Scientists are investigating whether an infusion of gut hormones could help people to stop over-eating, quit smoking, or give up alcohol. In the study, volunteers will have three infusions of hormones similar to GLP-1 and ghrelin, or a placebo infusion of salt water. There will be 90 participants in total -- 30 will be overweight and trying to lose weight, 30 will be ex-smokers and another 30 will be ex-drinkers.