Elderly people who are obese appear to have a lower risk of falling ill with tuberculosis compared with those who are underweight or of average weight, according to an extensive geriatric study in Hong Kong.

Although obesity has been linked to health problems such as hypertension, diabetes and cardiovascular disease, experts notice that among people suffering from the same ailments, those who are overweight tend to outlive those who are thin.

In the latest issue of the Archives of Internal Medicine, researchers in Hong Kong reported that elderly, obese people tend to be better protected from tuberculosis than slim people.

They studied a cohort of 42,116 people who registered in 18 geriatric health centres in 2000, and monitored them closely for a period of five years.

They were 65 years old and over, and were classified into five categories according to their body mass index (BMI), the ratio of weight to height commonly used to classify an individual as over-, under- or normal-weight.

Those who were underweight were six times more likely to fall sick with tuberculosis than those who were obese, while those who were of average weight were three times more likely, according to a member of the research team.

"This relationship is quite consistent, the heavier you are, the lower the risk," CC Leung, a consultant chest physician with the Department of Health in Hong Kong, said in an interview.

More than a third of the world's population is infected with TB and the infection rate is one every second. However, only one in 10 infected persons will develop symptoms and that usually happens when their immune systems are weak. Left untreated, Mycobacterium tuberculosis infection, kills half its victims.

Leung said studies should be done to establish the reasons for the apparent robustness of overweight people when it comes to TB, although it appears to be supported by a few theories.

"Our adipose (fatty) tissues not only control our metabolism, but also our endocrine and immunological systems. This is our conjecture, but it's not without basis. In our studies (in Hong Kong), in ICU wards, overweight people have lower death rates than thin people," Leung said. "We must find out why this is so."

Falling TB rates and the availability of food at more affordable prices in industrialising states in the late 18th and early 19th centuries may not be a mere coincidence, he added.

"The availability of food had a direct bearing on TB. And before we had a good TB cure, TB figures were already falling in places like Britain and Europe," he said.

The findings have important applications. Where resources are scarce, BMI may be used as a factor to help decide if a person should be screened or treated for tuberculosis.

"We have many latent infections, which we have to screen and decide whether to treat, which may bring side effects especially for the old. If BMI has such an important bearing, we may want to consider it when giving screening and treatment," he said.

"As for this thin fad, if a person's weight suddenly plunges, we have to be careful because it may raise chances of the person developing TB (if he is infected)," Leung added.