Hi Peter
This is (once again) the NHS talking cobblers!
When you consume insufficient carbs to power your body, it swaps to using fat. This is called Ketosis and you get ketones in your urine as a by product. Ill informed professionals get this muddled with Ketoacidosis, which is very dangerous. Ketosis isn’t! You can check for ketones by using Ketostix to test your urine – ketones mean the diet is working!
Hope this helps
TP