I heard a lot about a diet to eat only food like meat or fish and strongly recommended not to eat fruit or bread or potatoes anb so on. is it healthy?

Low carbohydrate : You might be referring to an atkins-type or low-carbohydrate diet. People do lose weight with this approach, but it may not be sustainable as a whole-life practice. In reality, you need lean protein, plenty of vegetables & fruits, healthy fats and cut down on processed foods like sweets, breads, etc. If you are insulin-resistant, you want any carbohydrates to be complex, & more slowly absorbed.