How a calorie deficit diet really works

·7-min read
Photo credit: marilyna - Getty Images
Photo credit: marilyna - Getty Images

When you want to lose weight, it only makes sense to do your research to see what the most popular diets are right now. But, while there are plenty of trendy eating plans floating around, there’s an old standby that’s suddenly become popular again: the calorie deficit diet.

The diet is simple at baseline: It doesn’t tell you which foods you can and can’t eat. Instead, it just focuses on calories, which are the amount of energy that’s released when your body breaks down food. The more calories food contains, the more energy it can give your body, explains Jessica Cording, R.D., author of The Little Book of Game-Changers.

Your body needs a certain amount of calories to function normally, but when you take in more calories that you need, the excess amount is stored in your body as fat, Cording says.

That’s where the calorie deficit diet comes in. But how does the calorie deficit diet work and how can you know if it’s right for you? Nutritionists break it all down.

So, what is a calorie deficit diet?

The concept behind the calorie deficit diet is pretty simple: You try to eat fewer calories than you burn. 'A calorie deficit is when we consume fewer calories than our bodies burn in a day, both in terms of our natural resting energy expenditure—your natural metabolic rate—and any exercise or physical activity we engage in on top of that,' says Dana Ellis Hunnes, Ph.D., MPH, R.D., adjunct assistant professor at the UCLA Fielding School of Public Health and author of the upcoming Recipe for Survival: What You Can Do To Live a Healthier and More Environmentally Friendly Life.

Can a calorie deficit diet help you lose weight?

If you calculate things right, a calorie deficit diet should help you lose weight. 'Calories are one of the primary factors in weight gain or weight loss,' Cording says. 'If somebody is consuming fewer calories than they burn, that can result in weight loss.'

So, if you typically eat 2,200 calories a day, on a calorie deficit diet, you should lose weight if you strive to cut that back to 2,000 or so calories a day.

There’s a reason why this happens. 'When we take in fewer calories than we burn, our body has to get the energy from somewhere to continue its natural functioning,' Hunnes explains. 'This additional energy to maintain bodily functions comes from stored energy within our own bodies.' Your body will usually burn fat first, followed by glycogen, which is what carbs are turned into in your body after you eat them, Hunnes says. And, when that fat is burned off, you end up losing weight.

How to do a calorie deficit diet

There are a few ways to go about this. 'The gold standard would be to find out how many calories you burn in a day,' Hunnes says. That can include having a test done known as indirect calorimetry, which measures the amount of carbon dioxide you exhale, combined with some complicated calculations to figure out how much energy you burn at rest (i.e. when you’re just sitting there, not doing anything). There’s also something called a Bod Pod that can measure how much energy you burn at rest, but both methods aren't easily accessible to everyone.

So, you can use online equations like the National Institutes of Health’s Body Weight Planner to try to figure out what your daily caloric intake should be in order to help you reach a certain weight goal. To use it, you enter in basic information like your height, current weight, age, and activity level, and then enter in your goal weight and how much time you’d like to take to reach it. From there, the planner will give you an estimate of how many calories you should eat to hit your goal weight in the amount of time you specified and how to maintain it.

But this isn’t an exact science and it doesn’t take into account things like how much muscle mass you have (muscle tissue burns more calories than fat) and your individual metabolism, Cording says. 'There are so many factors that go into energy expenditure,' she points out. Still, it can give you a rough estimate to work with.

If you want to make it easy on yourself and you know you want to lose about a pound a week, you can try to slash 500 calories from your existing dietary intake, says Keri Gans, R.D., author of The Small Change Diet. 'Safe weight loss is considered at a rate of one pound per week lost,' she says. 'One pound is equivalent to about a 500-calorie deficit per day for one week.'

But being able to cut that many calories and feeling comfortable with it depends on what you were eating before, Cording says. If you had plenty of high-calorie processed foods and sweets in your diet, swapping them out in favour of plenty of fruits and vegetables, lean proteins, and healthy fats may not feel like a huge deal in terms of your hunger levels, she says. If you were already eating pretty healthy to begin with, though, you may feel more comfortable cutting out 125 to 250 calories a day instead, with the understanding that it will likely take a little longer for you to lose the weight. ''I’ve seen people do really well with that,' Cording adds.

How much of a calorie deficit is too much?

The Academy of Nutrition and Dietetics recommends that adult women have between 1,600 and 2,400 calories a day, while men should strive to have between 2,000 and 3,200 daily calories. (The lower end is if you do little to no activity every day.) Dropping below the minimum amount is not considered healthy, so you’ll want to be mindful of having too much of a calorie deficit.

You also could be having too much of a calorie deficit if the pounds you’re losing are 'adding up too quickly, such as five pounds per week or more,' Gans says. Symptoms like fatigue, nausea, dizziness, and constipation can also be signs that you’ve gotten too overzealous with your weight loss pace, she says.

In general, Hunnes says, 'I strongly encourage not cutting out more than 15 to 20% of calories.'

What’s the healthiest way to do a calorie deficit diet?

In addition to not cutting out too many calories from your daily diet, experts say there are some other tips that can help you do a calorie deficit diet in a healthy way:

  • Focus on what you’re adding to your diet. 'If you are adding more fruits and veggies to your diet, you might naturally wind up eating less foods with lots of added sugar and fat,' Gans says. 'Therefore, you will lose weight.'

  • Strive for nutrient-rich foods. Those include fruits, vegetables, 100% whole grains, low-fat dairy, seafood, poultry, lean meats, and healthy fats, Gans says.

  • Minimise processed foods. 'I recommend incorporating the most natural forms of foods into your diet as possible— single-ingredient, as close to nature as can be found foods,' Hunnes says. 'The fewer things out of a package the better.'

  • Watch your portion sizes. While you could calculate exact calories at every meal, that can get stressful and time-consuming. 'Keeping portion sizes in check is most likely one of the best tools to help,' Gans says.

  • Pay attention to your macronutrients. Macronutrients, aka 'macros,' are the main components of food that you need to be healthy, Cording explains. 'Make sure you’re getting enough protein, healthy fats, and calories that feels good to you,' she says.

  • Eat in. 'Eating at home gives you greater control over what is in your food,' Hunnes says.

If you’re interested in doing a calorie deficit diet and aren’t sure where to start, check in with your primary healthcare provider or a registered dietitian—they should be able to help offer personalised guidance.

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