Calorie deficit explained: Is it a safe, sustainable approach to weight loss?

Looking to lose weight? There always seems to be a trendy new method. But whether you’re practicing intermittent fasting, tracking your meals, or taking a GLP-1 medication, you’re following the same principle that has guided nutrition science for decades: creating a calorie deficit. Here’s how to maintain a calorie deficit safely — and how it can help you achieve sustainable weight loss.

What is a calorie deficit?

Your body needs a baseline number of calories to fuel the functions that keep you alive, such as breathing, circulating blood, and regulating temperature. Known as the resting metabolic rate, this level of calorie burn varies considerably from person to person based on age, sex, body size, and genetics, “but for most people it accounts for about 60% to 70% of the total calories your body burns,” explains Meghan Salamon, a registered dietitian at the Weight Center at Harvard-affiliated Massachusetts General Hospital. The rest comes from calories you burn as you digest food, move around, exercise, and perform any other activities in the course of your day.

A calorie deficit occurs when you consume fewer calories than your body burns in total. This forces your body to tap into its stored energy reserves — that is, your body fat — to make up the difference.

Importantly, you’ll want to reach that deficit through a mix of eating less and being more physically active, says Salamon. “Exercising and cutting calories work in tandem as the most effective way to lose weight.”

How a calorie deficit leads to weight loss

Ideally, a safe calorie deficit produces a slow, steady rate of weight loss, says Salamon. “At our center we typically recommend losing no more than 1 to 2 pounds per week, to produce more sustainable weight loss and preserve muscle.” She usually recommends starting with a calorie deficit of 500 calories per day, then adjusting it later as needed.

“Losing weight too fast forces the body to break down not just fat for energy, but also lean muscle mass. The more muscle you have, the more calories you burn,” she explains. “The inverse is also true: if we lose muscle, we lose metabolic power — and that makes it hard to maintain weight loss in the long term.” The recommended slow weight-loss pace applies even if you’re taking a weight-loss medication, she adds.

How to calculate your calorie deficit

Here are three practical approaches to applying a calorie deficit to lose weight:

Indirect calorimetry. The most precise way to determine your calorie needs, it measures the gases you exhale to calculate how many calories your body is burning. At the Weight Center, Salamon uses a device similar to a breathalyzer to calculate a resting energy expenditure. “Then we’ll add in an activity factor, and we’ll usually subtract 500 calories from that amount to determine a calorie deficit to aim for.”

An online calculator. For a rough estimate, try the Body Weight Planner from the National Institute of Diabetes and Digestive and Kidney Diseases. It accounts for your height, weight, age, sex, and activity level to estimate how many calories you need per day to maintain or lose weight.

The general rule of thumb. The simplest (though least precise) method is to aim for a daily calorie amount that produces weight loss for most people: generally, 1,200 to 1,500 calories per day for the average woman without severe obesity, and 1,500 to 1,800 calories per day for the average man.

Common mistakes when cutting calories

A few pitfalls can derail your calorie deficit goals:

Cutting too many calories. Besides triggering metabolic slowdown and muscle loss, “it can cause your body to produce higher levels of ghrelin, the hunger hormone,” explains Salamon. “That’s going to make it more challenging to keep your calories down and could trigger unhealthy eating patterns.” Listen to your hunger cues: persistent fatigue, irritability, or an unusual preoccupation with food may be signs your deficit is too aggressive.

Not adjusting your calorie target as you lose weight. As you lose pounds, your caloric needs will decrease, says Salamon, so it’s important to adjust your intake accordingly. If you find your weight loss has stalled for more than a few weeks, it might be time to recalculate your calorie deficit goals.

Not including strength training. Regular resistance training is a win-win, as it helps stave off muscle loss as you lose pounds — and besides burning calories while you work out, your muscles actively consume calories later, during rest and recovery. One study of older adults with obesity found that resistance training could prevent nearly all of the muscle loss caused by calorie restriction. Aim to do at least two sessions of muscle-strengthening exercises per week.

Overestimating what exercise burns. Many people assume that a vigorous workout “earns” them extra food. The reality is more complicated, says Salamon. “While exercise can certainly contribute toward an energy deficit, research shows that calories from exercise can’t be traded off in a one-to-one way with caloric intake.” The body partially compensates for increased activity by reducing energy expended elsewhere, she explains. Instead, think of exercise as vital to maintaining health and preserving muscle, but not as a reliable calorie “bank account” you can freely spend.

Eating less, but not eating better. Since you’re taking in fewer calories, every bite counts a little more. Make sure you’re eating a well-balanced diet that includes plenty of fruits and vegetables, adequate protein, high-quality carbohydrates, and healthy fats.

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