEAT account for a large variation among humans in calories burned.
Hormones govern how your body uses and distributes energy over the long term.
For the above reasons, you can’t accurately measure “calories in” or “calories out.”
No medical advice
The Content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.