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Jusup does not understand why there is no debate on whether the laws of thermodynamics apply to humans, but he recognizes that caution must be exercised when using these laws to make predictions . "Using them to provide dietary or nutritional advice seems, in a sense, to use a wall clock to measure milliseconds," he says. Hall echoes this sentiment, claiming that applying the laws of thermodynamics to weight loss tips can be confusing. He points to the "3500 Calorie Rule" as a case in point. Preece also mentioned this rule as a source of concern.
The 3500 calorie ruler – written in nutrition textbooks and dieting websites – states that for every 3500 calories you lose from your diet, you will lose a pound of body fat. But the data do not match and this failure has led some to question the validity of any nutrition or weight loss model involving energy saving.
The 3500 calorie rule comes from the work of the American doctor Max Wishnofsky. In 1958, he calculated that the amount of energy stored in a pound of fat tissue was about 3,500 calories. [1]He then claimed is the amount that a person should give up to pay a pound of fat. But this thought poses two problems, Hall says. First, the calculation assumes that only fat disappears when you lose weight, which is wrong. Secondly, it assumes that the body's energy expenditure remains constant, while studies show that the body adapts to its lighter load by burning fewer calories. You have to reduce the growing number of calories to lose every extra kilo.
Hall thinks that the rule has gained ground because it is easy to memorize and that, on a time scale of a few weeks, the predictions are blown away. "People like simplicity," says Hall.
Hall says he and others have made progress in educating doctors and dieticians on the equivalence of calories from different macronutrients and on the 3500 calorie ruler. For example, they have developed tools that allow doctors to make more accurate predictions. Hall's NIH Body Weight Planner software encodes a simplified version of its energy flow model and can be used by patients to predict the amount of calories needed to reach a target weight. "The website has been used by millions of people, so the message is clear," he says. But, he adds, spreading diet myths among the general public is another ball game.
-Katherine Wright
Katherine Wright is a senior writer for Physics.
Correction (April 29, 2019): This story has been updated because an earlier version of the text was published by accident.
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