It’s time now to discuss yo-yo dieting or “weight cycling”. Many folks are unfortunately familiar with this phenomenon, which can lead not only to lasting physiological damage, but psychological damage as well. Yo-yo dieting refers to the “annoying” habit of losing five pounds, only to later gain back seven. A person in this situation will often then refocus their energies to lose another ten pounds, only to gain back twelve. You get the idea. Fluctuating body weight in this manner is associated with high blood pressure and triglyceride levels. It’s also understandably difficult from a mental and emotional perspective.
Resting metabolic rate
Resting metabolic rate (RMR) is the amount of energy your body expends to maintain physiological equilibrium at rest. In other words, RMR accounts for the number of calories (kcal) needed to maintain your heartbeat, brain activity, organ function, etc. while you sit in bed and stare at the ceiling all day. There are many elaborate formulas to estimate a person’s RMR, but for the sake of this article let’s approximate it at 10 kcal/pound/day. Eddie, for example, weighs 200 pounds and has an RMR of 2000kcal/day.
Here is how RMR relates to yo-yo dieting: the primary determinant of RMR is muscle mass. As a person loses weight they will likely lose both fat and muscle mass. Thus, as Eddie drops from 200lbs to 185lbs his muscle mass decreases, as does his RMR (from 2000kcal/day to 1850kcal/day). The good news is that Eddie managed to achieve his weight loss goal. The bad news is that he can never “go home again”, meaning that unless he increases his physical activity energy expenditure he can never eat what he used to eat at 200lbs and maintain his new, current weight. His body simply doesn’t need as many calories to maintain itself.
Although this makes perfect sense, many folks fall prey to yo-yo dieting. Eddie, for example, decides to enroll in a new fad diet. He loses a significant amount of weight in a short period of time, but then cracks, binges, and gains it all back. Or perhaps he decides to lose weight for a special occasion, succeeds, and then returns to his previous diet and lifestyle. In both scenarios Eddie’s weight loss results from a decrease in both fat and muscle mass; subsequent weight gain is associated primarily with an increase in fat mass. When Eddie returns to his original weight of 200lbs, his RMR is less than what it used to be (perhaps 1950kcal/day) because he does not have as much muscle mass. Unless Eddie increases his physical activity, it shouldn’t come as a surprise that over the next few months he will put on several additional pounds.
The take home message is two-fold. First, be realistic about weight loss goals – both the total amount you intend to lose as well as the rate. Two pounds per week may be the maximum sustainable rate of weight loss, but one pound per week (or less) is more manageable. Second, no diet will be successful long term unless it is also accompanied by a change in lifestyle; physical activity is key to maintaining muscle mass and/or increasing energy expenditure. However, lifestyle changes can be difficult to make. Next week we’ll explore some practical behavior modifications related to weigh loss and management.