Okay, so you resolved to lose weight. You reduced your calories. You added a few miles of walking or running to your daily to-do list. And what was the result? That’s right, you didn’t lose an ounce. Or maybe gained a pound. Or two. What’s up with that?
What’s up is that you probably miscalculated the unique and fickle calorie requirements of your metabolism. Your metabolism includes all the processes through which your body uses food to create energy and maintain bodily functions, and it accounts for the majority of calories you burn in a day. Metabolisms are also like snowflakes—no two are alike.
On the surface, losing weight should be easy. There are 3,500 calories in a pound of body fat, so losing a pound requires eating 3,500 calories less than you expend. That forces your body to get that energy from stored fat. Simple, right? Except that subtracting 3,500 calories from your diet requires that you first know how many calories you’re consuming to maintain your current weight. In other words, you have to know what number to subtract those 3,500 calories from. To calculate this, you need to consider three factors:
- Your body weight
- Your calorie expenditure during exercise
- Your basal metabolic rate, or the amount of calories expended by your metabolism to keep your body functioning while at rest.
Determining the first two factors is a cinch. For body weight, all you need is a bathroom scale. And for estimating calorie burn during exercise, you can use an online calculator. But calculating your metabolic calorie expenditure, well, that one’s tricky.
Your metabolism doesn’t have a generic, single setting. Two people with the exact same body weight can have completely different metabolic rates. That’s because your metabolism is affected by all sorts of factors, including muscle mass, age, gender, dietary deficiencies, genetics and body size, as well as by environmental factors such as hot and cold weather. There are formulas for determining your resting metabolic rate (e.g., the Mifflin-St. Jeor equation, which multiplies your weight in kilograms by your height in centimeters by your age in years, to which you add 5 if you’re a man and subtract 161 if you’re a woman). But as complicated as these formulas can be, they’re still only this: good guesses. What’s more, your metabolism is significantly affected by three additional factors that are intrinsic to any weight-loss plan:
Studies show that dieting itself lowers metabolism. A 1991 study published in the American Journal of Clinical Nutrition found that just three weeks of a restricted-calorie diet lowered metabolism to 82 percent of normal. And a 2000 study determined that lowered metabolism from dieting could be at least semi-permanent. Eight subjects were confined to a biosphere for two years, during which they lived on a restricted-calorie diet. Six months after returning to a normal diet, the subjects still had reduced metabolisms. In other words, feeding your body less causes your body to burn fewer calories, and this is an outcome that can continue long after your diet has been abandoned.
I know, it isn’t fair, but weight loss will lead to a lower metabolism. It makes sense when you think about it. When there’s less of you to maintain, you’ll need fewer calories to do it.
This is the good factor, the one that can partially (or wholly) offset the previous two. When you exercise, you increase your metabolism post-exercise by creating an “afterburn”—a period of time during which your metabolic calorie usage rises even though you are no longer exercising. If you engage in high-intensity exercise, such as weightlifting or interval training, the afterburn will be greater than that for aerobic exercise alone.
If all this has your head spinning, if you still have no idea how many calories you should eat and feel like maybe it’s time to grab a slice of comfort pizza and delay the diet until another time, relax. There are plenty of tools on the internet to help you get a ballpark figure for your daily calorie burn.
First, use an online calculator to estimate your metabolism’s daily caloric needs, based upon height, weight, age, and sex. Both of these calculators will do the trick:
Next, add the calories you expend working out, using this calculator to determine your exercise-induced calorie burn:
Once you know how many calories you require to maintain your current weight, you’ll need to adjust that total downward in order to lose weight. A reduction of 500 to 1,000 calories a day should lead to a healthy 1-2 pounds of weight loss per week. If you don’t see weight loss by the third or fourth week, you’ll need to adjust your calorie intake downward a little more until you do.
If you prefer to let an online tool calculate your daily calorie reduction, try the Body Weight Planner (BWP) provided by the National Institutes of Health. The BWP first calculates your metabolic rate and daily activity level. It then determines the calories needed to maintain your current weight and the calorie reduction required to reach your goal weight. Better still, you can input the exact date you want to reach your target weight, and the BWP will adjust your daily calorie total to reflect that goal.
Finally, don’t panic if you don’t see weight loss for the first couple weeks. If you’ve started a new exercise program in conjunction with your diet, your body will increase muscle glycogen stores (i.e., the energy supply in your muscles), which can add between 1-4 pounds to your weight (a combination of the stored glycogen and accompanying water weight gain). Also, according to “set point” theory, your body might attempt to maintain its current weight by temporarily lowering your metabolism, thereby offsetting any calorie reduction. In this staring contest with your body, you’ll need to practice patience. Eventually, faced with a combination of calorie restriction and exercise, your metabolism will blink, and you’ll be on your way to your weight loss goal.