WikiFindings

The drifty genotype hypothesis of obesity is not consistent with evolutionary theory

Background

Obesity is a major medical issue affecting millions of people worldwide. Whilst the immediate causes of obesity will clearly be consistently consuming more calories than are used, the reasons why humans (and other animals) have evolved to be disposed to this are not known. The thrifty genotype hypothesis proposes that in natural environments it pays to store lots of fat as an emergency energy supply during a food shortage. However, this doesn’t explain why not everyone is obese. The drifty genotype hypothesis is based on the idea that we have two separate critical levels of fat: if fat level drops to the lower one then we gain weight and if fat level gets to the upper one then we lose weight. This upper one is supposed to depend on the risk of predation, which is greater if fat reduces athleticism. Since we are no longer predated, people with mutations in the genes controlling the upper limit don’t get killed and so the levels have ‘drifted’ to different extents in different people.

Findings

We made a computational model which finds the best level of fat for an animal that may die of starvation or being predated. The animal can switch between activities that have different risks of predation and different amounts of food. If this switching is cost-free then the animal should have a single target level fat, which is determined by the environmental conditions suggesting that people should have a variable target. We predict that people will have poor control against over-eating because in human history it was never very costly in terms of survival. We explain that the incidence of death during famines cannot help assess the different explanations. If switching is costly, then the animal should have two critical levels, as in the drifty genotype hypothesis, but both of them should change if the predation risk changes, which suggests that this hypothesis is not consistent with evolutionary theory.

Implications

We made a computational model which finds the best level of fat for an animal that may die of starvation or being predated. The animal can switch between activities that have different risks of predation and different amounts of food. If this switching is cost-free then the animal should have a single target level fat, which is determined by the environmental conditions suggesting that people should have a variable target. We predict that people will have poor control against over-eating because in human history it was never very costly in terms of survival. We explain that the incidence of death during famines cannot help assess the different explanations. If switching is costly, then the animal should have two critical levels, as in the drifty genotype hypothesis, but both of them should change if the predation risk changes, which suggests that this hypothesis is not consistent with evolutionary theory.