Photo/IllutrationGo Kanzaki, an assistant professor at the Jikei University School of Medicine in Tokyo, right, and John F. Bertram, the head of the research team and a professor of Monash University in Melbourne, Australia (Provided by the Jikei University School of Medicine)

Japanese are born with fewer nephrons in their kidneys than other races, which could be why Japan has one of the world’s highest incidence of chronic kidney disease, an international research team found.

The relatively small body size of Japanese people could be behind the lower count of nephrons, which filter out waste in the blood to produce urine, according to the findings of the team, which included scientists from Japan and Australia.

Smaller bodies generally mean smaller kidneys and fewer nephrons, which the team connected to chronic kidney disease.

The salty diet could also make Japanese more susceptible to such diseases.

On average, one kidney contains about 1 million nephrons. But the number could vary from 200,000 to 2 million depending on race, according to the researchers.

In the first study on nephron numbers among Asians, the team examined the kidneys of 27 Japanese subjects and split them into three groups: normotensive, or subjects with normal blood pressure; hypertensive subjects; and subjects with chronic kidney disease.

The nine normotensive subjects had an estimated 640,000 nephrons per kidney on average, while the nine hypertensive subjects had 390,000, and those with chronic kidney disease had 270,000, according to the study.

In comparison, Europeans and U.S. citizens have an average of 900,000 nephrons.

“The number of nephrons is determined at birth,” said Go Kanzaki, an assistant professor at the Jikei University School of Medicine in Tokyo and a member of the team.

Kanzaki said he is especially concerned about the increasing number of babies born with lower body weights in recent years.

He urges Japanese to reduce salt intake and avoid obesity.

“It is necessary to keep healthy lifestyles and continuously check kidney functions,” Kanzaki said.

The results of the study were published in the Oct. 5 online edition of the Journal of Clinical Investigation Insight.

Around 13 million patients suffered from chronic kidney disease in Japan in 2008, according to estimates by the Japanese Society of Nephrology.

More than 320,000 kidney disease patients were undergoing artificial dialysis at the end of 2015.