Men who carry excess pounds may also carry an excess risk of pneumonia, a new study hints.

However, researchers note that the effect appears to be indirect. “This risk is driven by the development of chronic diseases related to obesity, rather than by obesity itself,” Dr. Jette Brommann Kornum of Aarhus University Hospital in Denmark, who led the study, told Reuters Health in an email.

Rates of hospital admission for pneumonia have been on the rise over the last decade - up 20 to 50 percent in Western countries. At the same time, more and more people worldwide are putting on unhealthy amounts of weight, which has fueled increased rates of chronic diseases such as diabetes and asthma.

The few studies that have attempted to link these trends in obesity and pneumonia have produced conflicting results, according to Kornum and colleagues. In search of a clearer answer, they looked to data from the large Danish Diet, Cancer and Health Study.

The team selected nearly 50,000 Danes between the ages of 50 and 64 with no previous history of chronic disease or hospital stay for pneumonia. Of the 22,578 men, 1,087 (4.8 percent) were admitted to a hospital with pneumonia between enrollment in the mid-1990s and the end of the study in April 2008. A slightly lower rate of pneumonia was seen among the women: 1,025 of 27,973 (3.7 percent).

They grouped participants according to body mass index, or “BMI” - a standard measure of weight in relation to height used to gauge whether someone is overweight or obese.

After accounting for factors such as lifestyle and education, moderately obese men - those with a BMI between 30 and 34.9 - had a 40 percent greater risk of pneumonia compared with those of normal weight (BMI of less than 24.9). Men who were severely obese, having a BMI greater than 35, were twice as likely to get pneumonia.

No differences were seen in risks among the women.

“The reasons for this sex difference are unclear,” said Kornum. “My best guess is that differences in fat distribution play an important role, as apple shape - being more common in men than in women - may cause reduced ventilation” in the lungs.

After the researchers accounted for chronic diseases diagnosed during the study, the effect of obesity among the men, too, disappeared. It seemed that obesity’s role was playing out through these related ailments. The authors note that diabetes, for example, has been associated with a 25 to 75 percent increased risk of hospitalization for pneumonia.

In addition to its relationship with chronic disease, Kornum suggests other routes by which obesity could impact pneumonia risk: a decrease in immunity, a higher risk of aspiration, reduced lung volume, and an altered ventilation pattern.

In a report in the European Respiratory Journal, the study team also acknowledge the possibility that obese individuals gained additional weight during the course of the study - a figure not recorded - and that physicians may have been more likely to hospitalize obese than lean patients with infection. Both situations could have led to overestimates of obesity’s effect on pneumonia risk.

But the size and rigor of the Danish study leads Kornum to translate their findings into advice for obese men: Be aware of symptoms suggestive of pneumonia - such as chills, fever, chest pain, shortness of breath or cough - and avoid other risk factors, including smoking and heavy alcohol drinking.

SOURCE: European Respiratory Journal, published online March 29, 2010.