I couldn't resist!
On my adventures around the interwebs, I've noticed the following:- "Humans aren't Calorimeters. Therefore calories are irrelevant to humans." While I agree with the first sentence, I don't agree with the second one.
Calorimeters burn (oxidise) foods at high temperatures with a flame using oxygen, which produces carbon dioxide, water (depending on what's being burned) & heat energy.
Humans burn (oxidise) foods at 37ºC with enzymes , charge transporters etc using oxygen, which produces carbon dioxide, water (depending on what's being burned), mechanical energy & heat energy.
As both oxygen & carbon dioxide are gases, these can be measured by a respiratory gas analyser, to establish the rate of burning and what's being burned at any instant. See It's all in a day's work (as measured in Joules). When resting, burning occurs at a rate of ~1kcal/minute and, as it's measured while fasted, ~0.11g/min of fat is burned, & ~0.01g/min of carbohydrate is burned. Also note that a lot of mechanical energy can be produced, which can increase the rate of burning by a factor of seventeen.
In conclusion, humans burn (oxidise) foods, though not with a flame, and they can produce mechanical energy in addition to heat energy. The rate of burning and what's being burned at any instant can be measured.
When we eat food, it's digested and absorbed. As a digested meal is absorbed, it appears in the blood as glucose, triglycerides & amino acids. These then disappear from the blood due to burning and storage. See Extended effects of evening meal carbohydrate-to-fat ratio on fasting and postprandial substrate metabolism.
Figure 1 shows the effects of a 100g Oral Glucose load or a 40g Oral Fat load on blood glucose level over a period of 360 minutes. Note that subjects are resting during the 360 minutes. As the 100g Oral Glucose load produces a large insulin response (See Figure 2), fat-burning temporarily stops. Therefore, the ~1kcal/minute resting burning rate is derived 100% from carbohydrate. Therefore, the carbohydrate-burning rate is ~0.25g/min. At this rate, it would take ~400 minutes to burn 100g of glucose. However, it actually takes ~180 minutes for blood glucose level to fall from maximum to minimum. Therefore, some of the glucose from the Oral Glucose load is stored (mostly as glycogen in muscles and liver).
Figure 3B shows the effects of a 100g Oral Glucose load or a 40g Oral Fat load on blood triglyceride (fat) level over a period of 360 minutes. Note that subjects are resting during the 360 minutes. As the 40g Oral Fat load produces no significant insulin response (See Figure 2), fat-burning is unaffected. Therefore, the fat-burning rate is ~0.11g/min. At this rate, it would take ~364 minutes to burn 40g of fat. However, it actually takes 180 to 240 minutes for blood triglyceride (fat) level to fall from maximum to minimum. Therefore, some of the fat from the Oral Fat load is stored (as fat in adipocytes), even though there is no significant insulin response.
Therefore there are times when stuff is stored (anabolism) and there are times when stuff is withdrawn from stores (catabolism). If more stuff is stored than is withdrawn over a period of time, weight goes up, and vice-versa.
After doing intense exercise e.g. sprinting, resistance training with weights etc, muscles become very sensitive to insulin. Therefore, if intense exercise is done just before stuff is stored, amino acids & glucose are preferentially stored in muscles rather than adipocytes. This increases muscle mass relative to fat mass.
If non-intense exercise is done at times when stuff is withdrawn from stores, this maximises the amount of fat withdrawn from adipocytes and minimises the amount of amino acids withdrawn from muscles. This decreases fat mass relative to muscle mass.
It's therefore possible to increase muscle mass at certain times and decrease fat mass at other times, while keeping overall mass relatively constant i.e. it's possible to gain muscle and lose body-fat without being in an overall caloric deficit. See Body Recomposition.