Mice experiments on caloric restriction compare caloric restricted mice with mice eating as they wish (ad libitum). By contrast, in human dieting, humans eat less calories than they were doing previously at their higher weight. How can the reduction in calories of the human, e.g. a reduction of 20%, be compared and calibrated with the forced reduction of the mice? What role does the reference-man calculation comparing caloric consumption with a same-BMI person of BMI 22 mean here?

Here is no truly objective way to calculate the degree of calorie restriction you are achieving on a CC diet regimen or other calorie restriction protocols. Obviously, you can calculate your percent reduction of calorie intake from baseline when you go on a CC regimen; this calculation will be most meaningful once your weight has equilibrated at a new lower set point, and you thus are on a sustainable regimen. However, if you had been substantially overweight and were really stoking in the calories, a 20% reduction from baseline in calorie intake — while no doubt beneficial for your body weight and your long-term health prospects — cannot be presumed to provide the same degree of calorie-restriction health benefit that a relatively lean and abstemious person would achieve if he likewise reduced his calorie intake 20% from baseline. That’s why some experts on calorie restriction suggest that we calculate our “true” level of calorie restriction by comparing our calorie intake to that of a person of comparable height who has a relatively lean, healthy BMI of 22 and is taking in sufficient calorie to maintain his weight. Choosing a BMI of 22 was rather arbitrary — 21 or 23 might have served as well. But by employing this standard, we can then seek to correlate one’s “true calorie” restrictions with health outcomes, and perhaps use the results of studies in rats or rhesus monkeys to estimate the life expectancy benefits we might hope to enjoy.

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