You mention you worked out a lot when younger, so did I and drank a lot of protein shakes, which I've read can be taxing for the kidneys. You too?
After recently taking and highly regretting taking a high one dose of doxycycline my function took a temporary (hopefully) hit falling 15 to 20 points(past three years I've maintained around 80/85 function out of 100). I can tell per itchy skin, minor joint pains (creatinine build up) or sitting too long feeling my leg has fallen asleep(before diagnosis when I was at 55 function I did a long drive got out of the car & my legs fell asleep so bad I could barely walk until they woke up per say). Ive changed my diet further as my most recent labs (get tested once to bi-monthly) showed my LDL cholesterol is Way too high..now eating meat (chicken..high in cholesterol)only once a day, no ice cream (high in cholesterol) , a reduction of cheese intake and no cheat days (once I got my function up to 80/85 I allowed myself a cheat day or two). If blood isn't flowing in a healthy manner to kidneys due to high LDL then I think that is another factor along with the one high dose of doxy I took that has currently has my function not where it was but I believe it's a temporary thing.
And high LDL isn't necessarily a problem by itself. The problem happens when cholesterol is formed into plaques as a reaction to arterial damage. Better to prevent that damage in the first place rather than worrying about cholesterol levels.
Here's an exert from one of the sites "American Heart Association's (AHA) "Life's Simple 7" steps for heart health were 10 times more likely to have low blood pressure and flexible arteries than those who didn't.
The AHA's steps include controlling cholesterol, keeping blood sugar low, being active, eating a healthy diet, keeping off extra weight, and not smoking. (The seventh step is maintaining normal blood pressure, which the researchers were already measuring.)"
My lifestyle (what i eat and many times a week exercise) is pretty close to following all those steps. I definitely could follow it better as well become a vegetarian.
All my posts here are just sharing what I did to increase my kidney function, lessen amount of gout attacks (5 to 6 in 8 years isnt a ton ... im not taking any meds for it) and try to keep my cholesterol in check.. all without getting on medications that my kidney doc says meds will damage your kidney. This is just what helped me and Im a huge believer in what you put in your body matters. Others can try to do the same thing if they want there is no harm in living a healthier lifestyle. Yet it may or may not work for them but they can try and stick to it and see. All what's important to them .. eating tasty unhealthy food and having their health decline further or better their health and in turn their quality of life hopefully over a longer period of time.
https://www.hsph.harvard.edu/nutritionsource/2015/02/25/ask-...
The other AHA recommendations are generally good, although phrases like "healthy diet" are rather meaningless.
Curious what is your interest in this subject? Do you have CKD, a researcher, a doctor or ?