A natural protein that some hoped would be a “magic bullet” for weight control may have a dark and dangerous side. New research links the compound with diabetes.

The protein, called leptin, received a flurry of publicity last year when studies showed that it caused extremely obese mice to lose up to 30 percent of their weight. The mice also exercised more and ate less. Some researchers raced to develop leptin or related proteins for use in humans.

But now a lab in Israel has found that leptin may play a role in development of Type II diabetes, a serious disorder that frequently strikes obese adults.

Menachem Rubinstein, a biochemist at the Weizmann Institute of Science in Rehovot, Israel, said that when leptin is put on human liver cells in the laboratory, it disrupts a normal action of insulin, the hormone essential for control of sugar in the blood.

“We know that obese individuals have a high level of leptin and we know that obese individuals have a tendency to develop diabetes,” Rubinstein said in an interview. “There might be a linkage. It might be that leptin is one of the agents that induces Type II diabetes.”

He said clinical studies with leptin should be approached with caution.