Please remember that a “calorie” is a unit of measurement and nothing more.
We do not eat calories… we eat food.
Food can have many different effects on our body and hormones
Weight loss and gain are more influenced by hormones than “calories”.
When your body becomes a bomb calorimeter then you can start talking about “calories”.
@Wotters … From your original post.
I respect your posing the question as I assume, like everyone else around here, you’re interested in learning more and exploring the research (as far as it goes).
It doesn’t appear that the responses provided above are leading you to reconsider your underlying assumptions. Which leaves me confused as to why you’ve bothered eating a ketogenic diet for the past 5+ years in the first place if you’re truly convinced that the composition of what you eat doesn’t matter.
Since I’m not sure what else I can add to this discussion, I’ll excuse myself.
He may not actually have lost fat at the same rate as someone who lost the same amount of weight on a keto diet. Studies have shown that even though people tend to lose about the same amount of weight over the same time periods, those on very low carb diets will lose more fat and retain lean mass, while those on simple calorie reduction/low-fat diets will lose more muscle.
Sorry @Wotters - you’ve provided a pretty clear message with the sum of your responses…
https://unlcms.unl.edu/engineering/james-hanson/trolls-and-their-impact-social-media
My n=1 having done both low calorie and keto diets.
I lost weight on both.
On low calorie diets I was hungry and tired all the time and my cravings never went away.
On keto after 4-6 weeks my energy level went up, my appetite so much less that I skipped meals because I forgot to eat, my aches and pains were significantly reduced and I felt better at 60 than I did at 40 or 50.
Wound I ever try a low calorie to lose weight again HELL NO.
My symptoms above are what we refer to when we mean fat-adapted. No way to measure it. No studies. Sorry.
If your friend is able to lose the weight on a low calories diet AND KEEP IT OFF FOR AT LEAST A YEAR and has no other symptoms of inflammation then good for him. No need for him to do anything different until it quits working for him
I think there is also the issue of muscle retention/preservation? Someone step in with the more specific science if you know it off the top of your head, but this is my understanding based on what I have watched and read.
So when fat-adapted instead of burning your muscle for fuel when you run out of carbs to burn, you will burn fat instead. My understanding is the metabolic process where we burn carbs would have us burning muscle for energy in the absence of carbs. When we become fat adapted, it means we start preferring fat and our bodies will no longer try to burn carbs or muscle, but will burn fat primarily before finding another source. Introduce too many carbs again in a consistent fashion and your body will switch back again.
I think about Edward Norton of all things when this comes up. When he did American History X he was super muscley. Then he turned right around and did Fight Club. To be the scrawny guy we saw in fight club he had to lose a lot of that muscle weight. He ate very little and did a lot of running and burned off all the muscle. If he had been fat adapted it may have taken longer? I think keto is muscle preserving in this way. You eat enough protein to preserve your muscle and then train your body to run on fat. When on a higher carb diet and you calorie restrict to lose weight, you body burns carbs, then muscle, then fat. When I did Weight Watchers years ago, I could see a noticeable difference in my strength levels and the composition of my body. I lost muscle, and when I put weight back on, its more fatty than before. There no muscle holding things up and together anymore.
Someone else chime in, like I said, if you know more of the technical explanation, but this is my general understanding of the science behind it.
This right here is the most comprehensive explanation I have seen about why it can take a while to become fat adapted and why energy can be low during that time. It confused me before I became fat adapted why I was eating well, feeling great otherwise, but didn’t have any energy.
To answer the OP original question technically: ‘Fat adaptation’ is the gradual replacement of the glycolitic energy state by the oxidative energy state. As others have already stated, in practical terms that means in the absence of sufficient carb intake to replenish glycogen, muscles learn to utilize ketones and fatty acids to function, even at intense and high levels of activity. For a pretty good explanation of how this works watch the video at the top of the following link:
PS @Wotters cite some sources instead of just claiming “that’s not true” to every response.
How old is he?
Statistics always allow for exceptions, ie 1 - 5% of people will maintain Weight loss in the long term on a low calorie diet. I wish him well!
You mean you started this debate with nothing but your anecdotal story about your brother? You claim to be firmly in the low carb camp yet all you do is say everything isn’t true? Yes, you can loose weight by calorie deprivation and nutritional absence. It was widely proved in Germany and Poland during WWII. Was it healthy, was it the same thing as eating a ketogenic muscle sparing diet? I am not going to answer anymore because you conveniently say everyone is wrong and you are right without anything to back it up. I realize you’ve been here a while but this is purely trolling and you have no desire for discussion of anything or acknowledging any of the many relevant posts with real answers including my own. This isn’t a discussion it’s rabble rousing. 
If we keep feeding @Wotters we’ll continue to get:
and he/she is completely ignoring all input around the title of this post.
Just the fact that he/she keeps saying everyone is wrong carries an implication that he/she knows what is right (so why ask).
People, there are many legitimate posts out there with people that have true concerns and issues that would love some of the above positive Ketogenic Forums bandwidth.
Yes. I will bow out of this gaggle. There is apparent trolling afoot. Good day folks.
Don’t forget that that enzyme that is needed for hepatic de novo lipogenesis is down-regulated when you’re on a ketogenic diet, and is slow to re-express. I would say you’re fat adapted once that enzyme is down-regulated.