Does "someone" have any information on that?

I know a number of people online who have reversed type II diabetes or insulin resistance by eating this way, for years. Or who experience less seizures or migraine. Whatever undesirable side effects of eating very low carb there might be, and I'm not claiming that there actually are any, they have to be weighed against benefits that are often quite substantial.

I can't speak to the person's claims without knowing what they're specifically claiming. There's a fair amount of nonsense out there. One claim is low carb, equals low leptin. This is sort of true. Lower fat mass means lower leptin. Especially if you're very lean. In obese people who've lost some weight and still have a bit to go--not so much. You can increase leptin by eating carbohydrate. Studies show insulin signalling is important to this--because the increase of leptin is dependent on the storage of fat in fat tissue. Stay fat, and you'll have increased resistance to getting fatter. I can see at least one problem with this strategy in the war against obesity.

More common is, oh, it's bad for your thyroid. There are like one and a half studies looking at the effect of eating carbohydrate versus fat or protein on thyroid levels.

Okay, make that two and a half. I've seen at least one carbohydrate overfeeding study, they wanted to see the capacity for de novo fatty acid synthesis in humans, so they fed thousands of extra carbohydrate calories, mostly as sugar. Thyroid hormone went up. Hooray for Ray Peat. Fasting, like low carb, can decrease t3 and increase reverse t3. Let's see, overeating sugar drives t3 up, fasting does the opposite. Fasting, we run mostly on fat and a bit of protein. Sounds like low carb. Sounds like maybe we need a bit more thyroid--or we're resistant to its effect or something--with increased carbs, or at least increased sugar.

Another claim I've seen is you need carbs versus stress. Cortisol and other stress hormones work to increase availability of energy--glycogen breakdown, lipolysis, both increase. Focusing on the glycogen/supporting a higher blood glucose part of the equation, if I understand the argument, increase carbohydrate coming in from exogenous sources, and you won't need an increase in stress hormones to provide it from endogenous stores--or maybe, if glycogen stores are higher, then it doesn't take as much of a stress response to dig into those stores. Okay--but characterizing stress hormones as just bringing up glucose levels is faulty, that isn't all that stress hormones do. Their effect depends on your metabolic state going in--if your carbohydrate adapted, and have high glycogen stores, glucose from glycogen might provide the feedback, that energy's available to run from the lion, or whatever. Low glycogen? You might get a similar stress response--but the needs are satisfied more by an increase in free fatty acids and ketones and lactic acid produced by muscle during some necessary sprinting, and less by liver glycogen.

Bipolars keep popping up saying their symptoms, and overall stress levels are lower, not higher, on a ketogenic plan. I tend to give that claim maybe more merit than necessary, because I live in it.