From Harry Rutter’s Where next for obesity? comment (free, registration required):
There is a seductive simplicity to the conceptualisation of obesity as a straightforward problem of energy balance—calories in versus calories out. But the physiological, behavioural, and environmental influences on this relation are asymmetrical. Therefore, although the basic arithmetic holds true, in practice it is much easier for people, and populations, to gain weight than to lose it.
Read other comments and papers in The Lancet’s recent series on obesity.
