The idea of negative calories can be confusing for sure, but it isn't new: that concept already applies to such food as celery, grapefruit and lettuce. None of them, of course, doesn't have real negative calories, but what about coffee?
To answer this question we need to dive deep into the nature of calories and find the truth!
What is a calorie?
A calorie is a unit of measure much closer to a British thermal unit (Btu) than to anything biology-related. To be clear, a calorie is actually a metric completely equal to Btu. The only difference between them is that a calorie represents an amount of energy needed to raise 1 kilogram of water by 1 degree celsius while a Btu represents an amount of energy needed to raise 1 pound of water by 1 degree celsius.
Strictly speaking, anything that burns can be defined by caloric value, including wood, propane, gasoline, etc. But because our bodies simply can't digest these calories, they were separated into a special group of indigestible calories.
Some categories of fiber that can be found in common fruits and vegetables are also indigestible. This fact leads to the popular opinion that fibrous foods cause increased weight loss. Simply put, 200 calories from fibrous foods, let's say kale, is totally different from 200 calories from meat, which is different from 200 calories of butter, and so on.
Most of the 200 kale calories are cellulose and other indigestible fiber which our bodies can't convert into valid energy and only a small amount of them can be metabolized properly. At the same time starchy, carbohydrate-laden foods are completely different because our bodies can store energy extracted from them as fat.
A cup of coffee has only about 5 calories and a large amount of them is indigestible. So coffee clearly belongs to the low-calorie food tier. But still, how can coffee have a negative calorie value?
Caffeine makes wonders
The main reason coffee has such a paradoxical calorie value is because coffee includes caffeine and other chemical compounds as well as chlorogenic acid. Those components (mainly caffeine) cause a complex chemical chain-reaction which as a result brings us the feeling of enjoyment. They also increase our metabolism, so our body burns much more calories than usual.
Moreover, caffeine suppresses appetite and chlorogenic acid prevents fat and carbohydrate absorption, which, combined with increased metabolism, helps with weight loss even more. But those effects are much harder to correctly evaluate than just simple metabolic rates.
We can say for sure that 100 milligrams of caffeine, which is an amount that a weak 8-ounce coffee includes, causes the human body to burn an extra 75-150 calories by increasing the metabolism rate. Now let's do simple math and subtract these numbers from a total of 5 calories per cup of coffee. The result would be a negative 70-145 calories.
Before you start soaking all your favorite "guilty pleasure" food in coffee, we must warn you: it doesn't work like that. The reason is tolerance and diminishing returns which appear when our body gets used to caffeine and the more regularly we consume coffee, the faster it happens. The ability of the human body to adapt its chemistry to Pavlovian stimuli is almost as incredible as coffee with a negative calorie value.