When you lose weight, fat cells don't disappear. They just get skinnier.

That's bad news for obese people, who already have made a lot more and often bigger fat cells. To stay trim, their fat cells have to be even leaner than other people's.

And if you gain weight, your body may create more fat cells to store the additional calories.

"We're trying to find out what turns on fat-cell growth," said Roy Martin, head of UGA's foods and nutrition department. "We think maxxed-out fat cells send a signal to convert [undifferentiated] cells into fat cells. "

"The more we understand about how fat cells are recruited, the more we'll be able to learn how to control them or prevent their development," said Dorothy Hausman, a research associate in Martin's lab.

Martin, Hausman and her husband, Gary Hausman, a researcher at the USDA Richard B. Russell Laboratory, study the factors that influence fat-cell development both in early life and in adulthood.

Dorothy Hausman studies fat at the cellular level. She examines what triggers undifferentiated cells, which are not yet developed, to become fat cells and which ones are destined to store fat. Starting with a growth medium that supplies all the required nutrients, she adds a variety of other ingredients such as hormones to see which ones stimulate or inhibit fat-cell growth. For example, she and her graduate students have shown that fat-cell extracts either from genetically obese rats or from overfed rats caused prolific growth of the cultured fat cells. "We think when fat cells get to a point where they can't hold any more lipid, they secrete something to trigger more fat-cell development," she said.

While Dorothy Hausman studies fat at the cellular level, Gary Hausman investigates what influences fat development in prenatal pigs. That's because most fat cells are formed early in life.

"Most fat cells are formed in fetal stages of development, and pigs and humans are more alike than they are different in their fat-cell biology," Martin said.

Their research, funded by the U.S. Department of Agriculture, may lead not only to leaner pork in the grocery store, but also to a better understanding of hormones that control the development of fat cells early in life.

Gary Hausman was the first to show that the pituitary gland, which regulates overall growth, development and metabolism, also secretes several hormones that stimulate fat-cell growth and development. He also showed that obesity may be associated with higher-than-normal levels of thyroid hormone combined with suppressed levels of growth hormone.

In other experiments he and Martin showed that glucose metabolism also may play a role in early fat-cell formation. Fetal pigs that later grew obese had a higher level of glucose metabolism than those that developed normal fat percentages.

The pair also observed differences between piglets born to obese sows and those that had lean mothers. Piglets whose mothers were fat had larger fat cells and absorbed more lipids from the blood stream.

"It's difficult to treat obesity," Dorothy Hausman said. "And it's not going to be a simple answer to the question of how to control fat-cell development."