As research progresses we’re beginning to discover that the theory of ‘calories in and calories out’ is an over-simplification of the way we use energy. What we eat and how we exercise can make all the difference to our weight loss goals – read on to find out more.


What is fat and why is it important?

Fat, such as carbohydrates and protein, is a macronutrient needed by the body. It has a number of roles, including building cell membranes, nerve tissue and hormones, and aiding the absorption of fat-soluble nutrients, including vitamins A and D. Our bodies use fat as energy and store any excess for future use; this stored fat acts as an insulator and helps cushion vital organs, bones and other tissues, protecting them from damage.

Although eating too much fat can be unhealthy, there are certain types of fat we must get from our diet because they’re essential to our health and well-being.

Discover our full range of health guides including the top health benefits of exercise and how much fat should I eat a day?. Plus, browse our low-fat recipes, including cod and olive tagine with brown rice and spicy spaghetti with garlic mushrooms.

Athletic man doing sit ups

Where do calories come from?

Your body uses nutrients in different ways, so it’s helpful to be aware of where your calories come from. Dietary fat, for example, is more ‘fattening’ than protein or carbs and it's less likely to be used to build your body. By weight it also contains more than double the calories of carbs and protein.

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The body uses protein, carbs and fat in the following ways:


These are the body’s main energy source and the source the body favours to fuel exercise.

1 gram of carbohydrate = 4 calories.


This helps to build muscle – the more muscle you build, the higher your metabolic rate. The body also uses more calories when it breaks down protein, this is known as the thermic effect of food.

1 gram of protein = 4 calories.


This supports vitamin absorption, and helps manufacture hormones and cell membranes. However, as with carbs and protein, if you consume too much fat, there’s little the body can do with it other than store it.

1 gram of fat = 9 calories.

Learn more about calories and how many you need.

Which type of exercise is best to burn fat?

Your total calorie burn will be greater when you exercise at high intensities, but your body burns fat more efficiently when exercising at low to moderate intensities. Lots of gym equipment, such as treadmills and indoor bikes, feature ‘fat-burning zones’ (essentially aerobic exercise) during which your body relies on both fat and carbohydrate stores for energy in relatively equal measure. As intensity increases, your body turns predominantly to easily accessed carbohydrates (glycogen) for quick energy.

If you aren't fit enough to really push yourself, or have a health issue or injury preventing you from going too hard, a lower intensity workout is ideal. It can take the form of any exercise you like that gets your heart pumping – a gym session, running outdoors, a bike ride, a swim session, and so on.

How do I know if I am working in the aerobic ‘fat burning’ zone?

In simple terms, you’ll feel like you’re doing exercise, but it will be pretty comfortable. The talk test is a helpful gauge – if you can hold a relatively short conversation without gasping for air you're in the right area.

What is HIIT and how can this help fat burning?

The opposite to a low intensity workout is a HIIT (high intensity interval training) workout. Just as it says on the tin, this is a workout performed at high to maximum intensity, with running, cycling and circuit training being typical activities. It’s impossible to work at near-maximum effort for long, so a HIIT session lasts anywhere up to 20 minutes. The obvious benefit is that you’ll get the same total calorie burn as a low intensity workout in far less time. So, a low intensity run that might take an hour could have the same calorie burn as a 15-minute HIIT session, making this form of exercise ideal if you’re strapped for time.

During a HIIT session your body will predominantly work anaerobically, meaning you'll generate energy for exercise without oxygen.

The benefit of using anaerobic sources for energy is that it causes physiological adaptations in your body that will help you run/swim/cycle faster and harder for longer.

Woman happy after workout

Is muscle mass important?

When it comes to burning fat, body composition makes all the difference. Muscle burns calories more than fat, even when at rest, so the more muscle mass you hold, the better. However, this doesn’t mean you need to build muscles like Popeye – simply toning up and changing the balance of your body composition from less fat to more muscle will help with weight loss.

Resistance training is the best way to build muscle and a strength training programme, specifically designed for weight loss, combined with a cardio routine will help you shed pounds and develop muscle tone rather than bulk.

How do I know how many calories I need?

When it comes to losing weight, and alongside an exercise programme, you need to work out your Total Daily Energy Expenditure. This can be as scientific as you want to make it but online calculation tools are a good place to start.

To get a rough idea of how many calories your favourite activities will burn, take a look at our guide to calorie expenditure.

Is exercise safe for everyone?

If you’re new to exercise, speak with your GP or healthcare practitioner to ensure the exercise you propose is appropriate for you. This is especially relevant if you have heart disease, high blood pressure, diabetes, osteoporosis, an existing muscle injury, arthritis or joint issues; or if you’re on prescribed medication or are pregnant.

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Are you trying to lose weight the healthy way and hitting a wall? Share your questions in the comments section below and we'll do our best to help.

Kerry Torrens BSc. (Hons) PgCert MBANT is a registered nutritionist with a post graduate diploma in personalised nutrition & nutritional therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the last two decades she has been a contributing author to a number of nutritional and cookery publications including BBC Good Food.


All health content on is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other healthcare professional. If you have any concerns about your general health, you should contact your local healthcare provider. See our website terms and conditions for more information.

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