I'm 5'10" 190 pounds, wanting to lose 15 pounds. (I have lost 7 pounds in my first 2 months of paleo).
I'm not diabetic, and I presume insulin is behaving normally.
I'm on a low-ish carb paleo (maybe 100g), where the 100g is primarily from fruit.
I presume while sleeping, and in-between meals, my insulin falls and I burn stored body fat.
If that's the case, is there any reason I would try to achieve ketosis?
I remember from 20 years ago when I tried the atkins diet, that ketosis breath is terrible!
Any & all thoughts are welcome as I'm trying really hard to understand this ketosis thing.
I have read the Taubes book, and browsed through the archives here and am learning a bunch!
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Short answer: no.
Long answer: There might be some benefit to spending some time in ketosis so that your body is better keto-adapted, should you ever find the need to spend some long period of time without access to a lot of carbs. Think of it like lifting weights - you're stressing yourself to produce an adaptation that might be beneficial. If you go on long hikes or backpacking or something like that, it's nice to not have to go through carbflu.
If your goal is to lose bodyfat, and you're down to the last couple of pounds, you'll either need to be patient and just let it work itself out over a period of months or years, or you'll need to spend some more effort investigating more involved interventions than a simple paleo diet shift. Not saying that paleo is bad for this, mind you, but it only gives you a framework within which to eat healthy, not a framework designed to minimize bodyfat.
Constant ketosis is not necessary to lose bodyfat and you will, in fact, burn stored bodyfat when not in ketosis. So your intuition here is correct: you don't NEED to be very low carb to lose fat. It might help in some situations and some of those more involved interventions I mentioned will utilize it, but it's not absolutely required.
Paleo isn't defined by macronutrient ratios. You can eat any level of carbohydrate that works for you. Ketogenic and high protein diets work primarily by blunting hunger. if you have hunger/appetite issues (tend to overeat), then a ketogenic diet might be helpful. Emphasis on might. If you are losing weight just eating paleo, then rock on and don't worry about it.
GCBC is a great book, but Taubes Carbohydrate-Insulin hypothesis of obesity is probably oversimplified and possibly just incorrect. (That doesn't mean that people don't fatten on refined flour and sugar - they do - it's just his explanation of the mechanism leaves something to be desired.)
There are reasons to avoid prolonged ketosis as well, many people end up with cold hands/feet, which could be indicative of a downregulated metabolism (thyroid). There are other reasons as well. There are also people who eat keto long term with no apparent side effects. But you shouldn't feel like you HAVE to be in ketosis to lose weight.
In general terms, the natural way to lose weight is to burn it off over winter when there's not much food around. Of course it's also natural to spend most of the time curled up round a fire sleeping. Still, in practical terms, it can be very hard to persuade the body to release fat stores when there's food constantly available. Fasting makes for an effective aid to weight loss for many people, and it's a lot easier to extend fasts when your body is used to running in ketosis.
Yes you burn fat while you sleep (and all the rest of the time). You also store it whenever you eat. Most advice is centered around tricking your body into more of the former than the latter without turning you into a grumpy sloth fighting off constant cravings. Ketosis is simply the natural state for that. Natural isn't always best.
Ketosis is not necessary to burn body fat. Many people have reduced body fat without being VLC. I think ketosis can suppress appetite which could be helpful. And obviously, to be in ketosis means you're cutting out processed carbs (which account for much of the processed food we eat in general). Everyone here would agree that cutting processed foods will help you burn fat (even if we don't all agree on the exact mechanism by which this works). But of course, you can also cut out processed foods without going VLC.
Also, I think we are all constantly burning and storing fat. It's more of a question of the ratio at which that is happening.