When it comes to stroke, being very overweight may improve odds of survival, a new study suggests.

While obesity is clearly associated with an increased risk of heart disease and stroke, a new study reveals a counterintuitive result: Patients who were severely obese were 62 percent less likely to die in the first three months after a stroke compared to those of normal weight, according to a report presented at the annual meeting of the American Academy of Neurology. In contrast, patients who were underweight were 67 percent more likely to die within three months of a stroke compared to those of normal weight.

The reason may be tied to the amount of energy stored in fat cells, said the study’s lead author Dr. Zuolu Liu, a stroke fellow in the department of neurology at the University of California, Los Angeles, Medical Center. “If you’re underweight you might not have the energy reserve to go through physical and occupational therapy.”

It’s also possible, Liu said, that many of the thinner patients had a different — more serious — kind of ischemic stroke than the severely obese ones.

On the surface, the study seems to bolster the theory known as the “obesity paradox,” which holds that obesity may actually result in better survival rates among particular groups of people, such as the elderly and those with certain chronic diseases. But experts say that when it comes to stroke, BMI may actually be a marker for something else.

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The new study followed 1,033 people with an average age of 71, who had an ischemic stroke, which occurs when blood flow to a part of the brain is blocked. The patients were part of a larger trial called the FAST-MAG study, which was originally designed to test the efficacy of magnesium given to stroke patients by first responders.

Body mass index, or BMI, was calculated for all study participants and turned out to be 27.5 on average. People are considered overweight if their BMI is 25 to 29, those with higher BMIs are considered to be obese. That means, a person who is 5 feet, 9 inches would hit the obesity threshold at 203 pounds.

In the three months following a stroke, patients who were severely obese were 62 percent less likely to die compared to those of normal weight; patients who were obese were 46 percent less likely to die and those who were overweight were 15 percent less likely to die. People who were underweight were 67 percent more likely to die, compared to those who were normal weight.

While it’s possible that obesity might be protective against death following a stroke, it’s also possible that being obese or being underweight in this study are simply markers for something else, experts said.

It may well be that having fat stores can help when a person undergoes a rigorous program of rehab, said Dr. Ashutosh Jadhav, an associate professor of neurology and neurosurgery and director of the stroke institute at the University of Pittsburgh School of Medicine.

There’s also the possibility that having some fat tissue can help with healing in the short term, Jadhav said. “There is some animal data showing that adipose tissue may have some anti-inflammatory properties. So in the short term there may be some gains.”

Causes of stroke

Still, Jadhav said, the researchers don’t have enough information about the patients’ strokes to be able to say that fat will protect you. The causes of stroke in the obese patients may be different from those in the underweight patients, he explained.

Among the obese patients strokes were most likely associated with cardiovascular disease, Jadhav said. In contrast, patients who were very thin may have had atrial fibrillation, a condition characterized by abnormal heart rhythms, which can lead to stroke caused by a clot blocking a major blood vessel. Strokes related to atrial fibrillation tend to do more extensive damage than those related to atherosclerosis, Jadhav said.

Dr. Rebecca Gottesman suspects that being underweight may indicate some other health issue, such as frailty.

“It’s extremely likely that in the group with the highest mortality, the underweight population, that weight is a marker of underlying disease, whether it’s frailty or something else.”

Still, the new results suggest that people who have a stroke and are underweight, “need careful follow-up in terms of rehab,” said Gottesman, a professor of neurology and epidemiology at Johns Hopkins Medicine and the Johns Hopkins School of Public Health. “We should be making sure they are strong enough to participate in rehab.”