Do carbohydrates cause weight gain?
Low carb and low-sugar diets are based on the notion that carbohydrates and sugar are specifically problematic for weight gain.
What does the evidence say? And is asking, ‘do carbohydrates cause weight gain’ even the right question?
Are low carb diets best?
There is evidence that low-carbohydrate, high-protein diets may induce greater weight loss, at least in the short term[i]. This is likely due to greater satiety effects and increased thermogenic energy expenditure. Yet extreme low-carb, high protein diets have not been proven safe over the long term [ii] [iii] and limit or exclude healthy foods recommended by Dietary Guidelines such as wholegrains, fruits, legumes and dairy foods. Very low-carbohydrate diets are also problematic from an environmental sustainability perspective because of their over-reliance on animal foods with a greater environmental impact[iv] .Fortunately, a diet more moderate in protein has been shown to be effective for longer term weight management. In the world’s largest diet study, DIOGENES, a moderate-protein (25% of energy) and low glycemic index (GI) was the most effective of the four diets tested, including a high protein diet with high GI carbohydrate sources[v]. Hence the glycemic nature of the carbohydrate is likely to be key to weight control.
Does added sugar cause weight gain?
Surprisingly, in several epidemiological studies sugar intake is inversely associated with body mass index (BMI)[vi] [vii] and the Institute of Medicine (USA) has concluded there is “no clear and consistent association between increasing intake of added sugars and BMI”[viii]. In the Dietary Guidelines for Australians, the evidence for any relationship between sugars and body weight or body fatness was rated as poor [ix]. Although the evidence is conflicting, intake of Sugar Sweetened Beverages (SSB) may cause weight gain, suggesting added sugar in this form may be obesogenic [x] [xi] In Australia, however, added sugar intake and SSB intake have been declining over the same period as obesity has increased – the so-called Australian sugar paradox – suggesting sugar intake is not a primary driver of population obesity levels [xii].
Foods, not nutrients
There is a paradigm shift away from nutrients to a focus on whole foods and dietary patterns to improve health. It is now understood that low-carb diets are too narrow in their focus and can have adverse effects, both in the diet and in the food supply. The traditional Mediterranean diet is an example of the inadequacy of macronutrient based approach to diet. It is moderately high in fat and moderately high in carbohydrate, yet is associated with increased longevity[xiii] and reduced obesity and chronic disease risk[xiv] [xv]. It works because it contains a healthful pattern of nutritious foods - and the carbohydrate content is part of the mix.
Asking whether carbohydrates or sugars uniquely contribute to weight gain is not the right question; asking whether carbohydrates and added sugars can be consumed from nutritious foods as part of a an overall healthy food pattern is a better question. No diet that detracts from the enjoyment of food will be sustainable long term. A healthy dietary pattern is a better approach for weight control as well. Surplus energy in any form (high or low in carbohydrate) will increase the risk of chronic disease.
This article was reviewed by Professor Jennie Brand Miller from the School of Molecular Biosciences and Charles Perkins Centre and Director, Sydney University Glycemic Index Research Service.