Finally! Our own Creatine study, Feigenbaum et al

[Jordan Feigenbaum]

Well then I have a question.

We "stress" our skin in order to adapt to the sun or to friction. We stress the cardiovascular system . We need (I've been told) some exposure to germs to stress the immune system. We all know about stressing muscular-skeletal stuff. Etc.

Is it, then, unreasonable to think any "stress" to all other systems is dangerous and should be avoided?

Any stress =/= intolerable levels of stress. Also, there has to be a homeostatic mechanism present in the organ system for some level of stress to be useful.

[tfranc]

Excreting more creatinine does not place "stress" on the kidneys, as that is a normal function that they are fully equipped to handle.

As much as I hate the phrase "putting stress on the kidneys," the most common ways to do so include uncontrolled hypertension, uncontrolled diabetes, and excessive use of nephrotoxic medications.

In other words, the rate at which filtration occurs doesn't change and because higher serum levels in the blood are not dangerous (at least not levels that synthetic creatine provide) no damage occurs (excluding what you pointed out about it being a natural function).

[ithryn]

My vague analogy is a funnel. If you pour water in so that it backs up, the flow rate is just going to stay constant. If you dump more water in, it's still going to be constant. If you sprinkle some creatine in, it's going to dissolve and flow out at a constant rate. If you dump three spoonfuls in, it's still going to pass through at a constant rate. It's probably a bad analogy. Heck, adding more water probably adds to the downward pressure and increases the flow. Well, my analogy-funnel doesn't do this. Dammit Jim I'm a forum shitposter, not a physicist.

Now the kidney has a magical mechanism to make creatinine pass through faster than it normally, and then stops when the level in the blood is, say, "1.5". Because that amount is healthy or something, who knows. So imagine the funnel has a creatinine pump that can push it out at a maximum of, say, 2x faster than it naturally would, until the level goes down to that "1.5" mark. Whether you put a little in or dump a crapload in, it's still going to just pass it through until it hits that maximum constant rate. It just takes a little longer to get back down to the level it wants. The kidney DGAF how much is in your blood, it's only going to do X amount of work. Period. So the level "backs up" in the blood a bit. If you have healthy kidneys it eventually takes care of the excess, just takes a little longer.

So a better question is probably: does the backup of creatinine in the blood harm anything else (your arteries, organs it's passing through, etc), not does it harm the kidney. But I don't think it does. I don't think there's any mechanism where it would harm something, or any studies that show this.

tl;dr you could chug a gallon of water right now. Aren't you afraid of stressing your kidney? No, because your kidney will just pass it through. It'll hit a max rate of water excretion and you'll be pissing your brains out for a bit until the water level in your blood gets back to normal. But nobody's worried about the "load" on the kidney. If you kept drinking gallons of water you'd die of electrolyte dilution before you'd die of kidney failure.

[tfranc]

My vague analogy is a funnel. If you pour water in so that it backs up, the flow rate is just going to stay constant. If you dump more water in, it's still going to be constant. If you sprinkle some creatine in, it's going to dissolve and flow out at a constant rate. If you dump three spoonfuls in, it's still going to pass through at a constant rate. It's probably a bad analogy. Heck, adding more water probably adds to the downward pressure and increases the flow. Well, my analogy-funnel doesn't do this. Dammit Jim I'm a forum shitposter, not a physicist.

Now the kidney has a magical mechanism to make creatinine pass through faster than it normally, and then stops when the level in the blood is, say, "1.5". Because that amount is healthy or something, who knows. So imagine the funnel has a creatinine pump that can push it out at a maximum of, say, 2x faster than it naturally would, until the level goes down to that "1.5" mark. Whether you put a little in or dump a crapload in, it's still going to just pass it through until it hits that maximum constant rate. It just takes a little longer to get back down to the level it wants. The kidney DGAF how much is in your blood, it's only going to do X amount of work. Period. So the level "backs up" in the blood a bit. If you have healthy kidneys it eventually takes care of the excess, just takes a little longer.

So a better question is probably: does the backup of creatinine in the blood harm anything else (your arteries, organs it's passing through, etc), not does it harm the kidney. But I don't think it does. I don't think there's any mechanism where it would harm something, or any studies that show this.

tl;dr you could chug a gallon of water right now. Aren't you afraid of stressing your kidney? No, because your kidney will just pass it through. It'll hit a max rate of water excretion and you'll be pissing your brains out for a bit until the water level in your blood gets back to normal. But nobody's worried about the "load" on the kidney. If you kept drinking gallons of water you'd die of electrolyte dilution before you'd die of kidney failure.

Totally makes sense. I understand your analogy completely. The blood pumps at a safe, fixed pressure, so the rate of filtration is constant. The only way that could change is with a change in blood pressure (like you were saying about adding more water/tubing), but high blood pressure is a whole other thing.

Thanks everyone for helping me understand.