I've done the ketogenic diet for some months last year. Some of this is thus anecdotal and highly personal, your mileage may vary.
First I'll go with some actual objective stuff. The ketogenic diet, before becoming a health fad (not necessarily a bad thing) had been used to treat epilepsy. This means you can find some medical literature about it and there's clinical info out there. There are possible side-effects, including:
- Constipation. Although limiting digestible carbs does not have to exclude fiber, it does usually imply low fiber intake. You might want to take some fiber supplements.
- Increased blood acidity.
- Decreased blood sugar level.
- Raised blood lipid levels (cholesterol).
- Possible deficiency in some micronutrients.
- A higher risk for the formation of kidney stones.
Here's how my experiences/opinions correlate:
- Constipation: kind of. When I started the diet I ended up not, eh, having a good number 2 for ten days. Needless to say I was very worried. I went to the doctor who didn't find evidence of something serious like bowel obstruction and got prescribed a fiber supplement which helped considerably. What I found out is that if you eat mostly fat and protein, which can have a considerable absorption rate in the body, coupled with a significant caloric deficit and limited food volume, you can actually absorb frightening amounts of your food.
- Didn't notice something indicating blood acidity.
- Don't know about blood lipid levels. I never had it checked.
- (opinion) Deficiency in micronutrients could occur on any diet. That said, by almost eliminating one of the three macronutrients, there's a good chance your nutritional variety takes a hit. Try to eat varied and consider supplements.
- Fortunately didn't get any kidney stones.
In order to feel energetic enough, I think there's little you can do besides not making the caloric deficit too great. Slow and steady wins the race, too fast is just gonna lead to a crash. For the keto diet specifically, you will want to make sure you consume enough electrolytes (sodium, potassium, magnesium) and consume more water than usual, since you'll experience more frequent urination and possible dehydration.
As for easing into the diet, I do see frequent suggestions that you don't just jump into the deep end, but reduce carbohydrate intake gradually. Eventually, though, whether ketosis occurs or not will depend on whether the glycogen and glucose in your body becomes depleted enough for it to turn to ketones as its energy source. You'll have to reach a low level of carb intake and keep it there for some time before this happens. Doing this more gradually may reduce the profoundness of the discomfort commonly known as "keto flu" but might also make it last longer.
The medical literature shows evidence in favour and against the ketogenic diet. There's some amazing-sounding things (lowered LDL cholesterol, increased HDL cholesterol, lowered blood glucose, normalization of insulin response) and scary-sounding things (kidney stones, skeletal erosion). If you want my 2 cents... It's not worth it. In the end, what will lead to weight loss is a caloric deficit. The keto diet has a nice advantage of eating foods that provide good satiety and that makes it somewhat easier to adhere to (in my opinion, the secret to its success) but it is also very difficult to maintain due to how strict you must be in carb avoidance. Also, doing sports on minimal glucose levels sucks and cardio is a great way of increasing the caloric deficit. I'd say limiting carbs but not to the extent of keto will do you more good.