Evolutionary aspects of fat storage
1Institute of Genetics and Developmental Biology, Chinese Academy of Sciences Chaoyang, Beijing, Peoples Republic of China; 2Institute of Biological and Environmental Science, University of Aberdeen, Scotland, UK
The use of energy is continuous. For most animals, however, the supply of energy (as food) is discontinuous. Animals therefore need mechanisms to store energy to get them over the periods when they are not feeding. Short-term energy stores include glucose and glycogen. For longer periods, animals may cache food outside their bodies. These caches may require defending and may be susceptible to rotting. Adipose tissue probably evolved as a specialised storage organ to deposit energy for longer periods without food that avoids these problems. The greater the amount of stored fat, the longer the period without food that an animal can survive. Storing fat, however, has costs in terms of the time needed to acquire the energy, the costs of moving around carrying a fat store and reduced mobility. These costs may elevate the risk of predation, hence the level of fat storage may be regulated according to the balance of risks between starvation and predation. Early studies in the 1950s suggested body fat might be regulated by a ‘set point’. The discovery of leptin and the hypothalamic food intake regulation system provided a molecular framework for such a system. Yet, despite its attractiveness, the set point system is incompatible with many observations suggesting that fat storage is not strongly regulated – for example the fact we have an obesity epidemic. The dual intervention point model provides a framework for reconciling these disparate views. It suggests that body fatness may vary over a range where it is not regulated but if it reaches either an upper or lower limit (the intervention points) then physiological regulation systems kick in to regulate the level of fatness. The suggestion is that the upper intervention point is responsive to predation risk while the lower intervention point is responsive to starvation risk. This idea, combined with an understanding of the evolutionary context of our development, provides a way to reconcile how body fatness can be regulated, and yet we can still end up with an obesity epidemic.