Urbanization may be contributing to a well-documented increase in the prevalence of kidney stones in the United States and elsewhere, according to a hypothesis proposed by a New York kidney stone specialist.

Migration to cities exposes people to urban “heat islands,” places where ambient temperatures are higher than in rural and suburban areas, said David S. Goldfarb, MD, professor of medicine and physiology at New York University (NYU) and chief of the nephrology section at the New York Harbor VA Healthcare System. Assisted by co-author Jacqueline Hirsch, a medical student at  NYU, Dr. Goldfarb provided details of the urban heat island theory in a paper published online in Medical Hypotheses.

Studies have demonstrated that higher temperatures are associated with an increased prevalence of kidney stones, the authors noted. For example, stone prevalence in the southeastern United States is nearly 50% greater than in the northeast, where the mean annual temperature is on average 8° C lower. Higher temperatures stimulate heat-related perspiration. Serum osmolality increases, and this stimulates release of the antidiurectic hormone vasopressin. Vasopressin increases urinary osmolality and decreases urine volume, thereby increasing the concentration of relatively insoluble salts such as calcium oxalate, the most common constituent of kidney stones, the authors explained.

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In rural landscapes, naturally vegetated surfaces promote evapotranspiration, he noted. As water is absorbed and released by plant roots, soil, and leaves, it absorbs solar or atmospheric heat that converts it from a liquid to a gas. “These processes together lower atomospheric and surface temperature.”

Urban landscapes, however, are dominated by less reflective surfaces such as buildings and pavement that cannot absorb water, they explained. Urban building materials absorb more heat than rural surroundings, which, after absorbing heat, also can release it because of the water content of soil and vegetation. Also, in urban settings, the proximity of buildings impedes release of stored heat in the evenings. United Nations data show that more people are living in cities than in rural settings for the first time in history.

In an interview with Renal & Urology News, Dr. Goldfarb noted that other investigators have proposed global warming as a factor in the growing prevalence of kidney stones, but he said he is skeptical that it has been relevant to the increases in stone prevalence seen over the last 30 years. “There is a big difference between rural settings and urban settings as a result of urban heat islands,” he said. “That may be more important than global warming.”

In underscoring the higher temperatures in cities, he pointed out that it is mainly in cities that heat-wave-related deaths occur (as in the 1995 Chicago heat wave that led to 739 heat-related deaths). Such deaths seldom occur in rural areas, he noted.

“The possibility that kidney stones are a more mild form of heat-related stress I think is real,” Dr. Goldfarb said.

He said he advises his kidney stone patients to keep their fluid intake up throughout the year, especially if they are exercising and engaging in outdoor activities in the summer.

Dr. Goldfarb acknowledged that proving urban heat islands increase kidney stone risk would be quite difficult because of the challenge of teasing apart the many variables that can influence development of kidney stones. For example, people migrating from the countryside to the city may experience changes in income, diet, and activities that affect kidney stone risk. Additionally, when individuals move to the city they may spend more time in air-conditioned environments and spend less time outdoors, thereby mitigating the effect of urban heat islands.