Celebrity molecules promised to transform our health, but haven't always lived up to their billing.

Notes endocrinologist Jeffrey Zigman of the University of Texas Southwestern Medical Center in Dallas, scientists aren't sure whether ghrelin normally regulates hunger. Although boosting its levels in the blood ramps up appetite, researchers still argue over whether the smaller amounts that are naturally present dictate when we want to eat. “What's less debatable,” Zigman says, “is that ghrelin protects against life-threatening falls in blood sugar.” If food is scarce and our blood glucose starts to decline, ghrelin intervenes, activating mechanisms that restore the levels to normal.

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