Hey @Zenjen! What you shared that your sister and the doctor said does not give substantial evidence as to say that keto is dangerous. I hope your sister is getting some blood and other tests done to try and see what’s really going on. There is nothing innately dangerous about keto – quite the opposite: it innately removes a lot of foods that have been proven to be dangerous. If it helps, I’ve been eating ketogenically for over 3 years and am doing really well.
There are so many variations within ketogenic eating based on genetics and physiology. For example, I can pack back the cream and eggs on keto while others find out they have an allergy to it and get sick, or, I found I need more active folate due to a genetic mutation, while others can function just fine on less of certain kinds of vegetables. Feeling bad and getting sick while on keto does not mean that keto is dangerous, but may be a good sign that something you are (or are not) eating is not right for you, and needs to be investigated.
If you are concerned, I would suggest you work with your doctor to get blood tests done, and make the way you eat a life-time experiment to see what is best for you. Your health and future are worth a deeper and personal investigation.
Is there anything within ketogenic eating that you are specifically worried about as dangerous?

