Obesity: evolution of a symptom of affluence.

This paper delineates the evolutionary background of the unprecedented epidemic of obesity that has evolved over the last century. Some two million years ago, a change of climate in the habitat of our primate ancestors triggered dietary adaptations which allowed our brain to grow. A shift from principally carbohydrate-based to fish- and meat-based eating habits provided sufficient fuel and building blocks to facilitate encephalisation. Insulin resistance may have evolved simultaneously as a means to avert the danger of hypoglycaemia to the brain (in view of the reduction of carbohydrate intake). Ensuing cognitive capacities enabled the control of fire and the manufacturing of tools, which increased energy yield from food even further and eased the defence against predators. The latter development relieved the selective pressure to maintain an upper level of bodyweight (driven by predation of overweight ndividuals). Since then, random mutations allowing bodyweight to increase spread in the human gene pool by genetic drift. Also, (seasonal) food insecurity in hunter-gatherer societies spurred the evolution of thrifty genes to maximise nutrient intake and energy storage when food was available. The agricultural and industrial revolutions rapidly changed our habitat: virtually unlimited stocks of (refined) foodstuffs and mechanical substitutes of physical efforts push up energy balance, particularly in those of us who are still adapted to former environmental conditions: i.e. who carry thrifty genes and lack (genetic) protection against weight gain. Intrauterine epigenetic mechanisms potentially reinforce the impact of these genes on the propensity to grow obese.