Milder forms of obesity may be a good evolutionary adaptation: 'Fitness First' hypothesis

The prevalence of obesity is steadily increasing and is considered mal- adaptive, as it is a risk factor for diabetes, hypertension, cardiovascular illnesses and cancer. However, contrary to popular be- lief and expectations, recent studies have shown that people with milder grades of adiposity survive better (obesity para- dox), both in normal and adverse condi- tions. Several new observations have been made on how insulin resistance accompa- nying obesity may be beneficial in selected situations. Insulin resistance operates at the post receptor level and selectively involves the phosphatidylinositol 3-kinase pathway controlling glucose metabolism while leav- ing the mitogen-activated protein kinase pathways intact, which promotes somatic growth. In insulin-resistant states, glucose is shunted away from the glycolytic pathways to the pentose phosphate pathway gener- ating more nicotinamide adenine dinucleo- tide phosphate (NADPH) for antioxidant enzymes for combating stress. Mild obesity improves survival probability but at the same time decreases fertility. Anthropological evidence shows that humans produce fewer children in resource-rich environ- ments, leading to improved biological fitness of progeny. This article examines the situ- ation of the obesity epidemic from a fresh evolutionary point of view, discusses and integrates the evidence from medicine, mo- lecular biology, evolution and anthropology, and hypothesizes that milder forms of adi- posity may be an evolutionary adaptation of humans to a resource-rich environment - a mechanism improving survival and promot- ing investment in fewer offspring, thereby improving the biological fitness of the race. However, this article does not recommend that readers maintain a bulging waistline. INTRODUCTION Obesity is a state in which excess fat is deposited at various sites in the body, gradually leading to a variety of health-related problems. The incidence of obesity is increasing world- wide; it is estimated that there are more than 1.4 billion people in the world who are overweight, of whom 500 million are obese 1 . In the USA, in 2012, it was esti- mated that 67.3% of people were obese or overweight 2 .