Well there's a bit more to it than what you're asking but in general long term low carb dieting, without any carb days isn't benefical, at least if you are a person who is actively engaging in weight lifting. Atkins diet has no carb ups so you eat low carbs indefinately. Unfortunately unless you're a couch potato this isn't the best route. Weight training requires glycogen and glycogen comes in the form of carbs. Protein can be converted to glycogen in certain circumstances, but for sake of argument we'll keep it simple.
If you want to weight train with any intensity then a carb up every week is recommended. Thus you are "cycling" as you put it. 5-6 days of low carbs followed by a 1-2 day of high carbs to refill glycogen stores. You can however do a shorter cycle in which you have 2 seperate carb ups a week. Personally I'd choose the latter due to hormonal benefits. There are many options in this case, you can do 3 days of low carbs followed by a 1 day carb up and just repeat this indefinately. Or you can switch up low carb days from 3-4 and alternate the cycles.
As far as your other concerns, low carbs diets don't have any special effect on slowing your metabolism as far as i know. Any diet, regardless of carb makeup will slow down your metabolism down eventually. Here are a few points to consider when low carbing:
- There hasn't been much research on the effects of long term ketogenic diets. So long as you are doing a CKD with carb ups then I don't think you have anything to worry about. Going Atkin's style where you low carb at all times may be a different matter. People will claim up and down that they've been on it for years without problems, but *years* isn't necessarily long term. If you smoke for 5 years straight and then quit you may never suffer any consequences from it. This doesn't mean continuing in that manner for the rest of your life will be safe. I'm not saying a long term keto diet is dangerous, only that there isn't enough research out there to really make any conclusions. My guess is that without enough fiber in your diet you could suffer from colon problems later in life. You could counter this with a colon supplement but do you really want to have to take this the rest of your life?
- CKDs are benefical even if you aren't a weight lifter. The hormone leptin which has been found to contribute greatly to weight loss is triggered almost exclusively by carbs. To not eat any carbs for long periods of time will ensure that leptin levels get low and your fat loss will be tougher. By incorporating a 1 day carb up every week you can make sure your leptin levels are optimal without slowing fat loss. In fact I'd bet that fat loss in the long run will be greater with this method.
- Last but not least, low carb shouldn't mean no carbs. A ketogenic diet isn't the only low carb choice, it is the most extreme. Limiting carbs to 30% of your daily calories is by no means extreme, however it is still considered low compared with the amount of fat and protein you consume. For most people ANY kind of carb restriction is a good thing. The average human probably gets 70% of their diet or more in carbs so cutting that in half alone would make a huge difference.
My suggestion is to begin your low carb experimentation with a simple restricted carb diet where you simply drop your intake well below it's current level. Play with different amounts and see how your body reacts. Most of the population is fully capable of getting to single digit bodyfat %s without a ketogenic diet, although most will swear up and down that they're in the minority that can't. On a similar note I think 90% of the population truly believe that they are hard gainers with shitty genetics because they can't put on muscle as fast as Jimmie Joebob. I'd guess most of these people would make dramatic improvements by simply sticking to a well designed program and not following Ronnie Coleman's mass building routine in the newest edition of Flex. If you don't believe me just scan the boards and read how many people claim "I can't lose fat unless I cut out all carbs!" or "I've been lifting for 5 years and I have only put on 10lbs of muscle, I must be a hardgainer!"
Dieting isn't easy and if you don't know what you're doing you can waste a lot of time. Ketogenic diets are extremely simple and therefore people find them easy to follow and thus get results. If people would follow a properly designed "normal" diet they could get results as well but most can't do this long term.