Japanese people are some of the healthiest people on the planet. Their life expectancy is sky high, they eat well, they’ve got an awesome healthcare system, and they enjoy (relatively) healthy lifestyles.

Despite this, scientists have been puzzled by the fact that one in eight people in Japan have chronic kidney disease (CKD), the second highest rate of the disease in the world. Research led by Monash University in Australia looked into this curious contradiction, only to find that Japanese people are born with significantly fewer nephrons in their kidneys. The findings were recently published in the journal JCI Insight.

Nephrons are the microscopic filters in our kidneys that help remove waste from the blood, transferring it into the urine. As you can imagine, the fewer nephrons your kidneys have, the worse your body is at draining out waste. This can lead to problems with your kidneys, like CKD, and your wider health, like hypertension, which is also known as high blood pressure.

The researchers carried out autopsies on 27 kidneys from Japanese men. They found they had around 640,000 nephrons, a figure that dropped to 392,000 if they were hypertensive and 268,000 if they had CKD. By contrast, Europeans have an average of about 900,000 nephrons in their kidneys, according to previous studies.

“This paper, the first such study in an Asian population, shows that Japanese with hypertension have significantly fewer nephrons than normotensive Japanese do – it's a clear-cut finding," said Professor John Bertram of Monash University's Biomedicine Discovery Institute.

There are a few reasons why Japanese people might be born with fewer nephrons. Firstly, the low nephron number in Japanese people could simply be in proportion to their smaller body size. Secondly, the individuals whose kidneys were looked at in the study were all born during and shortly after World War II, a time of poverty and poor living conditions in Japan. Thirdly, it could simply be a matter of genetics.

“There is a trend towards Japanese women staying thin and small during pregnancy to try to look beautiful but their babies are more likely to be born smaller and with smaller kidneys and therefore less nephrons – the number of nephrons is set at birth," said lead author Dr Go Kanzaki.

The researchers are hoping doctors could use a low nephron count to predict illnesses like hypertension or CKD, although this would require the development of a new non-invasive way to count the nephrons without cutting up the kidney.

"Ultimately, you would hope that health professionals will think more about low birth weight although the idea's not there yet," added Professor Bertram.