Originally Posted by Greg Condon
so i guess my basic question is...does your body always have to process every calorie that enters the body? does it just ramp up the process a bit if you overload it and then just tack on the extra if it can't handle it all?
I don't know about people 900 lb+ and consuming 30,000 calories/day...eating that much food in a day would be a herculean task in chewing and swallowing alone.
As for your body processing food calories, well I just happened to be reviewing my thermodynamics textbook yesterday:
...the average energy contents of the three basic food groups are determined by bomb calorimeter measurements to be 18.0 MJ/kg (4.3 Calories/g) for carbohydrates, 22.2 MJ/kg (5.3 Calories/g) for proteins, and 39.8 MJ/kg (9.5 Calories/g) for fats. These food groups are not entirely metabolized in the human body, however. The fraction of metabolized energy contents are 95.5 percent for carbohydrates, 77.5 percent for proteins, and 97.7 percent for fats. That is, the fats we eat are almost entirely metabolized in the body but close to one quarter of the protein we eat is discarded from the body unburned. This corresponds to 4.1 Calories/g for proteins and carbohydrates and 9.3 Calories/g for fat commonly seen in nutrition books and on food labels. The energy contents of the foods we normally eat are much lower than the values above because of the large water content (water adds bulk to the food but it cannot be metabolized or burned, and thus it has no energy value). Most vegetables, fruits, and meats, for example, are mostly water. The average metabolizable energy contents of the three basic food groups are 4.2 MJ/kg (1.0 Calories/g) for carbohydrates, 8.4 MJ/kg (2.0 Calories/g) for proteins, and 33.11 MJ/kg (7.9 Calories/g) for fats. Note that 1 kg of natural fat contains almost 8 times the metabolizable energy energy of 1 kg of natural carbohydrates. Thus, a person who fills his stomach with fatty foods is consuming much more energy than a person who fills his stomach with carbohydrates such as bread or rice.
Theoretically, if you measured and weighed everything you ate very carefully and knew the exact proportions of Carbs:Protein:Fat of it all, you could calculate exactly how much energy you consumed
, and this is what websites like FitDay and TheDailyPlate aim to do. But you would still be largely in the dark as to what your body does with that energy. That is, you would still know nothing of the hormonal and metabolic processes that determine whether the energy consumed will be stored as fat, excreted, or burned through physical activity and heat dissipation. And there is more to it than simply your Basal Metabolic Rate (BMR) and work, exercise, etc., because the proportion and particular type of macronutrients you consume also has an effect on the energy you burn, and some suggest even on your propensity to perform physical activity following meals (e.g. restlessness).
There are some limits on how much energy can be converted to fat mass in a typical 24 hour period, although I imagine it would be quite difficult and painful to even approach those limits. Your body wants to maintain homeostasis, i.e. its current "normal" state of operation, and any attempt to sway it either direction will be met with resistance. Hence, the difficulty for overweight/obese people to lose weight, and for skinny people who dream of muscle mag covers to pack on pounds.