A woman stands outside a sandwich shop
© AFP/File Paul Ellis

PARIS (AFP) - Harvard University researchers also made the startling discovery that these so-called brown fat cells -- which burn calories rather than store them -- originate from the same immature stem cells that produce muscle.

While many people would prefer to have less of it, fat is essential for health. It helps regulate our metabolism, and keeps our bodies warm.

But there are two kinds of blubber.

White fat is composed of molecules that hoard calories, and has contributed to a worldwide crescendo of obesity with consequences ranging from diabetes to heart disease.

Brown fat, more prevalent in infants than adults, is different -- in fact far more different that scientists realised.

To find out what chemicals in the body trigger its production, a team of researchers led by Yu-Hua Tseng of the Joslin Diabetes Center at Harvard Medical School experimented with genetically modified mice.

They discovered that a protein called BMP7 was critical to the process: without it, brown fat cells failed to develop, causing the mice to die. Added in artificially high doses, BMP7 had the opposite effect.

But white fat, Tseng found, relied on different albeit related chemicals to develop.

More importantly, he proved that white and brown fat do not originate from the same precursor cells.

In the early phase of their development, the two types of fat cells appear to be identical, so most scientists had assumed they derive from a common source.

In the second study, Bruce Spiegelman of the Dana Farber Cancer Institute, also at Harvard, found out -- to his "huge surprise" -- that brown fat comes actually from the same stem cells that produce muscle tissue.

Embryonic stem cells
© AFP/File Mauricio Lima

The key is a "master regulator" protein called PRDM16 that determines which way these adult stem cells will develop.

"I think we now have very convincing evidence that PRDM16 can turn cells into brown fat cells, with the possibility of combating obesity," he said.

Though mature humans have relatively little brown fat, it is thought to play a critical metabolising role.

Spiegelman said that finding a new potential source for this "good" fat -- the adult stem cells, or myoblasts, that exist to replace mature muscle cells -- open a path for boosting its calorie-burning action to combat obesity.

Obesity is occurring at epidemic rates worldwide and is a major risk factor for type 2 diabetes and metabolic syndrome, a bundle of health problems including clogged arteries, heart attack and stroke.

Both studies were published in the British science and medical journal Nature.

©AFP