Hm, yes, I can see your concern. Your fasting glucose is still within normal range. Your Hba1ac at 5.8 is also within normal. I wonder if they are assuming you are going to move past prediabetes to diabetes but am not sure I'd want to be treated just yet either. 800 calories a day hardly seems sustainable. Are you male or female? Some women that have PCOS who gain weight and have other issues are sometimes prescribed Metformin. But that would obviously only apply if you are female and surely your doctor would discuss that with you. I think you need to talk to your doctor to get a full picture of why he thinks the med is necessary.
Хуже не будет. Принимайте. translated to, it's not going to get any worse, take.