The Same Geometry Works in Both Directions: Decelerating Mass Gain as the Mirror Image of Decelerating Mass Loss

Recent analysis has shown that the progressive slowing of human mass loss under energy restriction follows from a mass reservoir whose outflow scales with surface area. The same geometry operates in the opposite direction. When mass increases, surface area increases, net outflow rises, and the rate of further accretion declines. Decelerating mass gain, like decelerating mass loss, therefore requires no direction-specific adaptive mechanism for its baseline trajectory. For fixed intake, the system approaches an equilibrium mass M* where intake equals outflow; gain from below and loss from above are mirror images of the same relation. Geometry accounts for the direction-symmetric component of deceleration; adaptive physiology, if present, must be inferred from the residual beyond that baseline. Literature's near-exclusive focus on adaptation during loss, while largely ignoring the corresponding deceleration during gain, is itself suggestive of an unevenly distributed explanatory burden. Geometry does not choose direction.