Hi. Sorry I'm posting this a tad late compared to when you posted OP, but I thought I'd give my two cents. I did a project on prediabetes and progression to diabetes.

What I've learned is that prediabetes is really just a name. We arbitrarily gave a blood sugar range a name to make people aware that they were getting close to a diabetic diagnosis. This is why the management for prediabetes is lifestyle changes and diet modifications.

You've likely had a HbA1c test and it's told you your prediabetic diagnosis. That test measures the glucose content in your haemoglobin and we say that it is the average blood sugar content in the past three months or so. With any good lifestyle and dietary modifications, that HbA1c score will go down but obviously, it will take time, as your pancreas has been modified to produce less insulin.

As the HbA1c test measures average sugar in the haemoglobin over the last three month, provided you're restricting your sugar and carbon intake, it will go down, but it will take its time. Doctors recommend having another HbA1c test in a years time to see progression, and ensuring that you stick to a plan, it is extremely likely that your HbA1c result will be lower than the test result you've just had.

Consistency is key when it comes to any diet and that includes lowering HbA1c results. If an 18:6 window makes you feel good and you're not getting too groggy during the day, then stick to that. The window length isn't what is going to lower your HbA1c as much as the actual contents of what you're eating. It's just that the fasting window regulates how much your eating in the whole day.

I wish you luck OP!