For people with high blood pressure, the condition can prove tougher to control in the winter, researchers said on Monday.

Veterans treated in the winter were less likely to see their blood pressure levels come down to a healthy level than those treated in the summer, researchers told an American Heart Association meeting.

The five-year study focused on blood pressure readings for 443,632 U.S. military veterans with hypertension, or high blood pressure, in 15 cities, including such far-flung locales as chilly Anchorage, Alaska, and warm San Juan, Puerto Rico.

In each place, the researchers found, fewer of the veterans - regardless or race or sex - returned to normal blood pressure levels while treated in winter months compared to the summer.

“We are seeing seasonal variation in all of the cities,” Department of Veterans Affairs researcher Dr. Ross Fletcher, chief of staff at the VA Medical Center in Washington who led the study, said in an interview.

High blood pressure typically has no symptoms, but it can lead to major health problems including stroke, heart failure, heart attack and kidney failure. It can be controlled with lifestyle changes and medications.

Eight percent fewer patients returned to a normal pressure reading during treatment in the winter compared to the summer, according to Fletcher.

About 60 percent of the patients had a marked change in blood pressure control in the winter, Fletcher added.

Fletcher said the researchers have not figured out just what is causing this, but it did not seem to be changes in temperature or daylight that occur in the winter or the latitude of the city.

Instead, Fletcher said, the trend may be driven by weight gain, different eating habits and less exercise during winter. He said the data showed the veterans did, in fact, gain weight during winter. Weight gain can contribute to hypertension.

“I don’t think it’s the blood pressure rise that creates the weight gain. I think it’s the weight gain that might create the blood pressure rise,” Fletcher said.

Fletcher said people might eat foods with more salt in the winter as well, which also could help drive up blood pressure.

“There have been data looking at seasonal variation in heart attacks and strokes. And they tend to be higher also in the winter,” added Dr. Robert Bonow of Northwestern University Feinberg School of Medicine, a spokesman for the American Heart Association.

People should be aware of the possibility their blood pressure may be harder to control in the winter and should be more vigilant during this time, Fletcher said.

“It does appear that if one wants to have very tight control of blood pressure, that the ‘winter dip’ in control should be addressed - if not by a change of lifestyle, perhaps even by a change of medication,” Fletcher said.