Health officials from the U.S., the U.K., Europe, and Japan have been warning residents to stay out of the sun as the northern hemisphere experiences some of the highest early summer temperatures ever recorded. It’s not just to prevent heat-stroke, but to prevent the long-term consequences as well. As climate change drives summer temperatures even higher than usual, medical researchers are starting to find links between sustained heat exposure and chronic health conditions ranging from diabetes to kidney stones, cardiovascular disease and even obesity. “While increased risk for heat stroke is an obvious manifestation of global warming, climate change is actually causing health problems today, in both direct and indirect ways,” says Richard J. Johnson, a medical professor and researcher at the University of Colorado Anschutz Medical Campus, and one of the world’s foremost experts on the intersection of heat stress and kidney disease.
Hotter days bring an elevated risk of dehydration, says Johnson, which in turn can cause cognitive dysfunction, high blood pressure, and acute kidney injuries. Over time, the chronically dehydrated are less able to excrete toxins, leaving a higher concentration of salts and glucose in the kidneys and blood serum. Those substances are linked with an increased risk for diabetes and metabolic syndrome, a medical term that describes some combination of high blood sugar, high blood pressure, high cholesterol, and abdominal obesity that is estimated to afflict nearly a quarter of U.S. adults. As temperatures rise, he says, it is likely that incidences of metabolic disease will too, along with the concurrent risk of heart attack and stroke.
Read More: What Extreme Heat Does to the Human Body
The increased development of kidney stones is another possible outcome of rising temperatures. A 2008 research article, published in the Proceedings of the National Academy of Sciences, argues that one unanticipated result of global warming is the likely northward expansion of the present-day south-eastern U.S. kidney stone “belt,” where heat and humidity are higher, and cases are currently concentrated. The risk of developing kidney stones is exacerbated by either low fluid intake or excessive fluid loss, both of which occur in high heat. The paper’s authors found that, based on projections of climate change-induced temperature gains, the percentage of the U.S. population living in high-risk zones for kidney stones will grow from 40% in 2000 to 56% by 2050, and to 70% by 2095. Even if kidney stones don’t develop, consistent exposure to high heat and dehydration—in agricultural laborers, for example—has been shown in some cases to cause irreversible kidney damage, as described in a 2015 case study co-authored by Johnson and published in ScienceDirect involving sugar cane workers in El Salvador. “The kidney is very sensitive to heat stress,” says Johnson. “It is a barometer for health and climate change.”
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Johnson, author of Nature Wants Us to Be Fat: The Surprising Science Behind Why We Gain Weight and How We Can Prevent—and Reverse—It, is about to publish a new paper looking at the links between dehydration and obesity, with obvious implications for those living in hotter locales. “When an animal starts developing dehydration, this triggers fructose production from carbs,” says Johnson. The fructose stimulates the production of vasopressin, which helps store water in the body. But vasopressin also stimulates the production of fat. Camels, he points out, don’t store water in their humps, they store fat. When the fat is burned, it produces water. “Fat is actually used by animals to survive when water is not available,” he says. Fat production is the body’s reaction to—and anticipation of—dehydration.
Johnson’s hypothesis is that “climate change is making it easier to get dehydrated and hot, and in so doing it will activate this chemical reaction so that when carbs are present, it will lead to more fructose and vasopressin being made,” he says. “You can actually create obesity in animals by making them slightly dehydrated, so there’s a very strong link between dehydration, heat stress, and obesity.”
Read More: What It’s Like Living in One of the Hottest Cities on Earth—Where It May Soon Be Uninhabitable
Dehydration, of course, is not an inevitable consequence of hot days. It is easily staved off by drinking water—not sugary drinks—staying rested, and finding shade. For those working and sweating in hot conditions it means frequent breaks and rehydrating with sports drinks or electrolyte solutions to replenish potassium, sodium, and other minerals lost through perspiration. “Wear a hat,” says Johnson. “Get out of the sun.” His advice sounds just like any other health official’s for a reason. Heat can kill. Sometimes quickly—heat waves kill more people annually in the U.S. than hurricanes, lightning, tornadoes, floods, and earthquakes combined—and sometimes slowly. “If you go to an ER with heat stress, it increases your risk for developing chronic kidney disease later on in life,” says Johnson.
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