The whole business of keeping track of calories for weight loss is so misused that I am astounded. As a scientist, I understand the concept of calories as a measure of energy. Energy in this case specifically refers to heat.
The Reality of Calories
Consider this regarding calories: they are not useful for metabolism. Once food or anything else is converted to heat, it is no longer useful metabolically. In fact, calories are only directly measurable as units of heat. For example, the calories that are stored in the chemical bonds of glucose are measurable when released in a device called a calorimeter (see figure).
When 1 gram of glucose is burned in a calorimeter, 4,000 calories are released. (Oh, by the way, reference to the potential energy of food is technically as ‘Calories’ – with a capital ‘C’ – one of which represents 1,000 lower-case ‘c’ calories. In other words, 1 Calorie equals 1,000 calories [also called a kilocalories or kcal for short].)
A calorie is the amount of heat that is required to raise 1 cc of water by 1 degree Celsius, at room temperature and at sea level. So 1 gram of glucose can yield enough heat to raise 4,000 cc (i.e., 4 liters) of water by 1 degree Celsius.
Now Here is the Kicker…
You will never, ever get all the energy out of a gram of glucose. The 4 Calories per gram statistic that has become dogma is only retrievable in a calorimeter. The efficiency of your fuel-harvesting metabolism is probably between 10 and 20%, and certainly never greater than 30%, of that potential. At least a dozen factors determine what the efficiency will be for you for any particular food at any particular time.
A Ridiculous Comparison
Consider this: in a calorimeter a gram of starch will yield the exact same number of calories as a gram of cellulose, which is indigestible fiber. As you and I both know, starch is a source of metabolic energy (i.e., food) for people. In contrast, cellulose is not.
I know that lots of folks want to focus on the concept of ‘available calories’ – whatever that means. So we can say that cellulose offers zero available calories. We can also say that starch offers available calories. We just have no idea how many. It is going to be in the neighborhood of 400 to 800 calories (or 0.4 to 0.8 Calories).
What’s Really Important
Instead of comparing the metabolism of food with a furnace or calorimeter, it is much more meaningful to talk about what happens to different foods when they are digested, how they get into different kinds of cells (e.g., fat vs. muscle), and what happens to them once they are there.
By the way, once you understand those factors, you will be very clear on why calories have nothing to do with obesity. I hope you chew on that comment for a while (pardon the pun), because this is the kind of thinking that will guide you to success in any weight loss or muscle-building program that truly works for a lifetime.
All the best with calories,