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The short-term spike in blood pressure that comes with moderate exercise does not harm the hearts of healthy older people with mild hypertension, new research shows.

"There has been some hesitation on the part of individuals when they have high blood pressure to do exercise," said Dr. Kerry Stewart, the study's lead author and director of clinical and research exercise physiology at Johns Hopkins University Heart Institute.

"Among people who are otherwise healthy except for mild hypertension — which represents an awful lot of people — the message is, after a proper screening, participating in an exercise program appears to be beneficial."

Long-term high blood pressure is known to cause enlargement and stiffening of the heart, but the brief rise in blood pressure from regular exercise does not have the same effect, researchers found.

The hearts of the most active study participants did increase modestly in size, but it was comparable to that experienced by athletes and was accompanied by improved heart function and not stiffening, Stewart said.

Although the benefits of exercise have long been known, the researchers believe the study is the first to evaluate the effects of exercise on heart function — how much blood fills the heart with each beat and how much it pumps.

For six months, researchers tracked 104 men and women between the ages of 55 and 75 with untreated mild high blood pressure. Half exercised for an hour three times a week while the others maintained their normal routine.

The exercise included an aerobic workout, either on a stationary bicycle or a treadmill, and weightlifting.

Exercisers showed no negative effects in 11 measures of diastolic heart function, the filling of the heart's main chamber between beats. As a whole, the exercise group also showed no increase in eight measures of heart size, although the most active showed a modest increase.

The exercisers' oxygen intake rose and they lost an average of four pounds, with fat loss offset by gains in muscle mass. Abdominal fat, meanwhile, was reduced 20 percent among exercisers, the researchers reported. Non-exercisers had either no improvement, or significantly less improvement.

The research is part of a larger study of the effects of exercise on blood pressure, heart structure and cardiovascular function. Results of the Johns Hopkins findings were published in the July issue of the journal Heart.

Dr. Ares Pasipoularides, a professor emeritus at Duke University who was formerly with Duke's Center for Emerging Cardiovascular Technologies, said it was not clear whether the exercise or the weight loss was responsible for the improved heart function in the most active group.

"When people are lean, among other things, it improves the respiration. When we improve the respiration you improve cardiac function by many different mechanisms," said Pasipoularides, who did not work on the study.

Stewart noted that the volunteers' exercise was equivalent to brisk walking with weight training.

"We're not talking about running a marathon and we're not talking about doing the Tour de France, it was about an hour of what we would call moderate exercise," Stewart said.