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But many people, particularly those with type 2 diabetes, struggle to decide which one is best.
University of Otago department of medicine researchers Adjunct Prof Mike Lean and Dr Andrew Reynolds, along with researchers from Thailand and the United Kingdom, have analysed published data on 19 diets, covering Mediterranean, vegetarian, vegan, high-fat, and low carbohydrate ketogenic diets, to determine which is best for achieving and then maintaining weight loss in people with type 2 diabetes.
They found a range of low-energy diets were suitable for weight loss, but the greatest weight loss was achieved with formula meal replacement diets for 8-12 weeks.
They led to an average of 6.6kg more weight loss compared with food-based low-energy diets.
Dr Reynolds said awareness of the benefits of weight loss for type 2 diabetes was high, but both patients and healthcare practitioners needed clearer guidance on how to achieve this.
"This research is important to inform choice for those looking to lose weight.
"We see 1001 diets advertised for weight loss, but it can be hard to know which ones work."
He hoped the research would inform discussions with physicians that were flexible to what patients wanted.
"So, if they want to lose weight on a plant-based diet, or a higher protein diet, or a meal-replacement diet, then they are supported to do so."
More than onemillion New Zealanders are affected by type 2 diabetes — about 7% of the population have the chronic disease, and a further 18% have pre-diabetes.
Dr Reynolds said low-carbohydrate diets were no better than higher-carbohydrate diets for weight loss, and macronutrient content had little impact compared with energy intake.
He said few trials had reported data on successful long-term maintenance of weight, and more research was required.
"Well-conducted research is needed to assess longer-term impacts on weight, blood sugar control, clinical outcomes and diabetes complications."