I am currently at the 27th Annual Scientific Meeting of The Obesity Society (TOS) here in Washington DC.
Among the many excellent presentations, the one that I found of particular interest, was a keynote talk on the weight-reduced state by Rudulph Leibel, Columbia University, New York). This presentation addressed the important issue of why weight loss is so difficult to maintain and relapse (or weight regain) is the rule rather than the exception.
As Leibel and colleagues demonstrated in a now classic paper published in the New England Journal of Medicine back in 1995, even a 5 or 10% weight loss can lead to a rapid and sometimes drastic reduction in total energy expenditure, thereby very quickly canceling out the energy deficit created by caloric restriction.
The clinical expression of this reduction in energy expenditure is of course the infamous weight-loss plateau, which ultimately limits the magnitude of virtually every weight-loss attempt.
In his talk Leibel attributed this weight-loss-associated fall in energy expenditure to an “asymmetric” physiological response to leptin. Thus, although leptin appears to have little effect on the homeostatic system across a wide range of levels, when leptin levels drop below a putative “threshold” (as in weight loss), it triggers a powerful anabolic response resulting in a reduction in energy expenditure and an increase in orexegenic signals.
In his presentation, Leibel described how administration of low-dose leptin, just enough to raise leptin back to the levels that existed prior to weight loss, can prevent this anabolic and orexogenic response.
Functional MRI studies now show that leptin administration after weight loss also reduces activation of the hypothalamus and limbic systems, thus restoring the function of both the homeostatic and hedonic systems to the preweight-loss state.
There is some evidence to suggest that obese individuals may have higher “leptin” thresholds making them more sensitive to weight loss and perhaps more efficient in defending their body weight. It appears that once a higher bodyweight is maintained this threshold is changed to a higher level, perhaps because higher leptin levels result in structural changes in hypothalamic excitory synapses.
Once this higher threshold is established, any attempt at weight loss essentially moves the patient into a “non-physiological” hypometabolic state which not only reduces energy expenditure but also activates counter-regulatory “anabolic” behaviour. This makes it so difficult, if not impossible, for patients to maintain weight-loss in the long-term without continued caloric restriction and considerable amounts of exercise.
Clearly, the ability to restore the anabolic physiological metabolic state following weight loss back to a eumetabolic state at a lower body weight would be the “Holy Grail” of weight loss maintenance.
Whether or not administration of leptin (or a leptin analogue) can ultimately provide a safe and effective solution to this conundrum remains to be seen.