there have been umpteen theories on why South Asians are more at risk of developing vascular diseases. Now a team of Indian, Canadian, Scottish and Iranian researchers have come up with a new theory based on the pattern of fat distribution in the bodies of South Asians to explain why they are more prone to diseases like diabetes, osteoporosis and heart problems.

The heightened risk is despite the fact that the total amount of fat in a South Asian's body is much lower than that among white populations or Europeans.

The group has put forward the "adipose tissue overflow' hypothesis to explain the phenomenon. Their paper has been published in the latest issue of the International Journal of Epidemiology (Vol 36, No 1).

Fat is mainly deposited in three regions of the body and functions differently. The superficial subcutaneous fat is present all over the body, just below the skin.It is organized into tightly packed lobules. The deep subcutaneous fat is present mainly in the upper body and is present in lobules that are larger, more irregular and less organized. Thirdly, fat deposits around organs in the abdominal cavity. Here the fat lobules are even less defined.

Fat cells can release free fatty acids into the blood. Though the visceral adipose tissue is the smallest of the three, it is most often linked to metabolic complications. The deep subcutaneous fat is also linked to higher risk of diabetes. More lower body adipose tissue is associated with better metabolic outcomes.

Superficial adipose tissue develops and matures first. When primary tissue can't store the excess calories, subsequent fat tissue is formed. The researchers say this superficial layer is less developed in South Asians. Secondary layers develop when primary layers lose capacity to store fat.

The hypothesis could help explain why white people are protected from metabolic syndrome and diabetes. "Unlike the other theories on obesity, this one is eminently testable,' says Shah Ebrahim in the journal's editorial.

"People need to change lifestyles to reduce risk,' says co-author D Prabhakaran of department of cardiology, All India Institute of Medical Sciences. This means avoiding tobacco, increasing physical activity , moderate diet, avoiding sugar, salt and saturated fats and consuming more fresh fruits and vegetables, he says.