Welcome to Just Answer and thanks for your question. How old are you? Male or femal? African-American or not?
You are doing all the right things for your kidneys. Improvement in GFR due to dietary changes hasn't been proven yet, but certainly the things you are doing have improved your overall health. That reduces the work that both your heart and kidneys need to do. The most important thing is to keep your blood pressure under excellent control (under 120/80). GFR decreases as one ages, so it is common to see elderly people in stage 2 and early stage 3 when they really don't have problem kidneys. you're not elderly though so you will need to keep a close eye on the blood pressure and continue your exercise and monitor your weight. One other small thing you can do is to use Tylenol instead of Advil or Motrin for routine aches and pains.
Yes, I'm aware you are waiting, but I was helping someone else, and I can only type so fast.
Only your doctor can test to determine what is causing the decrease in kidney function. That isn't something a person can tell you without an exam and testing. Certainly being pre-diabetic is one factor. Kidney disease often comes as a consequence of diabetes and most diabetics have microvascular changes for a good 5-10 years before they ever get a diagnosis of diabetes. You should aim to keep your blood sugar below 100 mg/dL fasting and below 140 mg/dL two hours after meals. Sticking to a low salt, diabetic diet and keeping your weight within normal limits (18.5-24.9 BMI, or about 140-183 for someone 6 feet tall). Here are some links that can help plan your diet.
I'm sorry I'm not clairvoyant. There is no way someone online who can't examine you and has no access to all your medical records can tell you about what caused your lowered kidney function. That is something that only your own treating doctor can, and perhaps only after some more extensive testing.
Thanks for the insults...that's always an incentive for a better answer. The problem is that there is no better answer, and you don't want to hear the correct one. So, one more time, and loudly: Your own doctor will need to determine if your kidney damage is from diabetes, or from a congenital kidney problem, or from an infection, high blood pressure at times when it isn't being taken by the doctor, problem with circulation other than microvascular problems from incipient diabetes, tumors, cancer, etc. There is nothing you can do on your own other than maintain a normal weight, eat right, exercise and the other things I've already suggested. There is no magic food, drink, pill or potion that will help. Improving kidney function is done by controlling whatever is causing it to malfunction. It will take your own doctor's delving into your kidney problem to tell what it is. No one sitting several hundred miles away with no access to medical history and no way to do an exam can tell you any more than this.
That is an accurate answer. If you don't want to pay for the truth, then please keep your $15 and have a nice life. I have spent all the time I care to trying to educate you and listening to your abuse for my share which is only $7.50.