The NHS estimates daily caloric requirements to be 2000kcals for women and 2500kcals for men, although more accurate figures are based upon height, weight, age and gender. This is made up of our basal metabolic rate (BMR), the energy we require to function at rest, plus the energy required for digestion, dietary-induced thermogenesis - we use 5 - 10% of the calories we consume in the digestive process. Regular exercisers also need additional calories to fuel activity, so a Physical Activity Level (PAL) is applied to caloric requirements to calculate the additional energy required - for moderate exercise you should multiply your BMR by 1.6 (females) or 1.7 (men).

However, these energy calculations illustrate what is required to maintain your current weight. If you wish to lose weight, you have to create a calorie deficit. This means taking in less energy than you are using up. A daily deficit of 500kcals would create a weekly deficit of 3500kcals, which is approximately equivalent to the energy in one pound of stored adipose tissue (fat). The big question is, how to create this deficit?

The simplest – and healthiest – way is simply to cut out empty calories that are providing little or no nutrition: sweets, cakes, biscuits, alcohol, and processed, sugary or fatty foods. If your diet contains little of these anyway, you need to consider portion sizes – you may be eating a healthy diet but simply eating too much. Did you know that a portion of protein (meat, fish, eggs, beans, tofu) should be around the size of your palm? When was the last time you weighed out rice/pasta or cereal to ensure you were getting no more than 75g/35g respectively?

But if cutting back on treats or addressing portion control are not your thing, maybe you’ve tried intermittent fasting, as in the 5-2 diet? This is where calorie intake is drastically reduced to 500kcals on just 2 days a week. If your average calorie intake is usually around 2000kcals a day, then two days at 500kcals will create a deficit of 3000kcals weekly, almost equivalent to a pound of fat lost – however, not all the weight lost is fat.

Research has shown that animals live up to twice as long on a very low calorie diet, and caloric restriction in humans is linked to longevity and reduced risk of type 2 diabetes, heart disease and some cancers. But you can go too far…if you reduce your caloric intake to less than your BMR (40 - 70% of total caloric requirement), over time, thyroid function can down-regulate to reduce the amount of calories your body uses. This is because you need a certain amount of energy to function, and if you don’t have that, your body reduces its needs in order to survive. Based upon a daily total requirement of 2000kcals, 40% equates to a BMR requirement of 800kcals and 70% to 1400kcals, so consuming only 500kcals is certainly less than the BMR for most people.

The benefit of intermittent fasting is that one or two days a week is not sufficient to down-regulate metabolic rate, so you can safely create a calorie deficit. However, it’s worth considering that we only store enough glycogen (carbohydrate energy) for approximately one day – after that we start to utilise other fuel sources, including muscle. Loss of muscle tissue is noted in all low calorie diets, including intermittent fasting. In the first few days of ‘starvation’ we can lose 75g of muscle daily as glycogenic amino acids are converted into glucose for energy, but after about 3 days this reduces to approximately 20g of protein loss daily and we begin to use more free fatty acids and ketone bodies for fuel. Muscle loss can be partially offset with resistance training during the ‘diet’.

You may weigh less, but if the weight loss is water, glycogen and/or muscle loss, this isn’t good news for long-term weight maintenance. The only type of weight loss that is beneficial is a reduction in adipose tissue (fat).

- Water loss is detrimental to good health and exercise performance. Just 4% dehydration can reduce aerobic performance by up to 25%, and water weight will simply return as soon as you drink fluids.

- Glycogen loss can result in a lack of energy and impact on calorie expenditure through exercise. Glycogen weight will simply return as soon as you consume carbohydrates. 

- Muscle loss reduces metabolic rate, resulting in lower calorie expenditure over 24 hours – this is not good for long-term fat loss!

However you choose to do it – reducing portion size, limiting treat foods or having fasting days – the end result should be a calorie deficit and weight loss. To ensure that weight loss is not water, consume at least 2 litres of water daily. To maintain muscle mass, engage in regular resistance training using body weight exercises or weight training, and to maintain glycogen storage and minimise loss of lean tissue, regularly consume small portions of carbohydrates and protein throughout the day. Healthy fats should also be consumed, which can make a 500kcal goal difficult to achieve.

As long as you have the energy for maximum effort in your workouts (otherwise it is detrimental to the end goal!), and are including all the essential nutrients required for on-going health in your diet, then you should be on track. The trick is staying there…