The easiest way to look at this is to look at the cooling rate of a corpse. The slope of the cooling is pretty much linear as long as the ambient temperature i

The easiest way to look at this is to look at the cooling rate of a corpse. The slope of the cooling is pretty much linear as long as the ambient temperature is much lower than body temperature (for example 20 C vs 37 C). This allows you to calculate the amount of energy necessary to maintain body temperature. Convert that to kcal and this gives you the minimum number of kcal you must eat. I've actually done it before (and even wrote a comment in HN with the results -- though it was a long time ago and I don't know if there is a way to search comments), but it takes more time than I have now. Unless my memory is incorrect or my calculations faulty, the answer is that it's very, very unlikely that a person who is over about 50 kg would be maintaining their weight with 1600, even if they were doing no exercise at all. Unless they are living in a very hot climate, it appears to be thermodynamically impossible. Fun fact: I just searched my gmail for the word "corpse" to see if I emailed that calculation to anyone. I got 19 unrelated results. What the heck do I talk about in email??? There is actually one other variable (that I'm aware of) that's unaccounted for: the amount of nutrient absorption from the food. "Calories" in food are measured by burning the food and measuring the amount of heat given off. This value is multiplied by a kind of fudge factor to approximate how much of the nutrients are absorbed. According to a quick Google search, 98% of carbs, 95% of fat and 92% of protein is absorbed by the gut. So if we imagined that someone became super absorbent, they might require up to 8% less calories. But not enough to significantly alter calorie counting. I considered replying to the original poster of that comment, but decided that they probably weren't in the mood to receive the news. Almost certainly they are missing things in their calorie counting. On the plus side, if they are getting 1600 calories consistently and maintaining their weight, then their error is systemic and might not matter (i.e. the actual value is meaningless as long as it is consistent -- they can still go down to 1400 calories and lose weight). I've helped quite a few people lose weight before and it's pretty easy to overlook calories -- especially from fat. I had one friend who ate a really modest amount of food every day: typically a bran muffin (with butter) and coffee (with cream) in the morning. Half a subway sandwich (the kind with lots of different kinds of salami and mayonaise), a small bag of potato chips, and a bottle of sweetened ice tea for lunch. Pasta (with a cream sauce) and a salad (with dressing) in the evening. No desert, but the grand total number of calories was astounding -- pushing 3000 kcal per day. It's really, really easy to do.