[lol636363]

No, all calories are same. This is in the domain of physics or chemistry.

But of course if you get all your calories from simple carbs, you will be deficient in other important nutrients like vitamins.

However, deficiency of certain nutrients may cause body to slow down or increase metabolic rate.

[corty]

No, all calories are not the same. The basic process for measurements is "burn it, measure the energy output". " Burning" as in "gas flame, high heat, oxygenated atmosphere". This is something your body doesn't do, even if you supposedly "burn" calories. Your body uses different reactions that are often less efficient, meaning the energy output will be different from the calories measured. Nowadays some of those errors are corrected for, but not all of them. And depending on your personal metabolism, from genetics, daily changes and nutrient status, your body will be less or more efficient with some foods.

[AnIdiotOnTheNet]

Irrelevant. If your body extracts only 50Cal from a 100Cal cube of sugar, but you counted that 100Cal value against your daily limit, then you're still achieving your goal. The problem is often that people set too high a limit and overestimate how much they burn during exercise.

[staticassertion]

Saying "all calories are the same" ignores a few really important things.

1) It ignores the processes involved in burning fats, sugars, and proteins, which can heavily impact where your body prioritizes getting its energy (or storing it).

2) It ignores other health impacts of 'calories' such as blood glucose levels, which absolutely will depend on what it is you're eating and not just the calorie count. Sugars, fats, and carbs are also processed in different parts of your body - so you might be following a "calories in calories out" and giving yourself non-alcoholic fatty liver disease.

3) It conflates calories, as measured via completely artificial process, with human digestion.

[lol636363]

That's what I said.

Calorie as a measure sets upper limit of maximum energy you can extract from a given food. If you want to gain 1lb of weight, you will need to eat at least 3500 calories. If you want lose 1lb, you have create deficits of at least 3500 cals.