Regarding long term side effects, the truth is that there isn't much research around it. One of the biggest worries would be dyslipidemia because of the very high fat intake. The research I have read showed that indeed in some cases there was an increase in the total cholesterol but in most of these cases, the increase was because of a higher HDL. When ketogenic diets are followed by obese people, their lipid profiles are actually improved. I haven't seen a study yet linking Ketogenic diets to increased risk of cardiovascular disease. Serious side effects can also happen in people with Type 1 Diabetes. These people should only do such diets under strict medical supervision because low-carb diets put them in a higher risk for ketoacidosis.

Now in the short term, once you start the keto diet, your body will first deplete all of it's glycogen storage. Once that happens, most people will go through a phase called ''keto flu''. You feel weaker, dizzy, your mood is low, and you will probably get headaches. This is actually normal as the body makes the transition from sugar to fat. I have been on and off the keto diet for some years now. The worst ''keto flu'' I went through was when I attempted the diet for the first time. It lasted for about 7 days and I felt I had no energy at all. But once it goes away, you return back to normal. I have noticed that in all subsequent times of re-starting the diet, the ''keto flu'' lasted for fewer days and the symptoms were much more mild. Even though the body starts to produce ketone bodies quickly, to become fully keto adapted may take months. That's when you truly start to enjoy the benefits of burning fat as your main fuel. It's important to make sure that you are eating enough fat. Most people are afraid of fat and therefore they end up eating too much protein which may result in gluconeogenesis. Also it's important to give your body enough electrolytes, especially during the keto flu. A bit more salt on the food and some magnesium tablets do the job.