The several testimonies about the efficacy of the keto diet (at least those I have read) recount regaling tales of how several amounts of kilograms were burned in such short time. Yes, many of them are true, probably all of them are. However, what many people fail to remember is that there simply cannot be any gain (or weight loss in this case) without some pain.
One of the common pains of shifting to a keto diet is what has been termed the keto flu. Very simply, the keto is just like any other flu with symptoms including headaches, nausea, upset stomach, Lack of mental clarity (brain fog), sleepiness, fatigue which usually within the first few days of starting the keto diet.
For most, the symptoms last for just a few days but they could last for up a week with the symptoms becoming very severe. The keto flu is usually caused by the transition by your body from carbohydrates as its main fuel source to fats. Enzymes required to burn fats, for example, are different than those needed to burn fats. So the body needs to adapt to the change. The process is usually called keto adaptation.
Drink water a lot so that you can remain hydrated as this is the main issue with the keto switch. In addition, always watch your electrolyte level. Fluids lost means electrolytes lost so try to beef up your intake of foods containing electrolytes like sodium, magnesium and potassium.
Be sure to take as much fat as possible as this will hasten the transition and make keto adaptation faster. Keep protein level at a minimum as the body can transform protein to glucose and defeat the whole aim of jumping on a keto diet.
All said and done, keto flu should not be so much of a problem and should disappear within a few days. Consider it as part of the pains to achieve the gains. However, if the symptoms become extreme or last longer than a week, please see a doctor.