The only way to lose weight effectively is through a calorie deficit: but why is this?

Amy Wood - Nutritionist | 01 Sep, 2023

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So, you've decided to lose weight, but finding the best way to do it can be tricky. Celebrities showing off dramatic weight loss transformations claim to have discovered the secret, and diets like 'Atkins', 'keto' and 'paleo' get bandied around. The truth is that all these diets have one thing in common – and that is a calorie deficit.

Regardless of which diet approach you choose to follow, creating a calorie deficit is the one rule that must be followed to lose weight effectively. But what actually is a calorie deficit? And how can we safely and effectively achieve one?

Calorie deficit

What is a calorie deficit?

To lose weight, your body needs to draw on its fat energy stores. You need to consume fewer calories in your food and drink than your body uses in the day for this to happen. This can be achieved by eating fewer calories, burning more calories through physical activity, or ideally, a combination of both.

1How should I calculate the right calorie deficit for myself?

Everybody's energy needs depend on several different factors, including your height, weight, sex, genetics, and how active you are. These factors should be taken into account when making this calculation.

2How much should my calorie deficit be?

We know that diets can feel restrictive, but the great thing about calorie counting is that you have control over how hard you want to make it. The larger the calorie deficit you create, the quicker your weight loss, but the harder it might be to maintain. A steadier rate of weight loss requires a smaller calorie deficit which will be a lot easier to sustain until you reach your goal.

It is generally recognised that one pound of fat tissue stores 3,500 calories. So, reducing our daily calorie intake to 500 calories below our maintenance level equates to a deficit of 3,500 calories across a week – enough to lose 1lb. To lose up to 1.5lbs a week, your calorie deficit would need to be increased to 750 calories each day. And to lose up to 2lbs, a deficit of 1,000 calories per day is required.

It's all about managing your expectations – if you want to lose weight quickly, your deficit will need to be bigger, but if you're happy to take things gradually and lose a little less each week, you'll have more calories to play with. We wouldn't recommend aiming to lose weight any faster than 2lbs per week – this tends to be the downfall of restrictive 'crash diets', where weight is lost very quickly and then gained back just as fast.

3Is there such a thing as 'too few calories'?

The primary use of calories is to keep all the cells in our bodies functioning normally, including those in our hearts and brains. This is known as our basal metabolism. The rest of our calories are used to fuel all our daily activities, such as walking, sitting, exercising, and even digesting food! If our calorie intake drops below our basal metabolic rate for several weeks or months, then the reactions in our cells start to change to conserve our energy supply, i.e., our fat tissue.

We are not a fan of the phrase 'starvation mode', but it is true that our bodies have a series of specialised mechanisms that kick in when we decrease our calorie intake to protect key bodily functions – our bodies have evolved to adapt and protect us from starving. This causes our metabolic rate to be reduced, meaning we burn fewer calories throughout the day. This is also combined with an increase in the signals that tell us when we are hungry, making it even harder to maintain a deficit. While this response can be lifesaving in situations of severe malnourishment, it certainly presents challenges to those of us on a mission to lose weight!

4What is the best way to stay within a calorie deficit?

We recommend a double-pronged approach to creating your calorie deficit. Combining changes to your food choices and exercise routine produces the best and most sustainable weight loss results.

Food tips

Food tips

1Keep track of everything you eat

Remember to keep track of absolutely everything you eat and drink. Commonly forgotten items include cooking oils, spreads, sauces and dressings. Tracking everything in a food diary will help you stay on top of your deficit.

2Increase your intake of protein and fibre

The key to sticking with a calorie-controlled diet to create your deficit is by choosing tasty foods that keep you full and satisfied for longer. Two of the best nutrients for this are protein and fibre. Foods high in protein include lean meats, fish, eggs, beans, pulses, tofu and dairy products. To get your fibre hit, look for foods that make 'high fibre' claims on the front of the pack, such as breakfast cereals and crackers. Fruit, vegetables, porridge oats, beans and pulses are also good sources. Where you can, opt for wholegrain versions of carbohydrates, such as bread, rice and pasta, over refined white versions.

3Stay portion-size savvy

It's not just the foods you're eating, but how much you're eating. When you are first getting used to what a serving looks like, try weighing your food or using measuring cups. This will make it easier for you to track in your food diary.


Training tips

1Get your heart rate up for an immediate boost to your calorie burn

Aside from reducing the calories going into your body, calorie deficits are best complemented by increasing the calories being used by your body. Exercises that raise your heart rate and cause you to get out of breath, such as running, cycling and swimming, are the best way to increase your calorie burn and boost weight loss. Even increasing your steps through brisk walking will help. Try and find something you enjoy doing and can easily add to your weekly routine – this will help you stick with it consistently. To meet the government guidelines for UK adults, try to get your heart rate up for at least 150 minutes per week – that's equal to 30 minutes every weekday. If you're able to do more, even better!

2Add some strength training to increase your metabolism long-term

Aside from cardio-based exercises, try and incorporate some strength-based training into your routine too. Think of it as an investment for the future – this type of training may not burn as many calories as some cardio exercises, but the aim is to change your body composition over time to build lean tissue. Unlike fat, muscle is a metabolically active tissue, meaning it increases your metabolic rate even when you aren't using it. So, by increasing your muscle mass through strength training, you'll be burning more calories all day, not just when you are exercising!

Boosting your metabolism in this way sets you up for success long-term once you come out of your calorie deficit and begin maintaining your goal weight. The UK government recommends including strength-based exercises at least twice a week. Press-ups, sit-ups, squats and lifting weights are all good examples.

Nutritionist Amy Wood (ANutr), MSci BSc Nutrition has a keen interest in the relationship between diet and health. Having been published in the European Journal of Nutrition, Amy is passionate about making evidence-based nutrition accessible to everyone and helping others to adopt a food-focused approach to taking control of their health.

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