1 January 2018


New year’s resolutions often include promises to lose weight. While increasing physical activity is commonly part of the equation, for most people, the burden of weight loss reduction focuses on the “diet”. The problem is, which diet? Around 2,500 new “diet” books are published each year – all proclaiming that their way is the only way to lose weight and keep it off. As there are only three major nutrients – fat, carbohydrate and protein – these “diets” typically manipulate (increase or decrease) one or two of these macronutrients to achieve the desired result. We only have to think about which diets have been popular over the past 50 years or so to understand what is going on: low-carb diets were fashionable in the 1970s; low-fat diets dominated the 80s and 90s; now “low carb” has made a comeback. Food is fad when it comes to “diets”.

But there are some good sound facts about “diets”. Over the past 50 years, nutrition scientists have carried out numerous randomized controlled trials investigating the effects of various weight-loss diets on body weight and mortality (yes, death) and the findings of these studies give us enough data to summarize their effects using systematic review and meta-analysis statistical techniques.

The most recent analysis looked at long-term studies (greater than one year) of adults with a BMI greater than 30kg/m2 in people from a European background and BMI greater than 25kg/m2 in people from other backgrounds including Asia and India. This meta-analysis clearly shows there is level 1 evidence that low-fat diets in combination with physical activity help people lose weight.

Active people

The researchers found 54 trials involving 30,206 people who were followed for up to 12 years. All but one of the trials included weight-loss interventions that were low fat (less than 30% of energy from fat), and most were also low in saturated fat. One study included the Mediterranean diet and one was very low carb (less than 50 grams of carb-containing foods a day). Most of the interventions included exercise advice or an exercise program.

After one year, the average weight reduction was 3.42kg (7.5 lb); at 2 years it was 2.51kg (5.5 lb); and at 3 years it was 2.56kg (5.6 lb). It may not sound like a lot of weight, but these results are all statistically significant and equivalent to each person losing and keeping off at least five (500g/1.1lb) tubs of margarine or butter. This amount of weight loss was also enough to have some significant effects on the risk of premature death – a nearly 20% reduction in fact, or 6 lives saved per 1000 people.

We are the first to agree that one-size does not fit all when it comes to dietary patterns. But it is important to note that at this point in time, there is not equivalent level 1 evidence for the efficacy of long-term use of low-carb diets.

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Dr Alan Barclay  
Alan Barclay, PhD is a consultant dietitian. 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).