When Rony discussed positive energy balance, he compared the situation with what happens in growing children. “The caloric balance is known to be positive in growing children,” he observed. But children do not grow because they eat voraciously; rather, they eat voraciously because they are growing. They require the excess calories to satisfy the requirements of growth; the result is positive energy balance. The growth is induced by hormones and, in particular, by growth hormone. This is the same path of cause and effect that would be taken by anyone who is driven to put on fat by a metabolic or hormonal disorder. The disorder will cause the excess growth—horizontal, in effect, rather than vertical. For every calorie stored as fat or lean tissue, the body will require that an extra calorie either be consumed or conserved. As a result, anyone driven to put on fat by such a metabolic or hormonal defect would be driven to excessive eating, physical inactivity, or some combination. Hunger and indolence would be side effects of such a hormonal defect, merely facilitating the drive to fatten. They would not be the fundamental cause. “Positive caloric balance may be regarded as the cause of fatness,” Rony explained, “when fatness is artificially produced in a normal person or animal by forced excessive feeding or forced rest, or both. But obesity ordinarily develops spontaneously; some intrinsic abnormality seems to induce the body to establish positive caloric balance leading to fat accumulation. Positive caloric balance would be, then, a result rather than a cause of the condition.”

An obvious example of this reverse causation would be pregnant women, who are driven to fatten by hormonal changes. This hormonal drive induces hunger and lethargy as a result. In the context of evolution, these expanded fat stores would assure the availability of the necessary calories to nurse the infants after birth and assure the viability of the offspring. The mother’s weight loss after birth may also be regulated by hormonal changes, just as it appears to be in animals.