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Efforts to identify a single optimal diet for the treatment of overweight and obesity have so far failed.


Overall, the body of scientific evidence from randomised controlled trials (RCTs) indicate that in order to lose weight, individuals need to consume less energy (kilojoules/calories) than they are expending through their physical activities and basal metabolic rate. Dietary energy can come from a range of different diets with varying macronutrient contents. The macronutrients are carbohydrate, fat, protein and arguably alcohol (although not officially a macronutrient, for some people it is a significant source of energy). One size does not fit all, and the best diet for most people is the one that they can stick to (the one that suits their cultural, religious, familial and personal food preferences, and budget) in the long run.

Due to the high cost of conducting RCTs, most are short-term (conducted over a period of 3-6 months) and few are long-term studies (at least 2 years). Short-term studies often do suggest that one dietary pattern is superior to another, but these results rarely hold true after 12 months in the rare instances that they last that long, and differences usually completely disappear by 2 years as people develop diet fatigue and are no longer able to maintain the disruptions to their family, religious and social lives.

Because they are rare, when a well-designed long-term RCT is conducted, we should pay attention to the results. Back in 2010, a group of American scientists randomised around 300 people to one of two diets and followed them up for 2 years: 

  • One of the diets was low-carbohydrate, which consisted of limited carbohydrate intake (20 g per day for 3 months) in the form of low–glycemic index vegetables with unrestricted consumption of fat and protein. After 3 months, people in the low-carbohydrate diet group increased their carbohydrate intake (5 g per day per week) until a stable and desired weight was achieved. 
  • The other was a reduced energy (1200 to 1800 calories (5000 to 7500 kJ) per day) low-fat diet that provided no more than 30% of calories from fat. 

Weight loss was significant at 1 year (a decrease of approximately 11 kg) and 2 years (7 kg), however, there were no differences in weight, body composition, or bone mineral density between the two different diet groups at any time point.

However, a group of Danish scientists worked with the original American scientists last year, re-analysing the study data according to whether the people in the study had prediabetes (blood glucose between 5.6-6.9 mmol/L) or high fasting blood insulin. After 2 years, participants with prediabetes and high fasting insulin lost 7.2 kg more with the low-fat compared to the low-carbohydrate diet, whereas those with prediabetes and low fasting insulin tended to lose 6.2 kg more on the low-carbohydrate diet than low-fat diet.

This new analysis of a well-designed long-term randomised controlled trial helps explain why there is not one single optimal diet for the treatment of overweight and obesity. Tailor-making dietary advice to suit the specific needs of individuals is the direction we need to be heading in.

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 Dr Alan Barclay
Alan Barclay, PhD is a consultant dietitian and chef (Cert III). He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of  The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).
Contact: You can follow him on Twitter, LinkedIn or check out his website.