John Speakman is Head of Integrative Physiology at the University of Aberdeen and is one of the world’s leading experts on metabolism and energy expenditure. His work in early 1990s involved many contributions to the development of isotope based techniques, notably the Doubly-labelled Water method. More recently, his studies on metabolism have provided novel insights into the process of ageing. His second major contribution that is relevant to clinical science concerns his work on the links between energy metabolism and regulation of body weight. This has led him to formulate a new hypothesis that modern obesity is the result of random drift once hominids were no longer at risk from predation. This concept of ‘predation release’ provides the first realistic alternative to the ‘thrifty gene’ hypothesis.