The calorie deficit is the backbone of modern weight-loss approaches — but does it really work? More importantly, is it safe?
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It has been long held that being in a calorie deficit is a common technique to utilize when trying to lose weight. (You've likely heard or seen the phrase "calories in < calories out" at some point, right?)

But what is a calorie deficit, anyway, and is cutting back on calories actually the best way to lose weight? Here's what nutrition experts and recent research say about calorie deficits, how to calculate a calorie deficit, and whether or not they're a good idea.

What Is a Calorie Deficit, and Is It Safe?
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What Is a Calorie?

In terms of the human body, a calorie is a measurement that denotes the amount of food with a specific energy-producing value. Essentially, this all means that the food and beverages you consume provide your body with energy, measured in terms of calories, to live.

However, your body needs more than just calories to stay alive — you also need nutrients — including vitamins and minerals — in order to keep your body functioning well. (For example, the mineral calcium found in milk helps strengthen bones while the mineral iron found in beans is needed for red blood cell function.)

How Many Calories Do You Need Per Day?

There are three factors that determine the total calories you need: your basal metabolic rate, physical activity, and the thermic effect of food.

Basal metabolic rate: Your basal metabolism is the amount of energy needed for your body to simply stay alive, such as for your heart to pump. A person's basal metabolic rate (BMR) depends on numerous factors including gender, age, height, and growth (i.e. in children). Basal metabolism accounts for about 50 to 70 percent of your calorie needs.

Physical activity: Physical activity accounts for between 25 and 40 percent of your calorie needs. This, of course, includes workouts, but it also includes non-exercise activity thermogenesis, or NEAT, the energy that you burn while doing everything that's not digesting, breathing, eating, or exercise, i.e cooking, cleaning, fidgeting, typing, etc.

Thermic effect of food: The thermic effect of food is the energy needed to digest and absorb the food you eat. It accounts for between 5 to 10 percent of your total calorie needs.

Dietitians use several formulas to determine a person's calorie needs. One of the most popular formulas is the Harris-Benedict Equation; first, you calculate your BMR using your weight, height, and age, and then your BMR is multiplied by an activity factor (ex: how much you typically move) to determine approximately how many calories you need per day. For example, someone who rarely or never exercises will multiply their BMR by 1.2, and someone who moderately exercises 3 to 5 days per week will multiply the BMR by 1.55. Instead of doing all the calculations yourself, you can use the USDA's calculator to determine your calorie needs.

USDA Calorie Needs Calculator

Calorie needs for adult women range from 1,600 to 2,400 per day, according to the Academy of Nutrition and Dietetics. If your lifestyle is more sedentary, you'd be on the lower end of that range, and if you're more active, you'd be on the higher end. (Note: As you age, your calorie need decreases, and if you're pregnant or breastfeeding, your needs may increase.)

What Is a Calorie Deficit?

Simply put, a calorie deficit is when you consume fewer calories than your body is using or burning.

It's commonly advised that, in order to lose weight, you should create a calorie deficit. For example, if a person needs 3,000 calories per day, lowering their calorie intake to 2,500 calories per day creates a calorie deficit of 500 calories per day. In 1558, researcher Max Wishnofsky, M.D., calculated that 1 pound of fat stores approximately 3,500 calories of energy, according to an article in Today's Dietitian. Since then, it's been accepted as common knowledge that — in terms of weight loss — 1 pound is equivalent to 3,500 calories. With that in mind, the idea is that a daily 500-calorie deficit can ultimately result in about 1 pound of weight loss per week. (See: How to Cut Calories to Lose Weight Safely)

If you're consuming more calories than your body is using, that's called a calorie surplus. If you remain in a calorie surplus for an extended period of time, it can often lead to weight gain. (Of course, significant weight gain isn't always due to overeating — it can also be due to metabolic problems or other health issues such as hypothyroidism. That's why it's important to go to annual medical exams and see a doctor if you start gaining weight suddenly.)

Using a Calorie Deficit for Weight Loss

The National Institute of Health (NIH) broadly recommends following a reduced-calorie diet (aka entering a caloric deficit) for people who are overweight or obese and are looking to lose weight, and the Academy of Nutrition and Dietetics also specifies in a 2016 report that a daily 500- to 750- calorie deficit is indeed recommended for weight loss.

Some oft-cited research backs up this approach: A 2007 study on calorie deficits showed that cutting back on about 500 calories per day does help achieve weight loss. However, the amount of weight lost depends on the person's initial body fat, according to the study. For example, someone who starts with a higher amount of body fat needs a greater calorie deficit overtime to lose weight. The study explains that this is why men can lose more weight than women for a given calorie deficit since women typically have more body fat than men of similar body weight.

However, a 2014 study published in the International Journal of Obesity points out that the guideline of a 3500-calorie deficit per week (or 500-calorie deficit per day) as a general rule of thumb that may be over-simplified. Researchers in the study wanted to see if the 3,500-calorie rule could predict the weight loss of subjects, but the results showed that most of the subjects lost substantially less weight than the amount predicted in this 3,500-calorie rule. The reality is that there are many more factors affecting weight loss results than just cutting back on calories. Various metabolic factors, such as internal satiety cues (meaning, when you feel hungry or satisfied), can also play a role. There's a lot of research currently being done to determine other things that may play a part.

How to Safely Lose Weight Using a Calorie Deficit

Although research shows the 500-calorie deficit rule may not be foolproof, it's still the recommended guideline for losing weight among public health organizations such as the NIH, Academy of Nutrition and Dietetics, and the Mayo Clinic. And in order to lose weight with a calorie deficit, you need to maintain it over a longer period of time, says Joan Salge Blake, Ed.D., R.D.N., nutrition professor at Boston University and host of the nutrition and wellness podcast Spot On!. You can create and maintain a calorie deficit by:

  1. Consuming fewer calories.
  2. Increasing your daily physical activity without increasing your calorie intake.
  3. A combination of both.

The truth is that both cutting back on calories in combination with being more active (and thus burning more calories) is typically the most successful way for folks to lose weight, says Salge Blake. (Think: If you cut out 250 calories from your day and also go on a run or walk that burns 250 calories, it may feel easier to reach that 500-calorie deficit compared to reducing your food intake by 500 calories alone.)

That said, you don't want to go any extremes by cutting out most food and over-exercising because it isn't sustainable or healthy long term. "Just because a little calorie deficit is good for weight loss does not mean that eating as little as possible to lose weight is a good idea," says sports dietitian Amy Goodson, M.S., R.D. You shouldn't eat less than 1,000 calories per day, according to the NIH. If you're eating too few calories, it can actually impede weight loss and keep the body from receiving the calories it needs to stay healthy. If you eat too few calories even for just a short period of time, you can feel hungry, irritable, fatigued, and experience diarrhea and/or constipation. This doesn't inform healthy eating habits that can be maintained. If you cut back on calories for a long period of time (think: months), this can lead to malnutrition, nutrient deficiencies, and basically leave you in a state of starvation. As a result, your body will hold onto fat as a means of back-up energy for your body, which would ultimately impede on weight loss and be counterproductive to your goal.

A safe rate of weight loss is 1 to 2 pounds per week, according to the NIH. Anything quicker than that can potentially be dangerous and have health consequences; quick weight loss increases demands on the body and can increase the risk of gall stones, malnutrition, electrolyte imbalance, and liver damage. If the rate of weight loss is quicker than 1 to 2 pounds per week (i.e. after bariatric surgery), then it should be under the supervision of medical professionals.

And it's worth noting that, if you have an underlying medical condition that may be causing weight gain (such as type 2 diabetes or hypothyroidism), trying to use a calorie deficit for weight loss may not work. That's why it is important to see your doctor if you recently gained a lot of weight or if you've been trying to lose weight and you can't.

Short- vs. Long-Term Weight Loss Goals

"Depending on the person and their specific weight-loss goal, a calorie deficit may need to be done for a shorter or longer amount of time," says Goodson. "While it's not always a perfect equation, typically the length of calorie deficit depends on the amount of weight that needs to be lost." However, after six months of using a calorie deficit to lose weight, it's important to switch to a weight-maintenance program — regardless of whether or not you've reached your goal, according to the NIH. Once you've lost weight, keeping it off is extremely difficult, and taking breaks for a few weeks or months to make sure you don't regain it can help maintain your weight loss long-term. According to the Transtheoretical Model (or Stages of Change) used to explain behavior change, the fifth phase is the maintenance phase where the behavior is sustained (for six months or more) and is intended to be maintained going forward. Going into this maintenance mode after several months of a calorie deficit isn't necessarily about how much you're eating, but allowing yourself time to keep up with your behavior modifications and, based on this model, six months tends to be the magic number. Then, once you've kept off the weight for several months, you can recalculate your calorie needs and create a new deficit to continue weight loss efforts.

On that note, when you experience a weight-loss plateau — which does happen, and is a normal part of the weight loss process — it's important to re-evaluate your calorie needs as another deficit (though maybe not as large) might be needed. For example, if you lost 10 pounds by eating 500 calories less per day over about 2 months, your calorie needs will be less since you're 10 pounds lighter. As such, you may need to re-evaluate your current calorie needs based on your BMR and activity level; you may end up eating 750 calories fewer than when you began.

Similarly, once your weight loss goal is met, you'll need to adjust your calorie intake accordingly. Staying physically active on a regular basis can also help keep the weight off by increasing your overall daily calorie burn (as well as also provide your body with tons of other important health benefits).

The ultimate goal is to maintain weight loss for a long period of time and not have your weight go right back up. That's why the NIH advises no more than a 500 to 1,000 calorie deficit per day. "The trick is to create small calorie deficits so that the weight you lose doesn't come back," explains Elizabeth Ward, M.S., R.D., author of Better is the New Perfect. If you try to maintain a large calorie deficit, you'll likely become extremely hungry and won't stick to your meal plan for as long a period of time — totally cutting off your weight loss efforts. Long-term sustained weight loss is much more challenging than losing the actual weight, according to a 2019 study. In a meta-analysis of 29 long-term weight loss studies, more than half the weight lost was regained within 2 years, and after 5 years, more than 80 percent of the lost weight was regained.

Calorie Deficit and Exercise

People who are very physically active may wonder if they can lose weight with a calorie deficit. "The key is making sure that you're fueling your body appropriately for the amount of activity being done," explains Goodson. "Consuming too few calories can have a negative impact on performance and energy levels." For example, if you're cutting calories or exercising intensely, you might be at risk for female athlete triad syndrome, which manifests as menstrual cycle disturbances and energy loss.

"Tracking calories may be an important piece of the equation to make sure active people consume enough calories for energy but are creating a deficit to help them meet their weight loss goal," says Goodson.

As a general recommendation, people can reduce their caloric intake and increase their activity level to create a caloric deficit. But if you're an athlete (think: training for a marathon or other vigorous event) or have a lot of weight to lose, it might be smart to touch base with a registered dietitian who specializes in sports.

Behavior Change

Cutting back on calories and exercising more is just half the battle. The NIH also recommends behavior therapy in conjunction with weight loss and increased physical activity. According to the Transtheoretical Model (or Stages of Change), once a behavior is sustained for more than six months, it then becomes a habit that someone continues to do. Creating healthy habits that last a lifetime is the ultimate goal to keep off the weight.

Salge Blake recommends meeting with a registered dietitian nutritionist (RDN) who specializes in this area to help you through your weight loss journey. The service may be covered by your insurance. You can find an RDN in your area by going to the Academy of Nutrition and Dietetics website and clicking on "Find an Expert."

Why a Calorie Deficit Doesn't Always Work

A calorie deficit can result in weight loss; however, "many folks over-estimate how many calories they are burning through physical activity and underestimate how many calories they are consuming," says Salge Blake. For example, research shows that cardio machines and other calories-burned calculators may overestimate. This makes trying to lose weight solely through increased physical activity — or by calculating your calorie deficit needs while also factoring in calories burned during exercise — more challenging.

If you're trying to track your calorie deficit, Salge Blake recommends using one of the many available weight loss apps to help keep a log of what you're eating. Keep in mind, "you need to understand portion sizes in order to put in the accurate amounts of the foods you're consuming daily," explains Salge Blake. "If your portions are incorrect — especially, underestimated — so will be the output [your total calorie count]."

In order to achieve your calorie needs and take in the nutrients needed to keep your body healthy, it's important to prioritize eating nutrient-dense foods and limit foods high in saturated fat and added sugars. The 2020-2025 dietary guidelines for Americans offers the 85/15 rule as guidance, where 85 percent of the calories consumed should be from nutrient-dense sources, and 15 percent can come from non-calorie-dense sources such as those higher in saturated fat and added sugars (think: cookies, candy, butter). (Related: These 5 Simple Nutrition Guidelines Are Undisputed By Experts and Research)

In addition, if you have a history of eating disorders (such as anorexia nervosa, bulimia, or orthorexia), you may not be a good candidate for calorie counting as the tedious nature of calorie counting may trigger a relapse. In addition, those with eating disorders spend many months or years relearning proper eating behavior skills and also work on eliminating eating disordered behavior, including calorie counting. According to a 2010 paper published in the Journal of Neuroscience, calorie restriction can lead to stress and binge eating even in those who don't have a history of eating disorder. (Related: Counting Calories Helped Me Lose Weight—But Then I Developed an Eating Disorder)

The Bottom Line

Using a calorie deficit for weight loss is certainly a technique that works, but not on its own. Increasing exercise, understanding how to estimate what you eat (like portions), and behavior change towards healthy, sustainable habits is also part of the equation. Although many folks have done this on their own, having the guidance of an RDN can certainly help with support and positive motivation towards healthy weight loss goals.