An estimated 12 percent of the population of the United States will experience one or more of these uninvited guests at some point in their lives.
Small stones can exit the body during urination with little or no fanfare. Larger stones, however, are a different story; although rarely fatal, they can be excruciating.
To give you some idea, the following comes from a first-hand account. "It was what I can imagine a knife stuck in my back being twisted all around would feel like." According to another, "A female nurse told me it's the worst pain a man can ever feel because a man can't go through labor."
Aside from the agony, kidney stones don't come cheap, either. In 2000, around $2.1 billion was spent treating them in U.S. hospitals. This figure marked a 50 percent increase since 1994.
Due to the cost, the steady increase of prevalence, and the intense pain, there is significant interest in understanding why these ghastly miniature boulders are formed.
Who gets kidney stones?
Certain risk factors are well-established. For instance, if you are male or have diabetes, you are more likely to develop them. Another factor that raises your chance of growing your very own kidney stone is your ZIP code.
A landmark study published in 1994 looked at data from more than 1 million men and women across the U.S. They established the so-called stone belt.
Their analysis uncovered a distinct geographic pattern of kidney stones. As they rolled further south and east, prevalence increased. Men in North Carolina, for example, had an almost three times greater risk of developing kidney stones than men in North Dakota.
At first glance, this map-based lottery seems strange and perhaps a little unfair. However, it makes more sense when the weather is taken into account.
People living in hotter climes are likely to sweat more, so it follows that they are also more likely to be dehydrated. And, being chronically dehydrated is a well-established risk factor for kidney stones.
But according to the authors of the latest study, temperature cannot be the only vagabond on the rocky road to internal stones. As they explain:
"[H]igher temperature does not fully account for the stone belt phenomenon. If temperature alone was the primary environmental driving factor, the American Southwest should have a similar stone prevalence to the Southeast."
...And it doesn't, so researchers from Stanford University School of Medicine in California set out to investigate another climate-based factor: precipitation. The results were recently published in the Journal of Endourology.
To test their theory, the researchers concentrated on the good folk of California, which is a state with a particularly varied climate. They dived into data from the California Office of Statewide Health Planning and Development databases.
They perused the facts and figures of 63,994 people who underwent kidney stone procedures in the state in 2010–2012. They also gathered climate information from the National Oceanic and Atmospheric Administration.
Each participant's ZIP code was available, allowing the team to pinpoint their location and identify what sort of weather they were likely to experience.
Once the numbers had been crunched, their hunch turned out to be spot on. Although the effect was relatively small, it remained significant even after controlling for factors such as sex, age, and the prevalence of obesity and diabetes.
They found that for every additional inch of rain, there were an extra 0.019 surgeries per 1,000 people. Likewise, each degree Fahrenheit increase in mean temperature caused an additional 0.029 surgeries per 1,000 people.
Why does moisture promote kidney stones?
The study authors believe that a warm and precipitous environment, compared with a warm and dry environment, gives rise to more kidney stones because the body's thermoregulation system handles it less well.
When it is humid, sweat cannot evaporate as easily, meaning that the body's primary defense against heat is toothless. So, more sweat is produced in a fruitless attempt to keep us cool.
Though at this stage, more work will be needed to get a proper picture of what's going on.
As a worrying aside, the study authors bring up the potential impact of global warming. With both average temperature and precipitation predicted to increase across the planet, kidney stone prevalence is also likely to increase — yet another reason to buy an electric car.
So, if you live somewhere both hot and wet, you are slightly more likely to develop kidney stones. This may be an unsettling thought, so you could always move to the Antarctic: the coldest, driest place on earth. Or, if that doesn't float your boat, just make sure you drink enough fluids.