To summarize 10 years of the Finnish twin research into obesity, adult BMI is a highly heritable trait, but environmental influences have a major role in the eventual development of obesity and associated comorbidities. Higher levels of physical activity in adolescence can prevent the development of obesity in adult age, and the genetic predisposition to weight gain can be counteracted by an active lifestyle. Even at an early age, several prediabetic and preatherosclerotic changes can be observed associated with obesity: insulin resistance, accumulation of fat in the liver, proatherogenic changes in the lipid profile and low-grade chronic inflammation of the fat tissue that may be initiated by proinflammatory changes in the lipid composition of the cell membrane, depletion of mitochondrial DNA copy number and concomitant downregulation of the BCAA catabolism and energy generating oxidative phosphorylation in mitochondria (Figure 5). Simultaneously physical fitness declines and the ability to participate in the very activities that promote healthy cardiovascular and endocrine functions are compromised, leading to a vicious cycle of more weight gain and an escalation in the pathological processes described above. The studies here reviewed continue to give more information on these processes and the ongoing follow-up of these rare, discordant twins is sure to provide even more insight into the continuing process of weight gain over an individual's lifetime.
