Shed fats, not carbs ~ new human study

In recent years, it has become quite fashionable to argue that the real culprit in obesity are carbohydrates and as such fats have gained considerably good marks in the hierarchy of villainous calories. Gary Taubes, a highly influential author on this topic wrote thus: ‘‘Any diet that succeeds does so because the dieter restricts fattening carbohydrates .Those who lose fat on a diet do so because of what they are not eating—the fattening carbohydrates’’. The argument is that carbohydrates cause a surge in insulin release and insulin is what keeps fat within fat cells. When carbohydrates are restricted, this insulin surge falls allowing fats to be released from fatty tissue for burning (oxidation) for energy. All very well in theory but now, the results of an impeccably designed human study will greatly challenge this recent view that fats are good and barbs are bad[1]. So what makes this study so solid.

The volunteers were extremely obese. The males had a BMI of 38 while that of females was 33. The study randomized the subjects into two arms for 11 days of dietary intervention and all of the period of dietary intervention was conducted in a metabolic ward with strict clinical supervision. For the first 5 days in each study arm, the subjects ate what is referred to as a eucaloric diet. That is, they received the exact amount of calories that they needed simply to neither gain nor lose weight. The nutritional composition of each subjects eucaloric diets was identical with 50% energy from carbohydrate, 35% from fat and 15% from protein. For the next 6 days, their caloric intake was reduced by 30% either through a very low fat diet or a very low carbohydrate diet. There were no other changes in the composition of the calorie reduced diets. The only foods available to the subjects were those prepared by the research team and all eating occasions were supervised.

 On days 2 and 5 of the eucaloric diet and on days 1, 4 and 6 of the energy restricted diets, the subjects spent 23 consecutive hours inside a metabolic chamber. This would be a small room with a bed, seat, bathroom and other facilities but which is specially constructed to measure the inflow and outflow of oxygen and carbon dioxide. By measuring the loss of oxygen (used for fuel) and the gain of expired carbon dioxide, it is possible to accurately tell what type of fuel is being burned by the body for energy.  When the six days of dieting ended, the subjects took a 2-4 week break before resuming the same protocol but switching from the low fat arm to the low carbohydrate arm and vice versa.

The main effects were as follows, as outlined by the authors: “Body fat loss was calculated as the difference between daily fat intake and net fat oxidation measured while residing in a metabolic chamber. Whereas carbohydrate restriction led to sustained increases in fat oxidation and loss of 53 g/day of body fat, fat oxidation was unchanged by fat restriction, leading to 89 g/day of fat loss, and was significantly greater than carbohydrate restriction”.

To those of us in experimental nutrition for some time, these results come as no surprise. Very similar results were found in a series of studies carried out at the UK Dunn Nutritional laboratory in the 1990s. The bottom line is this. If you want to lose fat from the body, first lose it from the plate but make sure that the caloric deficit of fat leads to an overall deficit of calories form the diet.  

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[1] Hall K et al (2015) Cell Metabolism, 22, 1-10