What is a calorie deficit?

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Trying to lose weight? You need to be in calorie deficit, and that means eating and drinking fewer calories than you burn.

There are two ways to create a calorie deficit: 

  • Reducing calorie intake – changing what and how much you eat
  • Increasing calorie expenditure – in the form of exercise

How does being in a calorie deficit help you lose weight? It puts you into a 'catabolic' state, when the body is breaking down muscle and fat for energy.

If you cut 500 calories a day from what you normally eat, you can expect to lose about 1 pound – 0.5kg – a week, which is a healthy amount as per CDC guidelines (opens in new tab). The best exercise machines to lose weight (opens in new tab) can also help reduce body fat.

Here's what the research – and the experts – have to say about calorie deficits. 

What is a calorie deficit?

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Calories are the units of energy we get from food and drink. To lose body fat, a calorie deficit is needed – that means eating less calories than you burn. 

Nutritionist and PT Alex Parren says: “A calorie deficit is when you take in fewer calories than you expend in a day. Every human has a unique Basal Metabolic Rate (BMR), which indicates how many calories their body burns just by doing normal everyday functions, such as breathing, digesting food, removing waste products and cognitive function. 

“Factors that affect BMR include age, gender, height and bodyweight. Imagine your BMR as the amount of calories you would burn if you stayed in bed and slept all day. It doesn’t take into account your lifestyle and most people’s BMR will be fairly low – around 1,500 calories for adults.”

On an average day, you'll burn through more calories than this as you'll be up and moving around. In general an average woman needs about 2,000 calories a day to maintain her weight, and 1,500 to lose 1 pound a week. The average man needs to consume around 2,500 to stay the same weight, or 2,000 to drop 1 pound a week. 

Research published in Frontiers in Physiology (opens in new tab) suggests that doing muscle-building exercises alongside a calorie deficit will also help the body prioritize the loss of body fat rather than just muscle. 

What role does it play in weight loss?

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The body uses the calories it gets from food to fuel your metabolism, digestion and physical activity. When the number of calories you’re eating equals the number of calories you’re burning for energy, your weight will remain stable.

Once your body’s energy needs are met, any extra calories get stored – some in the muscles as glycogen (the main source of fuel for our cells), but most as fat. So, if you eat more calories than you burn – known as a calorie surplus – this will cause weight gain. By contrast, if you consume less food than you need for energy, you will lose weight.

Parren says: “There are several studies that prove being in a calorie deficit is the only way to lose weight. Any type of diet – whether it’s the paleo diet (opens in new tab), the DASH diet (opens in new tab), or Keto (opens in new tab) – rely on a calorie deficit to work. It’s advisable not to exceed a deficit of 500 calories a day.

“However, I would advise people to achieve this deficit through burning more calories, rather than restricting food intake. That is to say, burn your 500 calories a day through cardiovascular exercise and resistance training, rather than reducing the amount you eat. The reason for this is that it is less likely to lead to cravings and disordered eating,” she says.

How can you achieve a calorie deficit?

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To achieve a calorie deficit you need to first work out your Basal Metabolic Rate (BMR) – the amount of calories your body needs to achieve its most basic (basal) life-sustaining needs at rest. There are lots of online calculators to help you do this.

“Once you know your BMR, you need to include factors like how active you are and how much exercise you do,” says Parren. This calculation is your Total Daily Energy Expenditure or TDEE, which is a more accurate representation of how many calories you actually burn in a day. 

“Your TDEE will take into account how many steps you do in a day, how active your job is (sitting at a desk all day versus working in manual labor), and even activities like gardening or walking your dog. Once you have a figure for your TDEE, you can calculate how many calories you’d need to eat to be in a deficit. 

“A calorie deficit is any number below your TDEE and one pound of fat roughly equates to 3,500 calories. So if you are in a deficit of 500 calories a day over seven days, you will lose roughly 1lb per week.

“To be more specific, if your TDEE is 2,500 calories, you would need to eat 2,000 calories per day to lose 1lb per week.”

Remember once you start losing weight you will need to recalculate your TDEE to work out your new daily calorie deficit figure. “Once you have lost some weight, eat at maintenance level – the same number of calories as your TDEE – to make sure you don’t gain the weight back,” says Parren.

Maddy is a freelance journalist and Level 3 personal trainer specializing in fitness, health and wellbeing content. She has been a writer and editor for 22 years, and has worked for some of the UK's bestselling newspapers and women’s magazines, including Marie Claire, The Sunday Times and Women's Health. Maddy loves HIIT training and can often be found working out while her two young daughters do matching burpees or star jumps. As a massive foodie, she loves cooking and trying out new healthy recipes (especially ones with hidden vegetables so the kids eat them).