Friday, January 9, 2015
This fallacy is based on the rathe simplistic notion that because 7 x 500 arithmetically happens to equal 3,500 Cal, which just happens to be approximately the caloric content of 1 lb of fat tissue, a reduction in weekly energy intake of 3,500 calories should lead to a 1 lb weight loss.
Thus, a recent rather unfortunate “Patient Information” page published in JAMA, states that,
“A total of 3500 calories equals 1 pound of body weight. This means if you decrease (or increase) your intake by 500 calories daily, you will lose (or gain) 1 pound per week. (500 calories per day × 7 days = 3500 calories.)”
Nothing could be further from the truth!
Now David Allison and colleagues, in a letter to the JAMA editors, points out just how nonsensical this “rule” actually is:
“For example, if a 5′6″, 30-year-old woman weighing 180 lb and consuming 2622 calories daily reduced her intake by 500 calories per day, the 3500-calorie rule would estimate her weight loss at 1 year to be almost 52 lb. At 10 years, the 3500-calorie rule would yield a negative body weight..”
Or, as I tell my patients, no one will continue losing weight till they disappear.
As regular reader of these page will appreciate, the simple reason why the 3500 Cal rule is wrong, is because the relationship between changes in caloric intake and weight loss is anything but linear. Rather, as daily caloric consumption decreases (or increases), metabolic demands adapt to compensate for this decrease (or increase), thereby limiting what happens to body weight.
This “dynamic” model of weight loss is nicely illustrated in the many validated predictors of weight loss available online (click here for an example).
Thus, based on such a model, in the above case, the actual expected weight loss at 1 year would only be around 12 lb with stabilization of a a 31-lb loss after 3 years.
Incidentally, these number, which in the above case correspond to about a 7% weight loss at one year and a 16% weight loss at 3 years are well in excess of what can generally be achieved with diet and exercise alone (those numbers being closer to 3-5% at 12-24 months).
As Allison and colleagues rightly point out, such inaccuracies (especially when propagated by reputable journals such as JAMA) mislead both health professionals and the public and do little more than promote unrealistic expectations about weight loss.
It is high time we abandoned simplistic “energy-in energy-out” models of obesity or weight management.