Pre diabetic and fasting while on Metformin

Well, I'm not sure of the wisdom of a five day fast in general, and it probably won't affect the A1C more than marginally because A1C measures an average of your last 90 days (plus or minus) of blood glucose, but it might be good for one or two tenths, maybe.

I'd be curious just to see your A1C after the diet (and metformin).

Or is the fasting something you've done periodically for a long time?

One other thing I don't see you mention and that's exercise. Doctors seldom give that the emphasis they should. And the exercise can be very modest, mine is usually walking, and my standard is just a one-mile walk, that's basically two blocks up and three blocks over, and back. Do that before lunch and before dinner (even before breakfast, if you can manage it, LOL). If your blood glucose is already well controlled this should improve things several tenths on the A1C, and if things are poorly controlled it will help get them well controlled, could be a full point or more on the A1C!

Actually that brings up a question - are you taking your blood glucose readings at home? If you are, you should be able to guess your likely A1C with pretty good accuracy! If you're not, I suggest you start, frankly I now recommend that to people who aren't even diabetic or pre-diabetic, it gives you such a great view of what's going on it helps you keep to your diet, and even shows what you can cheat with without doing much damage!

@uvdogz welcome to Mayo Clinic Connect. Congratulations on making the choice to tend to your health, diet and exercise! I’m sorry about what happened with your A1C going from 6.9 to 6.1 to 8.4! I hate it when that happens. It can be frustrating to be doing everything correctly (as far as you know) and then see bad results. Please don’t give up. This reminds me of some over used cliches: “things have to get worse before they get better”, “always darkest before the dawn” or as my grandmother used to say “sometimes you have to make a mess in order to clean up a mess”.
I agree with @carbcounter that exercise is an important piece of the plan. It is also helpful to have a system to check your blood sugars and keep records of them. There are lots of devices and software to help you do this. Ask your provider, a diabetic educator or pharmacist for recommendations.
We members are here to give hope and encouragement based on our own health journey experiences. We shouldn’t be diagnosing or prescribing. Will you update us on how you are coming along?

You need to do everything possible NOW -- Prediabetes is a strong warning -- who wants a life of blindness, amputation, and lifetime use of metformin?

You're absolutely right to get rid of Every extra pound --- why would you want to carry weight that (unlike mine, condemned by gods to carry!) is not just useless burden, it's worse: It a symptom of early onset of a disease you don't want your enemies have.
Also equally -- and Simultaneously -- important is physical activity. If my 82 years any help, I carry no extra weight -- I can tell myself my waist-to-hip ratio is about 0.8. Weight alone won't tell you much,you need BMI, The healthy range is under 25. Exactly what is Right-for-You your body will tell you: It will reach Optimal shape-and strength/flexibility.

To lose Anyone person to its clutches is no fun ... and too much trouble.
I wish you embrace the vitality and life-with-purpose and joy that awaits you.
Let's hear from you!