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There is evidence that public health measures aimed at reducing the burden of obesity have not been effective. Whilst there are several possible reasons for this, including a multitude of competing messages, the overabundance of energy dense food and the near absence of obligatory physical activity, it is likely that the more important reason is the fact that weight appears to be physiologically regulated and defended. This means that using public health measures to combat obesity is not likely to work for obese subjects. What is not yet known is if subjects who are overweight (as opposed to obese), defend weight loss, nor is it known if modest (~5%) weight loss is defended. If overweight or modest weight losses are not defended, it would allow the targeting of public health messages towards the overweight and to focus on modest weight loss in the obese. This study aims to investigate if overweight people defend weight loss as vigorously as obese people, and to determine at what degree of weight loss these physiological changes begin. Overweight and obese people will be recruited and placed on a VLED (very low energy diet) for a period of 12 weeks, aiming to decrease body weight by 15%. Measurements in body weight and a number of biomarkers will be taken at proscribed intervals. All participants will be followed up for a period of two years, to observe weight regain (if any). Gut hormone and other biomarker changes over the trial will be compared between the overweight cohort and the obese cohort, to determine any differences. For both cohorts, changes in gut hormones and other biomarkers will be compared at 5%, 10% and 15% baseline body weight lost, to determine when physiological defences increase.