Urea

Every day your body breaks down protein, from your food and from your own tissue being renewed. That releases nitrogen in the form of ammonia, which is toxic. Your liver turns it into urea, a harmless, readily soluble substance that travels through the blood to the kidneys and leaves the body in the urine. Urea is therefore not a poison but the solution to one, and the value in your blood is the balance of production in the liver and removal by the kidneys. That balance moves with more than your kidneys alone. Anyone who eats a lot of protein makes more urea. Anyone who drinks little holds on to more of it, because with little fluid the kidneys take back water and urea together. That is why urea can rise while nothing is wrong with your kidneys, and that is exactly the difference with creatinine, which mainly follows filtration. Your report shows urea in mmol/L. More than for most values, the range differs per lab, mainly over whether the upper limit rises with age: for an adult it sits between about 6.4 and 8.6 mmol/L, and at one lab it runs on past eighty to 14.2. So compare your value with the range on your own report.

Urea is the kidney value that tells you something about your day: how much protein you ate and how much you drank. That makes it unreliable as a kidney value and informative as a lifestyle value. A high urea with a normal creatinine is almost always too little fluid or plenty of protein, and it is one of the few results you can read with a glass of water in your hand. For anyone who exercises a lot and eats a lot of protein, that is worth knowing. A protein-rich diet, strength training and protein shakes lift urea without harming healthy kidneys; the number only shows that more nitrogen is being cleared. A low urea, conversely, goes with little protein, and sometimes with a plant-based pattern that runs on the lean side. Where it really is about the kidneys, urea joins in as the second voice beside creatinine and eGFR. With a genuinely reduced kidney function all three rise. In general practice kidney damage is diagnosed on eGFR and on protein in the urine, not on urea; the number is a supplement, not a diagnosis. In advanced kidney disease it is the substance that causes the symptoms, and then it is followed closely.

A high urea comes, in order of how often it occurs, from drinking too little or losing fluid, in heat, exercise, diarrhoea or fever; from eating a lot of protein; and from slower kidney function. Then from anything that breaks down your own tissue: heavy training, fever, prednisone, and prolonged fasting, in which the body draws on muscle. Bleeding in the stomach or gut raises it too, because the blood is digested as protein. A low urea is usually harmless: little protein in the diet or drinking plenty. In pregnancy it comes with the territory, because the kidneys filter harder. Only a diseased liver can really make production falter, and then the liver values beside it deviate as well. The lever sits in your glass and on your plate. With a high urea and a normal creatinine: drink regularly through the day, certainly around exercise and in warm weather, and check whether your protein intake is not higher than you need; for most athletes one and a half to two grams per kilo of body weight a day is plenty. With a low urea: check whether you get enough protein, especially if you eat plant-based or are older, because keeping muscle takes protein. Always read urea beside creatinine and eGFR. If those are normal, it is about fluid and food; if they deviate with it, it is about the kidneys, and then eGFR is the measure. Uric acid leaves the body by the same exit and often runs alongside.