Objective: To investigate the effect of commercial weight loss programmes on macronutrient composition and micronutrient adequacy over a 2 month period. Design: Adults were randomly allocated to follow the Slim Fast Plan, Weight Watchers Pure Points Programme, Dr Atkins' New Diet Revolution, or Rosemary Conley's `` Eat Yourself Slim'' Diet & Fitness Plan. Setting: A multi-centre randomised controlled trial. Subjects: 293 adults, mean age 40.3 years and a mean BMI 31.7 (range 27 - 38) were allocated to follow one of the four diets or control group. Subjects completed a 7-day food and activity diary at baseline (prior to randomisation) and after 2 months. Diet records were analysed for nutrient composition using WinDiets (research version). Results: A significant shift in the macronutrient composition of the diet with concurrent alteration of the micronutrient profile was apparent with all diets. There was no evidence to suggest micronutrient deficiency in subjects on any of the dietary regimens. However, those sub- groups with higher needs for specific micronutrients, such as folate, iron or calcium may benefit from tailored dietary advice. Conclusion: The diets tested all resulted in considerable macronutrient change and resulted in an energy deficit indicating dietary compliance. Health professionals and those working in community and public health should be reassured of the nutritional adequacy of the diets tested.