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Thursday, January 23, 2014

Sorry Dr Lustig: A calorie is still a calorie

In his book: “Fat chance: The bitter truth about sugar”, Dr Robert Lustig argues that a calorie is not a calorie or basically, not all calories from different foods are equal. He makes three arguments to support his idea. This part of his argument is central to the subsequent claim he makes that sugar and in particular fructose is the villain of obesity.
He begins by pointing out that weight loss frequently reaches a plateau because as we lose weight our resting energy expenditure falls. This resting energy expenditure is the energy required to keep our heart beating, our kidney’s filtering, our lungs breathing, our brain thinking and so on. It’s the calories you burn when you are asleep. The fall in resting energy expenditure is one of several adaptions the body makes when energy intake is restricted. Another adaptation is that the brain agrees to reduce its insistence on glucose as its sole fuel and agrees to start burning fats for fuel. If it did not do this, then the body would have to make glucose from amino acids, which would deplete body protein stores. So, several adaptations are made when energy is restricted but this has absolutely nothing to do with the calorific value of the fuels used by the body. The calorific value of amino acids, glucose, fats and ethanol are determined by metabolic pathways that allow energy to be extracted from these metabolites and these pathways are not amenable to change. Adaptations do occur but the calorific value of nutrients remains absolutely constant. A major US trial of diet composition on diet-induced reduction in resting energy expenditure found no evidence that this drop in resting energy expenditure was related to variation in dietary composition[1]
The second argument made is that within the categories of nutrients (carbohydrate, fat and protein) there is considerable variation. Thus Lustig points out that there are good fats and bad fats, that proteins vary in their quality or nutritional value and that carbohydrates range from complex molecules such as starch to simple molecules such as sugar. So lets consider fats. A typical dietary fat is made up of one molecule of glycerol (a sugar alcohol) and three fatty acids. Each fatty acid can vary in length typically from12 to 22 carbons long and the amount of hydrogen attached to each carbon can vary between 1 and 2. So, of course there is a wide variety of fats but that has no bearing on their calorific value. Since hydrogen is the atom that is central to the extraction of biological energy, we can predict exactly what the energy value of a fat can be based on its hydrogen count. A gram of fat can be made up of a lot of small chain fats or a a lesser amount of long chain fats. The calorific content will not change. Some fats are primarily designed to contribute to the structure of the body and their energy potential is not their primary function. Thus the fatty acid arachidonic acid (which the body synthesizes from fats found in vegetable oils) plays a major role in cell wall architecture and in the regulation of blood clotting and inflammation. The fatty acid eicospentaenoic acid (EPA derived from fatty fish) plays a role in the structure of nerves and in the transmission of nerve signals. Sure, fats vary in their structure and function but this has nothing to do with the theory that not all calories are the same.
The third argument he makes is that that our diet quality has changed and that we have reduced our fat intake and increased our sugar intake. However, the calorific value of fats and sugars remain constant. A calorie is still a calorie. Indeed in a major dietary intervention study of weight-reducing diets involving 811 obese subjects using the following 4 radically diets found no significant difference in weight loss.
% calories from
 So its calories that count and a calorie really is just that irrespective of the nutrient or food it comes from.[2]

[1] De Jonge L et al (2012) Obesity,20(12):2384-9. Effect of diet composition and weight loss on resting energy expenditure in the POUNDS LOST study
[2] de Souza RJ et al (2012) Am J Clin Nutr. (2012) 95(3):614-25 Effects of 4 weight-loss diets differing in fat, protein, and carbohydrate on fat mass, lean mass, visceral adipose tissue, and hepatic fat: results from the POUNDS LOST trial.

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