Hi,
I found following a kidney friendly diet is all I have had to do. I stick with healthy and fresh foods not off the shelf man adulterated foods, staying clear of sodas and sugar substitues as they can damage kidneys. I cut back on red meat and only have it 2 weekly going with chicken and fish instead. I'm 73 and have been able to keep my GFR between 38 and 45 for a few years now. Kidney function does decrease with age naturally. Some people need to change their lifestyles but I have been lucky that my lifestyle was good to start with. GFR 51 is pretty good for the age anyway so no need to panic just be mindful of what you eat and how you treat your kidneys. I make a point of having my quarterly bloods done at around the same time of day and after doing and eating the same things so I get a consistant measure of what my kidneys are doing. What you injest and are doing before blood is drawn can make a big change in your GFR, vigorous exercise and it can drop a lot. I tried it once to see the responce of my doctors, made them sit up and panic at the big drop. When my GFR drops I look at what I have been eating and doing that might have caused the drop and don't do it again. Medications can also effect the kidneys and most Dr's will mitigate your health without consulting you, I refuse to let them do that to me. My stats prove I'm doing the right thing by me kidneys. I watch the colour of my urine to tell me if my kidneys are doing the job required. The colour should be clear to light straw which indicates your kidneys are working fine and you are getting enough fluids to flush them. Don't be fooled into thinking you have to drink, drink and drink, to much fluid can be harmful as well to the kidneys. As kidneys decrease so does the immune system so maybe scripted vitamin D3 should be looked at to help.
Cheers