WASHINGTON (AP) - For people who get migraine headaches, a burst of intense exercise can be a time bomb in the brain, setting off a migraine hours later, a study finds.

The fuse apparently is a chemical cascade released by the exercise, said researcher Joao Araujo e Sa of the Institute of Biomedical Research of Light and Image in Coimbra, Portugal. He presented his findings in June at the International Headache Congress in New York City.

It's a paradoxical result, because doctors commonly recommend exercise as a way to reduce the risk or severity of migraine.

"In our patients, we triggered that by exercise," Sa said. "We see exercise as probably a strong factor."

In Sa's study, 21 women with a history of migraine were compared to 12 who never got the headaches. All were given Wingate tests, an exercise test in which intensity is quickly ramped up to the point at which the participants can do no more. These women reached their limits in 30 seconds.

Of the women who never had a migraine, all remained migraine-free after the exercise test. However, of the 21 with a history of migraines, 11 had an attack. The migraines began 4 1/2 hours to 5 1/2 hours after the exercise, the study said.

The headache-prone women who didn't get a migraine after exercise might not have been working out hard enough, Sa said. Their blood samples showed lower levels of lactic acid, a waste chemical created in muscle by hard exercise, he said.

What seemed to trigger the headache was a spike in nitric oxide, a chemical that has been implicated before in migraine. After 30 minutes, nitric oxide levels in the migrainous women were more than double what they were at the start. Nitric oxide can dilate blood vessels. And migraine researchers believe that the dilated vessels can put pressure on nerve cells next to them in the brain, causing the headache.

It's been known that exertion can trigger exercise, and the amount of exercise varies from person to person, said Dr. Judy Lane, medical director at the Head Pain Center of the Colorado Neurological Institute in Englewood. Usually, the headaches come after longer periods of exertion, she said.

"I see patients all the time who say, 'I don't exercise because it gives me a headache,"' said Lane, who was not connected to the study.

"I thought the results were surprising but very interesting," said Dr. Alan M. Rapoport, director and cofounder of The New England Center for Headache, Stamford, Conn.

One surprise was the shortness of the exercise duration needed, Rapoport said. And one interesting implication is the support that the study gives to the idea that nitric oxide, as a blood vessel dilator, creates conditions for a migraine attack, Rapoport said. It's possible that science may find a way to reduce nitric oxide levels as a treatment for migraines, he said.

This is not to say exercise is bad for migraine sufferers; it's just a question of approach, Sa said. An adequate warmup can help them avoid migraine, he said.

Sa said his current research supports the idea that warming up slowly alleviates the sudden nitric acid dump that he blamed for the migraines in the 11 women in the study he reported in New York. But the new study is not completed, he said.

Migraine sufferers who exercise regularly report fewer and less severe headaches, Rapoport said. But people who start exercising as a way to control their migraines need to build up slowly - maybe from a start of only 3 minutes of light activity, he said.

However, the study does not prove the exercise or the nitric oxide caused the migraines, said Dr. Michael Welsh, vice chancellor for research at the University of Kansas. Migraines can have many triggers, depending on the individual, and they can be as common as bright light.

In the hours between the exercise and the attack, Sa's subjects could have run into one of their usual trigger factors, Welsh said.

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