We’ve heard anecdotal evidence of this but I haven’t seen any science. Does it happen at all? To what extent? Is there a level of ketosis where more excess fat is excreted than stored? What’s the mechanism? @DaveKeto? @erdoke? @thefatemperor?
Excreting excess dietary fat when in ketosis
DaveKeto
(Dave)
#2
I haven’t actually done that much research into the literature just yet, so I can’t speak to what others have already discovered.
But I do feel personally confident there is a 400-600 calorie difference between my previous intake and my current one to maintain weight. So whether it is a rise in basal metabolic rate, excretion, or some other means of offloading, I do feel something is happening that is acting as a limiter of some fashion. (At least for me)
Naturally, the scientist in me would love to have a special… uh… john that would track… output and its composition. But as I mentioned before, I’m probably already pushing it with the Mrs with how much I do already. 
carl
(carl)
#3
I think this is a really good opportunity to do some science that nobody else wants to do for obvious reasons. If in fact it turns out that a certain percentage of excess dietary fat gets excreted under ketosis, and we can find the equation, that’s a huge win and could change the dialog.
Donna
(Meeping up the Science!)
#5
To my knowledge, the only reason the human body excretes fat to a very large degree is due to intestinal malabsorption or incomplete digestion. Otherwise, my assumption is that it is all processed. Diseases like Celiac and some infections can cause the inability to absorb macronutrients, as well as some types of surgery. Fat and protein both slow down gastric emptying and motility, though, so what’s far more likely anatomy-wise is that this occurs so as much as digested as possible. It would still be interesting to check.
What would be interesting to test would be excretions of carbon dioxide from the lungs, as most fat is excreted from the body via this route after it’s broken down. I forget, but I think it’s > 75% or so? Gabor would likely remember.
amber
(L. Amber O'Hearn)
#6
Yes, I would guess the difference is in metabolic rate, “futile cycles”, uncoupling, etc.
cbingel
(Christopher Bingel)
#7
If there’s a test, let me know. I’ve upped my calorie/quantity intake in order to lower my triglycerides for blood work on Friday. Mostly as an avoidance for the nagging he usually gives me, but also as a potential conversation starter.
jmbundy
(John)
#8
I don’t know about keto, but messing up my friends couch on Ally was an awkward conversation.
richard
(Richard Morris)
#10
Normal fat absorption rates are indicated by a fecal fat test under 7g of fat from 100g intake. So it’s apparently normal to not absorb all of the fat.
I don’t believe that a ketogenic diet will specifically cause an increase in fecal fat, but I suspect a high fat diet will because there will be an upper limit to how much you can absorb and process.
I suspect Amber has the right of it as far as increased energy USE when in a low insulin state. Namely futile cycling (converting glucose into pyruvate and back into glucose over and over to generate heat) but also energetic processes optional to short term survival being undertaken when energy is apparently abundant.
We have a prodigious ability to burn more energy instead of storing it.
Researcher Ethan Sims once tried to increase the body fat of volunteer prisoners by one quarter by over feeding them. One guy he could not get over an 18% increase even with a 10,000 kCal/day intake. And as soon as the overfeeding phase finished he lost all appetite and lost weight dramatically until he got close to his starting weight - and then he spontaneously resumed a normal 2000 kCal/day diet.
Interestingly 2 of his prisoners who WERE able to easily gain weight, had hard trouble taking it off. Both had a family history of diabetes.
richard
(Richard Morris)
#11
0.70 Respiratory Quotient (VO2/VCO2) means you are burning only fat. But that doesn’t tell you how much fat you absorb.