Extremely obese people are nearly three times more likely to die from H1N1 flu than people of normal weight, according to a new study.

And obese people are twice as likely to require hospitalization for H1N1 flu than people of normal weight, the study said.

"Lots of factors may play a role in this, such as medical conditions that are associated with severe influenza infection, like diabetes or lung disease," said study researcher Dr. Janice K. Louie, of the California Department of Public Health.

Obese people are more likely to have impaired lung function, leaving them vulnerable to influenza infections and making it difficult for them to fight off infection , Louie told MyHealthNewsDaily.

And once people who are obese are sick enough to require mechanical ventilation to help them breathe, research has shown that they tend to be on the ventilator longer and stay in intensive care longer than patients who are not obese, she said.

"This may be because obese persons have extra weight and pressure on the lungs, and so it takes much more energy and effort to get air into the lungs of an obese person when they are ill," Louie said.

The results demonstrate the need for people who are already at risk for health problems — such as those who are obese — to get a flu shot , she said.

The study will be published Feb. 1 in the journal Clinical Infectious Diseases.

The hard numbers

Louie and her colleagues looked at the medical data of 500 adults in California who were hospitalized with H1N1 flu, also known as the swine flu , during the first four months of 2009. People are considered to be obese if their body mass index (BMI) is 30 or more.

They found that obese adults made up 51 percent of people over age 20 who were hospitalized for H1N1, but only 23 percent of the general population in California — which means obese people were 2.2 times more likely to be hospitalized with the disease than other Californians.

The researchers also found that people who were extremely obese — defined as those whose BMI is 40 or more — were nearly three times more likely to die from H1N1 than other adults. Of the 17 percent of hospitalized H1N1 patients in the study who died from the disease, 61 percent had a BMI of 30 or greater, and 30 percent had a BMI of 40 or greater.

People classified as  obese, but not extremely obese, did not have an increased risk of dying from H1N1 flu, Louie said. This was likely because extremely obese people usually have more pre-existing health problems.

Flu’s age complex

The finding goes against the previously held idea among health care professionals that the elderly are the group with the highest risk of death from the flu. And elderly people who are frail are also more likely to be underweight than overweight, Louie said.

"The 2009 H1N1 influenza virus had a much greater impact on relatively young individuals, when compared to the old seasonal influenza," said Dr. Steven J. Lawrence, assistant professor of medicine at Washington University at St. Louis School of Medicine, who was not involved with the study.

In 2009, only about 5 percent of H1N1 hospitalizations and 10 percent of deaths were in people ages 65 and older, Lawrence told MyHealthNewsDaily. But in previous years, that age group accounted for 95 percent of flu-related deaths.

A major reason for this is that at least one-third of older adults already had pre-existing immunity against the 2009 H1N1 virus, he said.

As a result, the virus infected more of young people who didn't have immunity from a previous exposure to the flu strain. And extreme obesity may be more common in people younger than 65, Lawrence said.

"In prior pandemics, where younger age groups were at the highest risk … obesity was rare, so the phenomenon may not have been noticed then even if it truly existed," he said.

Pass it on: People who are extremely obese — with a BMI of 40 or higher — are at greater risk of death from the H1N1 flu than people of normal weight.

Reach MyHealthNewsDaily staff writer Amanda Chan on Twitter @AmandaLChan.