ABSTRACT
Over the past 100 years, twin and family studies have produced new knowledge on the role of genetic and environmental factors behind individual differences in obesity. Early twin and family studies showed the importance of genetic factors on the variation of body mass index (BMI) and other obesity indicators. Recent large-scale twin studies have established that the heritability of BMI has a characteristic age pattern. The heritability of BMI is highest in early adulthood when genetic factors explain around 80% of BMI variation and lowest in early childhood and old age when this proportion is between 40% and 60%. In early childhood, the lower heritability is because of the influence of environmental factors shared by co-twins, and in old age, environmental factors are unique to each individual. The effect of genetic factors on obesity can be moderated by eating behavior, physical activity, and metabolic factors. However, environmental factors can also modify the genetic variation of BMI, providing evidence on gene–environment interactions. Genetic variation is highest in obesogenic environments measured by several society-, family-, and individual-level indicators. This indicates that especially those having the highest genetic susceptibility to gaining weight are most prone to the influence of environmental exposures.
