The following article says a recent study shows that while kidney function tends to decline with age in more developed societies, this is not necessarily the case in less developed societies. That is, it seems like there may be something (or many things) about our "advanced" lifestyle which causes kidney degradation with age. No clues as to what that might be yet but adopting a more "primitive" diet (less processed food, more fruits and vegetables) certainly seems like it would probably benefit the kidneys.
https://www.economist.com/science-and-technology/2019/02/13/loss-of-kidney-function-in-old-age-is-not-inevitableSince being diagnosed with chronic kidney disease (CKD) almost 10 years ago I have fortunately so far managed to avoid further measurable kidney degradation. My diet has basically been:
- Relatively small amounts of animal proteins (no beef/pork/chicken/etc., fish/seafood maybe twice a month, eggs once week, yogurt and cheese a couple of times a week, no milk).
- Limited wheat (I found that wheat seems to cause me abdominal swelling).
- Lots of fruit, vegetales and nuts (bananas, avocados, broccoli, carrots, apples, almonds, unsalted peanuts, beetroot, lettuce, tomato, beans, etc.).
- Low sugar and salt (no added salt in cooking. Avoid high salt foods).
- A certain amount of starchy foods every day: Rice, corn, oatmeal, sweet potatoes, potatoes.
