MAYWOOD, Ill., July 23 (UPI) -- To stay hydrated in the summer drinking water is best but for those at risk for kidney stones adding lemon to water is a good idea, a U.S. expert says.

John Milner of Loyola University Chicago Stritch School of Medicine, Maywood, Ill., says kidney stones are a common disorder of the urinary tract that affects about 10 percent of the U.S. population. Kidney stones are crystals that form in the kidneys or the small tubes that drain the urine from the kidney to the bladder. Men are four times more likely to develop kidney stones than women and their risk rises dramatically once they reach their 40s, Milner says.

The most common cause of kidney stones is the failure to drink enough fluids but flavoring water with lemon slices or lemonade can help to ward off kidney stones.

"Lemons are very high in citrates, which inhibit the growth of kidney stones," Milner said in a statement. "Lemonade, not the powdered variety that uses artificial flavoring, slows the development of kidney stones for those who are prone to the development of kidney stones."