On one side you have people who say weight loss is just calories in vs calories out. The other claims that food quality is all that matters. So what’s the truth, and how should we tackle body composition?
A calorie is the amount of energy required to heat a gram of water by 1ºC. (Technically, on food nutrition panels each calorie is actually a kilocalorie.)
It was originally derived by burning food in a bomb calorimeter and measuring the temperature of the surrounding water.
Calories in vs calories out
Food contains energy. Body fat is stored food energy. Burning that stored energy requires less energy into the body (through eating) than is going out (through activity and staying alive).
We can use calories to measure how much energy is coming in. In theory that means we can limit it to be less than what we burn, thus leading to weight loss.
The fiction side
The most extreme calories-in-vs-calories-out deniers I’ve met are adherents of the carbohydrate-insulin model. They claim body composition is completely unrelated to energy intake. People get fat solely because their diet contains too much refined carbohydrate.
Anyone who’s got fat eating eating low-carb will find this hard to take seriously. (As will the starving people in various countries who eat mostly white rice.)
The more reasonable argument is that foods have different effects on body composition unrelated to their energy content (ie, “a calorie is not a calorie”). That makes calories in vs calories out equations imprecise to the point of uselessness for weight loss.
Practically this means food quality is the better focus. That is, avoidance of sugar and other highly processed/refined foods.
The fact side
The extreme version of calories in vs calories out is that the caloric value of a food is all that matters. A calorie’s worth of any food or macronutrient will have the exact same effect on body composition. That means you can substitute any food for any other food, and provided the calories match, the outcome will be identical.
The absurdity of this idea makes it easy to disprove. For instance, some types of fiber aren’t able to be used for energy in the body but technically still have caloric value. Thus there’s no likelihood of storage as body fat compared to the same calories of butter.
Yet we’re still left with the reality that losing weight requires fewer calories in than going out. That makes calories in vs calories out accurate as far as it goes.
It’s undeniably true that a given calorie of steak will have a different effect in the body than a calorie of sugar, broccoli, or butter. Therefore a pedantic, uncharitable interpretation of calories in vs calories out is false.
Weight gain or loss is caused by the net energy the body receives after digestion of food, bodily maintenance, and activity. The tricky part of using a calories in vs calories out calculation is the calories out side. It can be highly changeable even on a daily basis, and is determined by many variables we can’t quantify. These include the foods you eat, how well you slept, how much you moved, and stress levels.
But that complexity doesn’t make calories in vs calories out untrue, it just introduces imprecision. The critical question then: is it accurate enough to be useful for fat loss?
The answer is overwhelmingly yes. Millions have lost weight using calorie-tracking apps, calorie spreadsheets, and popular calorie-based eating systems such as Weight Watchers.
Horses for courses
The heart of the disagreement is more about the lack of nuance in the phrase “calories in vs calories out” itself. “Energy balance” is a better term with less room for misinterpretation.
Practically, weight loss can be achieved through regulating diet through caloric calculations, regardless of any absolute technical imprecision.
It can also be achieved through a focus on unprocessed, low-energy-density foods, which it turns out is really just a simple way of reducing calories in.
But calories in vs calories out is a fact, and if you want to lose weight, you absolutely must find a way to tip the balance in favor of the calories out side.