I started my first low carb "diet" late spring of this year. I had intentions of going full-on carnivore, but that's more restrictive than what I've done. I also eat cheese, bacon, some varieties of nuts, and low carb vegetables every now and then. My wife started doing this as well. I've lost 40 pounds. I haven't asked her what she's lost so far, but she's down a few clothing sizes and looks great.
I only went this route because low fat seems to no longer work for me like it did when I was younger. I used to be able to lose 5 pounds/week with low fat and exercise. Not anymore. Perhaps I have "insulin resistance" in my older age, not sure.
Other motivations for me were my blood pressure creeping up, and also being borderline "pre-diabetic" (according to the official stats, if you buy into the narrative on those things). Now my BP is down, A1C is down, and of course my fasting blood glucose is down. I'm no longer "pre-diabetic" even by health industry standards. Not sure about cholesterol, as my doc didn't request that on my last blood work. I suspect both LDL and HDL will be up, but whether that's really a concern or not is a whole other discussion.
If you do a lot of research online, you'll see many different reports of what people go through in the transition period. Things like lightheadedness, diarrhea, constapation, etc.
I had one day of diarrhea. That's it. Some people report a week of it. I didn't experience that. I had no constatpation at all. I think people mistake no longer going one or more times every day, to only going once every couple, or every few days, as being "constapation". That's not constapation.
I did have a period of a sort of "fogginess". Not sure I'd call it lightheadedness. Hard to describe. I could think clearly. I just felt...different. That was fairly short-lived and is gone now.
In terms of cravings, I really don't have any. My wife does have some, but she tends to fixate on things she can't have, while I do not. I just sat through two family Christmas dinners with the table loaded with carbs...filling, stuffing, potatoes, corn, bread, deserts, etc. Didn't crave any of them. I had no problem not eating those things. The only thing I really get a craving for (and it's only if I see it and smell it) is pizza. That and beer. When I see all those craft brew taps in the restaurant, I do have to divert my attention! Yuengling Flight at least allows me to have a beer every now and then, but let's face it...it kinda sucks if you're a beer lover. (Note...Straub Brite is a much better low carb option, but you'll almost certainly need to order it) But remember alcohol is converted to glucose, so go easy even on alcohol even with low carb options. An occasional drink is another reason I didn't go hard-core carniore.
As far as energy, I haven't had the great energy boom that others here and online have reported. My results aren't bad, though. I'd say I have a somewhat even energy. I don't get nearly as tired after eating like I used to with insulin spikes following a high carb meal. In my younger days when I ate a traditional "healthy", low fat diet, my energy would skyrocket. Compared to that, this doesn't seem to be nearly as good (for me, anyway). But again, that style diet no longer has the same effect for me that it once did, so a direct comparison may be apples to oranges now. My energy on this diet is a net positive from where I was immediately prior, but I can't say I had the huge increase others have experienced. I relate any increase in my energy more to no longer carrying around 40 extra pounds than I do as a direct result of the food I'm eating. That's just my opinion.
To sum it up, the upsides are 1.) It absolutely works 2.) You don't need to go hungry. I eat to satisfaction, and only eat twice a day now, which has a side benefit of intermittent fasting. I really could drop to one meal a day, and have done so on occasion, but that doesn't fit very easily into my wife's lifestyle and we still eat a dinner together. If you can eat one big meal at lunch time, or in the afternoon, it's easily doable as far as hunger is concerned. High fat food is very satiating, and you could definitely do just one meal. Believe me on this. I used to eat 5 times/day. If I can do it, anyone can.
The downsides have already been mentioned. It's very restrictive, and difficult to eat out, and even more difficult to eat out in a hurry. Not impossible, but difficult. You'll have few options on most restaurant menus, and you'll have to overlook probably 90% of the items on grocery store shelves. If you're the type that needs a lot of variety, it may not work for you very long. Then again, the results give you motivation, and most people don't really crave those other foods anymore.
I don't know if I can maintain a ketosis-level low carb diet for years to come, or not. For now I'm having no problems maintaining the lifestyle. Looking better, feeling better overall, and better blood work are powerful motivators. People have made eating an almost celebratory type of thing. If you look at eating more as a function of survival, and not for pleasure, perhaps it makes the restrctive nature of this lifestyle a little easier.
