It’s pretty common to see construction workers and other manual laborers stop into a convenience store on lunch to grab a refreshing cold beverage on a hot day. And more often than not, the product they’re reaching for is a soft drink.
That could be putting them at risk for kidney disease, according to the results of a new University at Buffalo study. The study suggests that drinking sugary, caffeinated soft drinks while exercising in hot weather may increase the risk of kidney disease. The study was published ahead of print in the American Journal of Physiology—Regulatory, Integrative and Comparative Physiology.
“Our data suggest that consuming a high-fructose, caffeinated soft drink increases the blood pressure response to exercise in the heat. Additionally, these beverages elicit a mild dehydration and acutely elevates biomarkers of acute kidney injury compared to water consumption during and following exercise in the heat,” says Christopher Chapman, the study’s lead author. Chapman is a PhD candidate in exercise and nutrition sciences in UB’s School of Public Health and Health Professions.
UB researchers studied healthy adults in a laboratory environment that mimicked working at an agricultural site on a hot day (95 degrees Fahrenheit). The study was inspired by recent reports of acute kidney injury among agricultural workers performing manual labor in hot outdoor environments in the U.S., Chapman explains, adding that there’s an epidemic of chronic kidney disease, the cause of which remains unknown, in farm workers in countries near the equator.
“Epidemiological data suggest that these issues are due to recurrent dehydration caused by performing physically strenuous work in a hot environment, which can induce acute kidney injury and potentially progress to kidney disease over the long-term,” Chapman says.
Chapman also points out that animal data suggest that the type of rehydration beverage matters: Consuming a high-fructose beverage following recurrent heat-induced dehydration exacerbates injury in the kidneys. “However, to the best of our knowledge, the acute effects of high-fructose beverage consumption during and following exercise in the heat on biomarkers of acute kidney injury has not been previously investigated in humans in a controlled study.”
Study volunteers completed an hour-long exercise cycle consisting of a 30-minute treadmill workout followed by three different five-minute lifting, dexterity and sledgehammer swinging activities. The study was performed in the Center for Research and Education in Special Environments (CRESE) at UB.
After 45 minutes of exercise, the volunteers rested for 15 minutes in the same room while drinking 16 ounces of either a high-fructose, caffeinated soft drink — in this case, Mountain Dew — or water. After the break, they repeated the cycle three more times for a total of four hours. Before leaving the laboratory, the volunteers were given more of their assigned beverage to drink before consuming any further fluids. The volume was either 1 liter or a volume equal to 115 percent of their body weight lost through sweating, if that amount was greater.
The researchers measured the participants’ core body temperature, heart rate, blood pressure, body weight and markers of kidney injury before, immediately after, and 24 hours after each trial. All volunteers participated in both soft drink and water trials separated by at least seven days.
The research team found higher levels of creatinine in the blood and a lower glomerular filtration rate — markers for kidney injury — after the soft drink trial. These temporary changes did not occur when the participants drank water. In addition, the participants’ blood levels of vasopressin, an anti-diuretic hormone that raises blood pressure, was higher and they were mildly dehydrated during and after the soft drink trial.
“The consumption of soft drinks during and following exercise in the heat does not rehydrate,” the researchers explained. “Thus, consuming soft drinks as a rehydration beverage during exercise in the heat may not be ideal. Further work will need to discern the long-term effects of soft drink consumption during exercise in the heat, and its relation to the risk of [kidney disease].”
While the study was designed specifically to mimic the physical demands and environmental conditions of a four-hour morning work shift, it’s likely that the findings have even bigger implications for manual laborers spending an entire day outside in the heat.
“It is possible that the results of our study are underestimating the effect of consuming a high-fructose, caffeinated soft drink during a longer work shift, or as the heat load increases throughout the day,” said Chapman.