When Lynette Sylvester weighed 259 lbs., she hated Jane Fonda. The movie star was everything that Sylvester, 42, wanted to be and wasn't: among other things, svelte and athletic. In fact, Sylvester's weight problem had become so severe that her physician recommended she have her stomach stapled. "I realized that if I didn't change my behavior, I would die," recalls the store owner from Burnsville, Minn. Determined, she went on a liquid-protein diet and lost 120 lbs. Now, a year later, down from a size 44 dress to size 10, Sylvester regards Fonda as her personal exercise guru.
Liquid diets, which...