Margaret McCartney: Hydration, common sense, and evidence.

“Remember—healthy pee is 1 to 3, 4 to 8 you must hydrate!” So goes a rhyming couplet from the Think Kidneys website, the online presence of the Transforming Participation in Chronic Kidney Disease programme. It comes with a colour chart, not unlike those available in shops selling paint, ranging from the off-white shade 1 (good) to shade 8, which I’d describe as orange with a hint of rust (severely dehydrated). I suspect that the normal colour of most doctors’ pee when at work—if they get to the loo at all—will range from “dehydrated” to “severely dehydrated” (that is, from mellow yellow to that classic dark orange). But is this actually pathological? Dehydration is surely by definition pathological and would need blood tests to determine it. A physiological adjustment of our renal output shouldn’t be classed as dehydration but as a body that needs fluid to continue functioning normally. Human physiology is amazing: drink more, and we pee out more urine; drink less, and our kidneys concentrate it. This ballet is orchestrated through our osmoreceptors and baroreceptors, by antidiuretic, angiotensin, and aldosterone hormones, and by the sensation of thirst, which everyone remembers from tutorials in medical school. So, when does physiology become pathology?