Ruth Harris wants to understand fat, in all of its intricacies: how fat increases and decreases; why our bodies sometimes disregard internal signals to quit eating; why we may respond to stress by not eating; and why sometimes we regain the weight we lose during stress and other times we don't.
It has been known since the early 1990s that the hormone leptin is involved in the loss of fat. When genetically obese mice that do not make leptin were given low doses of the hormone, they lost 30 percent of their body weight within two weeks. Researchers still don't know how the weight loss actually occurs, but Harris is confident that there are factors beyond leptin that are necessary. For example, there may be still-unidentified hormones that are triggered after leptin reaches the brain, or they may be released when leptin binds with receptors elsewhere in the body.
Harris hopes that the results of two studies currently under way will shed light on these questions. One project focuses on preadipocytes, undifferentiated cells that will grow into fat cells. The other looks at mature fat cells. With the assistance of UGA protein chemist Lance Wells in the biochemistry department, the Harris lab is separating various proteins out of blood serum that contains leptin. The preadipocytes and fat cells are then exposed to the blood serum. The results are expected to show whether leptin-laced blood serum affects lipid synthesis in mature fat cells or the proliferation of preadipocytes. It may also lead to the identification of other factors necessary for leptin-related fat loss.
Although Harris' team spends a great deal of time studying leptin, that's not the only focus of their research. The scientists are also conducting a series of experiments on the impact of stress on body weight.
"I started studying stress because the loss of weight that occurs from it isn't consistent with what we would have expected," she said. "When a rat experiences extreme stress it loses weight. When the stress is relieved, the animal resumes its normal eating pattern, but it never makes up for the weight that was lost. That is, it never weighs the same as a rat that did not experience extreme stress."
Interestingly, Harris has determined that the weight-loss phenomenon is limited to extreme stress-caused, for example, by confining a rat in a small space. Mild stress-induced, say, by moving the rat to a new but amply sized cage, will cause it to lose a small amount of weight, which it usually regains within 24 hours.
"What these different responses demonstrate," says Harris, "is that although extreme stress and mild stress both cause weight loss, they appear to be acting through different pathways in the brain and have different long-term consequences."