Lustig, R.H. (2006) Nature Clinical Practice Endocrinology & Metabolism 2 447-458
Childhood obesity has become epidemic over the past 30 years. The First Law of Thermodynamics is routinely interpreted to imply that weight gain is secondary to increased caloric intake and/or decreased energy expenditure, two behaviors that have been documented during this interval; nonetheless, lifestyle interventions are notoriously ineffective at promoting weight loss.
Obesity is characterized by hyperinsulinemia. Although hyperinsulinemia is usually thought to be secondary to obesity, it can instead be primary, due to autonomic dysfunction.
Obesity is also a state of leptin resistance, in which defective leptin signal transduction promotes excess energy intake, to maintain normal energy expenditure. Insulin and leptin share a common central signaling pathway, and it seems that insulin functions as an endogenous leptin antagonist.
Suppressing insulin ameliorates leptin resistance, with ensuing reduction of caloric intake, increased spontaneous activity, and improved quality of life. Hyperinsulinemia also interferes with dopamine clearance in the ventral tegmental area and nucleus accumbens, promoting increased food reward.
Accordingly, the First Law of Thermodynamics can be reinterpreted, such that the behaviors of increased caloric intake and decreased energy expenditure are secondary to obligate weight gain. This weight gain is driven by the hyperinsulinemic state, through three mechanisms: energy partitioning into adipose tissue; interference with leptin signal transduction; and interference with extinction of the hedonic response to food.
This important paper reviews the mechanisms underlying blood sugar regulation and appetite, showing how a high-sugar, low-fibre diet can lead to excessive production of insulin, which can in turn affect the body's sensitivity to the hormone leptin. The resulting disturbances of both appetite and energy expenditure help to create a vicious spiral of further overeating and weight gain.
Overweight individuals have long been told to 'Eat less and exercise more'. But as this paper makes clear, unless they also change the *kinds* of foods that they eat, this will be easier said than done, because the high-sugar foods and drinks they crave can actually create fundamental disturbances in their own biochemistry and metabolism.
If this analysis is correct, then consuming sugar to excess will effectively make people both greedier and lazier - which in turn could explain why the standard advice to 'eat less and exercise more' in order to lose weight appears in practice to be so ineffective.
See also the BBC News article on this research: 'Toxic Diets' Fuel Child Obesity