It’s ok to assume that you absorb 100% of what you eat, unless you see evidence to the contrary, and no corn kernel poop doesn’t count. Frequent diarrhea, weight loss, skin rash, and basically any symptom of vitamin or mineral deficiency.
That's not really true. If you've ever done the keto diet, you know that your body expels unburned ketones through your breath, sweat, and urine. Protein can be used to repair structures rather than burned or stored for energy.
There's also something called the "thermic effect of feeding". Your body requires more energy to process protein (20-30% of calories consumed) than it does carbs (5-10%) than it does fats (0-3%).
In other words, "calories out".
> It’s ok to assume that you absorb 100% of what you eat
Your gut absorbed it. That is literally what digestive absorption is about. Expelling ketones is not digestive malabsorption.
I don't think it's unreasonable to think that different bodies absorb food in different ways (or proportions), particularly given what we've seen about the gut microbiome
Apparently even just changing the time of food intake can affect obesity (in rats) if this study is to be believed: https://pubmed.ncbi.nlm.nih.gov/22608008/
In response to, "but those are rats", I think it's a lot easier to cast doubt on "100% of food is always absorbed" vs "I don't think that always holds true"
I mean, heck: if there are no residual calories in human waste, how can it burn?
About the rat thing: the cico hypothesis point of view might look at whether meal timing affecting energy expenditure first, rather than assuming meal timing change digestive absorption.
There is not much point in getting in the weeds about how much you absorb, unless you're running trials on yourself like changing when you eat, or what you eat, and leaving all other things equal like calorie intake and expenditure.
The best dieting strategies I've seen track calories in and weight change. From their you derive calorie expenditure, and it really doesn't matter if you burned it or pooped it out, does it?
https://www.latimes.com/science/story/2020-02-04/calorie-cou...
Being wrong about the number of calories in almonds doesn’t count as evidence that skinny people are skinny because they poop out undigested calories.
Also, I’m not saying digestive malabsorption is impossible, just that you shouldn’t assume it unless you have strong evidence to the contrary that doesn’t have another simpler explanation.