Global Warming Could Lead to More Kidney Stones

Higher temperatures can cause dehydration, which increases the likelihood of nephrolithiasis

BY JODY A. CHARNOW

Climate change is thought to be responsible for much of the extreme weather affecting people around the world, such as stronger hurricanes and other storms that dump increasing amounts of rain and cause catastrophic flooding, unprecedented heat waves, and prolonged droughts that imperil agriculture and human health. Extreme weather events generate headlines and dramatic images, so some of the subtle effects of rising global temperatures on human health may get little attention.

One plausible effect could be an increase in kidney stone risk. Higher temperatures can increase water loss through perspiration and cause dehydration. This lowers urine volume and concentrates substances in urine that can form kidney stones, such as calcium, oxalate, phosphate, and uric acid.

“So it’s not hard to conclude that greater exposure to heat, even for relatively short periods of time, is likely to increase kidney stone prevalence,” said nephrologist David S. Goldfarb, MD, Professor of Medicine at the NYU Grossman School of Medicine and Director of the Kidney Stone Prevention Program at NYU Langone Health in New York. Studies have clearly demonstrated a link between higher temperatures and elevated kidney stone risk. Pediatric urologist Gregory E. Tasian, MD, MSc, Associate Professor of Surgery at the Perelman School of Medicine of the University of Pennsylvania in Philadelphia, led one of those studies. He and his colleagues looked at the effect of temperature and kidney stone presentation in 5 major US cities with diverse climates. They discovered that a daily mean temperature of 30 vs 10 degrees C (86 vs 50 degrees F) was significantly associated with a 38%, 37%, 36%, and 47% increased risk for kidney stone presentations in Atlanta, Chicago, Dallas, and Philadelphia, respectively, according to their 2014 report in Environmental Health Perspectives. 1 The investigators found a nonsignificant 11% increased risk in Los Angeles. The study also revealed a short lag between daily temperatures and medical visits for kidney stones, with the maximum risk occurring within 3 days of temperature exposure.

Other studies in the United States, the Middle East, and elsewhere have documented that emergency department visits for kidney stones occur more frequently in summer than winter.

Kidney stone prevalence already has been on the rise. In the United States, the overall prevalence of kidney stones increased from 3.2% in 1980 to 10.1% in 2016, Api Chewcharat, MD, and Gary

Curhan, MD, ScD, of the Harvard T.H. Chan School of Public Health in Boston, Massachusetts, reported in a 2020 paper in Urolithiasis 2 At the American Urological Association’s 2022 annual meeting, Shirley Y. Zhang, a medical student at the University of Alabama School of Medicine in Birmingham, reported on a study by her and her collaborators showing that the prevalence of symptomatic kidney stones among children in the southeastern United States increased 84.4% from 2006 to 2020.

The precise reasons for the upward trend are unclear, but researchers have cited dietary changes and an increasing prevalence of diabetes and obesity among the underlying causes. Now, global warming is considered to be in the mix of risk factors.

The effect of higher temperatures on kidney stone risk would be especially pronounced in urban areas because they tend to be warmer than rural environments, said Dr Goldfarb, who has advanced a hypothesis that urban heat islands contribute to a growing prevalence of kidney stones.3 As he explained in an interview with Renal & Urology News, the asphalt, concrete, and other building materials that make up the urban milieu absorb more heat during the day and give up that heat to the atmosphere at night compared with rural settings, he said. This results in areas characterized not only by higher daytime temperatures but less dipping of temperatures at night compared with rural areas. Consequently, urbanites may have prolonged exposure to higher temperatures compared with their rural counterparts, Dr Goldfarb said.

“Our urban heat island hypothesis suggests that the temperatures in cities are really much more of an issue than the average temperature of the Earth,” Dr Goldfarb said. “There’s no question that global warming and climate change contribute to increasing temperatures in urban environments.”

And cities are where most of the world’s people live. According to the United Nations Department of Economic and Social Affairs, 55% of the world’s population live in urban areas, and this proportion is expected to rise to 68% by 2050. The proportion of city dwellers is much higher in the United States, where 80.7% of the nation’s population lived in urban areas in 2010, according US Census data.

Dr Goldfarb acknowledged that it is difficult to disentangle the effect of heat from other factors that influence development of kidney stones. He noted, for example, that city residents may have different diets, activities, and occupations that increase their risk for kidney stones compared with people who live in rural communities. In addition, access to medical and imaging modalities and intensity of medical surveillance for kidney stones may be greater in cities, suggesting that evidence for increased kidney stone prevalence in urban settings could be related to greater detection rather than a true increase in prevalence.

Dr Tasian, who also is Surgical Director of the Pediatric Kidney Stone Center at the Children’s Hospital of Philadelphia, pointed out that established associations between temperature and kidney stone presentations could be complicated by changes in human behavior in response to higher temperature. For example, he and his collaborators observed a ceiling effect in Dallas, where temperatures above 30 degrees C did not result in an increased risk for kidney stone presentations. It could be that the city’s population adapted to the local climate such that residents spend more time indoors in air conditioned rooms and increase their fluid intake at higher temperatures, Dr Tasian postulated.

What would happen, then, in places where air conditioning is not widely used, such as in Europe? According to Inaba-Denko, a company that manufactures air conditioning products, air conditioning is present in only 20% of households in Europe, with much lower proportions in some countries, such as the United Kingdom (about 3% of residential homes), France (about 5%), and Germany (about 3%). Much of Europe has experienced record-breaking heat waves with temperatures above 100 degrees F this summer, with climate change widely blamed for the phenomenon. Dr Goldfarb and Dr Tasian say it is plausible that the effect of global warming on kidney stone prevalence could be more pronounced in these and other places where air conditioning is uncommon. Although stronger storms, massive floods and wildfires, and property destruction are among the attentiongrabbing events attributed to global warming, an increase in the prevalence of kidney stones brought on by rising temperatures could be among the underappreciated threats to human health. n