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My friend and I were having a discussion over how "efficient" human digestion is. If a human ate a 1000 calorie hamburger, how many of those calories (how much energy) does the body process into usable energy (i.e. fat or other stores). I've heard things like "it's better to workout on an empty stomach because you will burn fat instead of energy from the food you just ate" - so from this I presume that storing energy as fat is less efficient than using the energy from digestion immediately (though in all honesty I'm not sure if that even makes sense.)

So, to summarize: how many calories of food does your body turn into usable calories via digestions? Percentages would be nice.

Bonus question: If the body does not process at 100%, does the FDA take this into account when labeling food products on their calorie content? If my Basal Metabolic Rate is 1800 calories a day, do I need to eat more than that since my body won't process all of the calories into usable energy?

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    $\begingroup$ The calories listed on nutrition labels are calculated so that they are representative of the amount of "digestible energy" - that is the amount of energy that actually enters the blood stream. $\endgroup$ – canadianer Sep 8 '14 at 0:24
  • $\begingroup$ Different foods are diested very differently, too. Richard Wrangham's "Catching Fire" goes into some of the research on the topic. $\endgroup$ – Amory Sep 8 '14 at 1:19
  • $\begingroup$ Do you have a source for "digestible energy"? I thought they used bomb calorimeters to establish the number of kilocalories in an item, which would figure a raw amount. Do they calibrate the calorimeters for a certain % of utilization for a human? If so, that would be the % I would be looking for for "efficiency." $\endgroup$ – WannabeCoder Sep 8 '14 at 1:55
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    $\begingroup$ On nutrition labels, they determine the constituents (protein, fat, etc) by mass in the food and use a standardized factor to convert these masses into energy. They do this to address the concerns you raised in your question: that not all energy in food is available for metabolism. $\endgroup$ – canadianer Sep 8 '14 at 3:15
  • $\begingroup$ For example, see this: legislation.gov.uk/uksi/1996/1499/schedule/7/made $\endgroup$ – canadianer Sep 8 '14 at 3:23
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I suspect that what you are actually looking for is the following:

- 1 gram of fat = 9 kcal
- 1 gram of protein = 4 kcal
- 1 gram of sugar = 4 kcal
- 1 gram of alcohol = 7 kcal

Those are general and inexact values. They're just often used to give a rough idea of the amount of energy we get from different types of food.

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  • $\begingroup$ I was under the impression that sorbitol was able to be reduced to fructose through the action of sorbitol dehydrogenase and aldose reductase. I believe that is part of the reason sugar alcohols are not *great sugar replacements for those who have T2DM. $\endgroup$ – Little White Lithe Sep 15 '14 at 16:02
  • $\begingroup$ Also, you might look up enhanced nutrient shuttling in the gut from enterobacteria. Depending on the microbiome that's been developed some bacteria will shuttle more calories from the chyme(found typically in the chronically obese). The question the OP asks is multifaceted. A good answer (in my opinion) should touch on host nutritional factors, gut microbiome colonies, impact of nutrient cycling, and type of nutrients being metabolized. $\endgroup$ – Little White Lithe Sep 15 '14 at 16:15
  • $\begingroup$ @LittleWhiteLithe Hmmm... actually, my biochem is too far behind for me to confirm or infirm your statement. You are probably right, now that I think about it. So I'll edit in consequence. The first part of the answer still holds, though. It was destined to be extremely simplified, beacause I was under the impression that the OP was looking for such a "rule of thumb". However, if you prefer to go into details, please do. It will be instructive for all of us. $\endgroup$ – Raoul Sep 15 '14 at 19:37
  • $\begingroup$ Unfortunately, I was looking for more than a rule of thumb. The details are what interest me. This answer is certainly closer, but to use your example: how many Kcals does 1 gram of fat/protein/sugar/alcohol yield in a blast calorimeter? I can't believe it's the same number as the body produces from absorbing the food (and indeed, the fact that we produce physical waste proves that we do not turn 100% of it into energy). In a blast calorimeter (what I am lead to believe gives us the calorie counts we so rely on), all of the matter is burnt to find this number. $\endgroup$ – WannabeCoder Sep 23 '14 at 19:05
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A year and a half later, I stumbled upon this article by Ars Technica.

To summarize, research/experts acknowledge that the calorie system is a poor way to measure human metabolic performance. Gut fauna, calorie measuring device differences, and caloric differences produced by different levels of cooking all contribute to this system being unreliable.

To address the original question specifically: there isn't a clear answer or "rule of thumb", because it depends on far too many variables, many of which are directly attributable to the "calorie" being an inappropriate way to gauge food consumption and weight loss.

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