In regular human red blood cells, about 40% of the creatine that gets inside does so using a special sodium-powered process, which means this system plays a big role in controlling how much creatine is inside the cells.

People with kidney disease have red blood cells that pull in more creatine using sodium than healthy people do, but their cells work the same when sodium isn’t involved — this suggests their bodies handle creatine differently because of the disease.

Even though blood cells from people with kidney disease let in more creatine, they don’t end up with more creatine inside — probably because something in their blood is blocking it from building up.

Red blood cells keep much more creatine inside them than is floating in the blood, even if blood levels change — it’s like they have a special pump that keeps the levels steady.

Your muscles use a special door called SLC6A8 to let creatine in, and this door only works when there’s more sodium outside the cell than inside — like a pump that needs salt to open.

Your muscles need sodium to pull in creatine, kind of like a battery-powered door — if the battery's dead or there's no sodium around, creatine can't get inside, even if you're drinking plenty of water.