Scientists have revealed deep insights into the role that a little-understood human hormone plays in regulating body weight. Named Growth and Differentiation Factor 15 (GDF15), this hormone is typically active only when the body experiences acute or prolonged stress, including following exposure to tissue-damaging toxins, such as chemotherapy, or during chronic disease, such as obesity or cancer. As a result, the GDF15 pathway holds promise for the development of potential therapeutics for diseases of both excess and insufficient body weight.