I have one very interesting question and I will appreciate if you someone can clarify on this:
Creatinine is waste product of muscle breakdown.
They also say that creatinine goesup due to ageing and they also say that muscle mass goes down due to age.
So how come creatinine goes up due to age
They also say that starting at age 45 you lose 1% of your kidney filtering each year.
I agree muscle mass goes down with aging. Here is link to info on creatinine from National Kidney Foundation. It does mention relation of creatinine to aging.
kidney.org/atoz/content/ser...
my point is why there is this contradiction:
Creatinine goes up due to ageing and muscle mass goes down due to ageing , so since muscle mass goes down, creatinine should go down
Totally depends upon the rate of loss of nephrons compared to loss of muscle mass. It is not a perfect relationship. If you take your theory to its natural endpoint, the more muscle mass you lose, to the point of detrimental muscle wasting, the more likely you'll remain in homeostasis with your nephron losses due to aging. Again, it doesn't work that way. It would be just as bad to say that the more muscle you put on, the worse and worse your kidney function will become. 
This makes me sad
The higher Creatine Levels is reflecting that your muscle mass is "melting"...and that your kidneys can no longer clean it from your blood as well as it could before
As Muscles Breakdown they release Creatine. As you get older the amount of Muscle Breakdown INCREASES. The result will be more Creatine as you Age. Simple Cause and Effect.
I think there is some confusion here. When we have blood labs done and a renal function panel (or a comprehensive metabolic panel) is done, they are not measuring the total creatinine produced by muscle activity, they are measuring the amount of creatinine remaining in the blood after the kidneys have filtered the blood.
The creatinine number is simply a marker of how well the kidneys are functioning at the time the test is done.
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