August 13, 2015

expert reaction to study comparing low-fat and low-carb diets and body fat loss

Publishing in the journal Cell Metabolism, a group of researchers has described their work into the differences between diets which are low in fat or those which are low in carbohydrates. They report that in the 19 obese people which they studied both diets resulted in loss of body fat, but that this loss was greater in the low fat diet.

Prof. Susan Jebb, Professor of Diet and Population Health, University of Oxford, said:

“This is a well conducted experimental study to study the metabolic effects of diet composition on body fat. It shows that, once total energy (calorie) intake is controlled, there are only small differences in fat oxidation (burning) as the body adapts its metabolism to closely match the composition of the diet that is eaten. The very small differences they observe may even be due to the short period of study, just 2 weeks, since it takes a short while for carbohydrate stores to adapt to the new carbohydrate intake. The similarity in the results observed from those predicted by the mathematical model reaffirms that the changes in body composition under controlled conditions can largely be predicted.

“But life in the confines of a metabolic ward with strictly limited access to food can only address the metabolic response to food and not the behavioural response to the environment. The investigators rightly conclude that given the minimal metabolic differences between low fat or low carb diets, the best diet for weight loss is the diet you can stick to. The real challenge for science is not the nutritional composition of the diet, but the behavioural strategies to promote adherence. All diets “work” if you stick to an eating plan that cuts calories, whether from fat or carbohydrate, but sticking to a diet is easier said than done, especially given the prolonged time it takes to lose weight.”

Prof. Nick Finer, Honorary Professor, National Centre for Cardiovascular Prevention and Outcomes, UCL Institute of Cardiovascular Science, UCL, said:

“Is it better to reduce fat or carbohydrates in your diet to lose weight? That is the question this study set out to explore by studying obese men and women in a metabolic chamber following a 30% cut in their energy (calorie) intake either from carbohydrates or fat over a six-day period from a period on a ‘healthy’ diet. These were big changes – cutting either carbohydrate intake by 60% or fat by 85% (leaving protein intake unaltered) or about 800 kcals each day, something unlikely to be achieved in a real life setting. However, big changes were needed to allow the study set out to explore the metabolic impact of changing the macronutrient content of the diet and also allowed the authors to validate a computer model that could be used to predict weight changes in a ‘real-life’ setting.

“Energy expenditure fell by about 90-100 kcals/day, something known to occur when energy intake is cut, and when weight is lost. Cutting carbohydrates, but not fat, increased the rate at which fat was burned, but at the expense of reducing the amount of carbohydrate used as a fuel source. However, the effect of cutting fat intake still outweighed this effect such that the reduced fat diet still led to greater fat loss – almost double. Weight changes of course are influenced by whether you are losing fat (about 9000 kcals in 1 kg), carbohydrate about (4,500 kcal/kg) or water (no kcals/kg) and the study was not well designed to measure these changes. It was still not surprising that weight loss was paradoxically slightly greater in the low carbohydrate diet as there was less of the high energy density fat being burnt.

“The study elegantly confirms that certainly in the short-term ‘calorie for calorie, restriction of dietary fat led to greater body fat loss than restriction of dietary carbohydrate’ in these obese adults and that theories that carbohydrate restriction are needed to lose body fat are wrong and that reducing insulin (as is seen with low-carbohydrate diets) is not necessary to increase fat-burning.

“In practice, evidence points to reduced fat diets being the most successful long-term, while increasing protein may enhance satiety or fullness. What really determines the success or failure of weight loss diets is how well people can adhere to them and this goes beyond just their nutrient composition involving issues of palatability, convenience, hunger, personal beliefs.”

‘Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity’ by Hall et al. published in Cell Metabolism on Thursday 13th August. 

Declared interests

Prof. Susan Jebb: My institution has received research support, either funding or in kind from the Cambridge Weight Plan, Weight Watchers and Slimming World who provide weight management services.

Prof. Nick Finer: No interests to declare.