Many books and articles on dieting, weight gain, and obesity often quote the rule of “3500 Calories is a pound”. By this, they mean that for every 3500 “extra” Calories consumed, you will gain one pound. So if you ate 100 extra Calories a day (e.g. a cookie), you would gain a pound in 35 days. In metric units, this is equivalent to 7700 Calories per kilogram. As I will show below, this rule is confusing and wrong on two accounts, although by sheer coincidence, it can be used as a mnemonic to estimate how extra food will lead to increased body weight. The basis of this rule is that the metabolizable energy density of fat is about 9300 Calories per kilogram and protein is about 4000 Calories per kilogram, and if most of our weight gain is in fat then you can persuade yourself that 7700 Calories per kilogram is a reasonable number. I should note that the dietary Calorie is equivalent to one kilocalorie or about 4.2 kilojoules of energy.

The first reason this rule is wrong is that the energy density of deposited tissue is not fixed at 3500 Calories per pound because the composition of that tissue is not fixed. It depends on how much you weigh and possibly diet composition. Kevin Hall has a nice article on this. The second problem is that this rule completely ignores the fact that as you gain weight your energetic needs change so not all of the extra 100 Calories can go to new tissue. If you apply the rule, it says that if you eat an extra 100 Calories per day, or one cookie, then in 35 days you’ll have gained a pound and in one year you’ll have gained 10 pounds. This is actually reasonable and close to being correct. However, if you continue to apply the rule then you’ll also predict that in 10 years you’ll gain 100 pounds and your weight will increase linearly forever. This is clearly not correct. What really happens is that as you gain weight you also burn more energy and eventually you’ll reach a new steady state where you burn what you eat.

The rule is also confusing because there are at least three ways of interpreting “eating an extra cookie per day”. What it was originally intended to address is the situation where everything else in your life is constant (i.e. eat the same thing, do the same amount of physical activity, and have no change in health), and then you eat an extra cookie. The presumption is that previously you were burning everything you eat and now you have an extra 100 Calories per day that you store as new body tissue. A second interpretation is that you are always exactly one cookie or 100 Calories out of energy balance every day. So, whatever amount of energy you are burning at this moment, you eat 100 Calories extra. This would imply that you are always out of energy balance and your weight would indeed increase linearly in time. The third interpretation is that you eat an extra cookie per day over what you ate the previous day. This would imply that you eat one cookie today, two tomorrow, three the next day and so forth. Because, the rule is inherently wrong and that it can be interpreted in multiple ways, it has led to great confusion and myths about dieting and how much you need to eat to lose or maintain weight. Below, I will try to make all of these concepts precise.

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