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PeerGynt comments on How I Lost 100 Pounds Using TDT - Less Wrong
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By which mechanism do these nutrients get excreted? Urine? Bile? Non-absorption?
My impression is that carbohydrates in urine is something that we only see to a significant extent when blood glucose concentration is at diabetic levels. Protein and fat in urine occurs, but it doesn't seem to me that this happens to an extent where it can make a difference to the total energy picture
I don't think excreting them through bile would work, the nutrients would just be reabsorbed further down the gastrointestinal tract.
It is possible that at very high intake levels, there is significant non-absorption of fat. Maybe this happens in competitive eaters, but I am not convinced it can explain much at fat intake levels seen in ordinary people..
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1793018/
(Bam.) So, under normal conditions you're pretty efficient (~4% of your calories just get pooped back out), meaning that something like metabolic rate just swamps poop-energy-content as an interpersonal variable.
This article says that there is some non-absorption of fat in healthy people, and much greater non-absorption in people with cystic fibrosis.
If you want to convince me that I should consider this when I choose the fat/carbohydrate/protein content of my diet, you would have to make an argument that the percentage of fat that is not absorbed is a function of my diet, ie, causally related to what I choose to eat.
I'm not saying this is not theoretically possible, but my intuition tells me that the variation in absorbtion that is caused by diet, is unlikely to have a major impact in the final analysis
So, Tim Ferris has done a couple of demonstrations where he ate about 20,000 calories in the course of 24 hours. The vast majority of that is not absorbed.
You may have had in mind the limited claim that macronutrient ratio has a small effect on the percentage of calories absorbed, which seems reasonable for normal macronutrient ratios, but quantity seems important, as well as more detailed chemical composition. For example, I don't produce enough lactase to digest normal American quantities of milk consumption without chemical assistance, and so if I continued to drink a glass of milk each day, the amount of calories that made it into my bloodstream would be predictably lower than the amount of calories put into my mouth.
So while the CI calculation can be complex, it seems obvious to me that the amount of calories you put in your mouth is a good upper bound. (I don't think this is seriously contested by anyone, but it's worthwhile to establish that it's not seriously contested.) The system dynamics may mean that in some cases a higher total number of calories in leads to a lower maintenance weight, and so just lowering intake is not always the right solution.