What Is a Calorie Deficit—and Is It Something You Should Use to Lose Weight? Here's What a Nutritionist Says
You're probably familiar with the notion of a deficit as it relates to money. In that sense, a deficit occurs when the amount you owe is more than what you have. In other words, it's not a good thing.
But when it comes to calories, a deficit is something many people strive for as a way to lose weight. Yet the concept isn't as straightforward when it applies to your body as it is with your bank account. As a nutritionist, here's how I explain to my clients what "calorie deficit" means. (Plus, I include a few notes about some common pitfalls to avoid if you use a calorie-based approach to weight loss.)
What is a calorie deficit?
There are two ways to achieve a calorie deficit. The first is to eat fewer calories than it takes to maintain your current weight. You can start by using an equation to estimate the number of calories your body needs to maintain your weight. There are several research-based formulas that determine this, including the Mifflin-St Jeor formula. This has been referred to by researchers and the nutrition community as the most reliable at predicting your resting metabolic rate, or the total number of calories you burn when your body is completely at rest.
If you don't want to do the math yourself, use a handy online calorie-needs calculator, like the one provided by Mayo Clinic. The tool asks for your age, height, weight, sex, and activity level, since each of these factors determines your calorie needs. For example, a 40-year-old, 5'4" woman who weighs 150 pounds and is inactive needs about 1,700 calories to maintain her weight, according to the calculator. If she consumes fewer than 1,700 calories, she has created a calorie deficit.
The second way to create a calorie deficit is to burn more calories than you consume. To use this method, the 40-year-old woman from above could eat 1,700 calories, but transition from being inactive to active. If she burns an additional 300 calories by adding a brisk walk to her day, she's created a calorie deficit.
Here's how to create a calorie deficit—safely
One of the biggest concerns I have when people try to lose weight on their own is that they create a calorie deficit that's too big for too long. For example, if it takes 1,700 calories to maintain your weight, and you start eating 1,000 calories a day, that's a pretty drastic drop. Add workouts to the mix, and now you're seriously stressing your body. You're also depriving your cells of nutrients needed for vital daily functions and for the recovery from the wear and tear exercise puts on the body. Over time, such a big deficit can result in a number of unwanted side effects, including the loss of muscle tissue, reduced immune function, poor digestive health, and irritability.
My personal rule of thumb is this: don't consume less than it takes to support your ideal or healthy weight for an extended period of time. If you weigh 150 pounds and your ideal weight is 130 pounds, enter 130 into the calorie-needs calculator. This ensures that you won't drop below the number of calories needed to support overall wellness at a healthy weight. In my experience, if you consistently eat only enough to support your target weight, you'll eventually reach and maintain that weight. (That is, if your calorie-based strategy is paired with paying attention to other key elements, like food quality and management of your medical conditions.) Not only is this approach healthier, but it's also much more practical in the long term.
You can't necessarily depend on a calorie deficit alone to lose weight
For years, experts relied on the notion that 3,500 calories equals one pound. That led to the idea that producing a deficit of 500 calories per day would lead to a one-pound weight loss per week (since 500 x 7 = 3,500), or that cutting 500 calories and burning an additional 500 with exercise each day would result in a two-pound loss per week.
Unfortunately, if you've ever experimented with this approach, you've probably found that it's not quite that simple. Nutrition and metabolism are far more complex than a simple calories-in-vs-calories-out equation.
Many factors impact weight-loss results and the rate of weight loss, including the quality of the calories consumed (whole vs processed foods), macronutrient balance, meal timing, hormones, stress, sleep, genetics, gut microbiota makeup, underlying health conditions, and medications.
I wish I could say that simply creating a specific calorie deficit per day will lead to a predictable weight loss result, but I can't. I've seen clients break a weight loss plateau by changing what and when they eat without cutting a single calorie. Plus, not all calories are created equal. A 500-calorie blueberry muffin made with white flour and refined sugar will have a very different effect on your body than a 500-calorie bowl of cooked oats topped with blueberries and walnuts. That's not to say that calories don't matter, but they're not the sole determinant of weight management.
Be patient with weight loss
You don't need to starve yourself in order to lose weight. In fact, people who lose one to two pounds per week are more successful at keeping weight off (the ultimate goal), according to the Centers for Disease Control and Prevention. While many people would like to lose weight faster, even modest weight loss has been shown to result in health benefits, such as improvements in blood pressure, cholesterol, and blood sugars.
Keep in mind that the closer you are to your weight goal, the smaller the deficit between the calories needed to maintain your weight and the number required to support your ideal weight. That means even slower results, perhaps even less than a pound per week. But if you remain focused on feeling well along the way, it's well worth the wait.
Bottom line: Our bodies are complex. If you're a math person, or you like to use food and/or exercise trackers, you're probably very aware of calories. That's fine, but please keep in mind that calories are only one piece of the weight management puzzle.
Cynthia Sass, MPH, RD, is Health's contributing nutrition editor, a New York Times best-selling author, and a private practice performance nutritionist who has consulted for five professional sports teams.
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