The Fitness Zone

Everything You Need To Know About Calorie Deficits

Jul 11, 2021 | by Lacey Blackman

If you’re focused on your health and fitness, it’s likely you’ve heard the term ‘calorie deficit’, with many people attributing their weight loss to it. But what exactly IS a calorie deficit, and what are the long and short term effects of striving to achieve it? Lacey Blackman, Pro Bikini competitor, Personal Trainer and Online Coach at the Australian Institute of Fitness, gives us the lowdown on everything you need to know about calorie deficits.

What is a calorie deficit?

The term ‘calorie deficit’ is often used in reference to the disruption in energy balance, whereby the body either expends more calories than it consumes, or less calories are consumed compared to what is needed to maintain current body weight.

The laws of thermodynamics dictate that ‘energy cannot be created or destroyed’, but rather transformed or transferred. This means that in order to lose weight (namely body fat), a disruption in body composition needs to occur, ideally through a healthy combination of calorie expenditure via exercise and calorie deficit via food intake.

How do I calculate what my daily caloric intake should be?

Calculating how many calories you should be consuming each day involves a bit of number crunching. If maths is not your forte, there are various online calculators that will do the hard work for you (such as the Eat For Health Energy Requirements calculator). However, if you are keen to take on the challenge yourself, keep reading!

  1. Calculate your Basal Metabolic Rate (BMR)

The first step in determining daily caloric intake is to calculate the energy you require to keep your body functioning at rest. This is known as your Basal Metabolic Rate (BMR) and can be estimated using the following formulas:

Age (Years)BMR: (kJ/ day)

[0.074(BW)+2.754] x1000
[0.063(BW)+2.896] x1000
[0.048(BW)+3.653] x1000
[0.049(BW)+2.459] x1000

[0.056(BW)+2.898] x1000
[0.062(BW)+2.036] x1000
[0.034(BW)+3.538] x1000
[0.038(BW)+2.755] x1000

For example, for a 33-year old female who weighs 70kg:

[(0.034 x 70) + 3.538] x 1000 = 5918kJ/day

Remember to press ‘equals’ after each portion of the equation, and to keep in mind that the answer will be based on kilojoules (kJ) rather than kilocalories (kCal).

  1. Factor in the Physical Activity Level

The second step is to factor in how much energy you typically expend, using the Physical Activity Level (PAL) scale. Simply multiply the BMR by the estimated PAL:

Physical Activity LevelDescription
1.2Bed rest
1.4Very sedentary
1.6Light activity
1.8Moderate Activity
2.0Heavy Activity
2.2Vigorous Activity

For example, if the PAL is of Moderate Activity, the calculation will be:

5918 x 1.8 = 10,652kJ/day

NB: To convert kilojoules into kilocalories (if you’re more familiar with using this unit of energy, which many of us are), simply divide kJ by 4.184:

10,652kJ / 4.184 = 2,545kCal

Therefore, the 33-year old female with a moderate activity level needs to consume 2,545kCal a day to maintain her current weight. If fat loss is desired, a calorie deficit can be achieved by reducing energy intake and/or increasing energy expenditure.

How much of a deficit do I need to be in to lose weight?

A deficit of up to 500 calories per day is considered safe for healthy and sustainable fat loss, which can equate to 0.5-1kg of weight reduction per week (depending on nutritional consistency, training frequency/intensity, etc). It is recommended to ‘start small’, around 200 calories for example, to assess whether this deficit prompts a favourable response.

How long should I be in a calorie deficit?

It is not ideal to be in a calorie deficit for an extended duration of time due to various physiological and psychological impacts and potential difficulties in maintaining weight loss efforts. Individuals should monitor both the success of their body composition goals and their mental health to determine whether a ‘diet break’ should be implemented periodically (every 8-12 weeks) to help break up their goals into smaller and more attainable chunks.

Is it possible to be in a calorie deficit without tracking food intake?

It is certainly possible to be in calorie deficit without tracking food, however changes in body composition using this approach may not always be consistent. It is worth keeping a food journal and reading nutritional labels to gain a more thorough understanding of the nutritional content of food, in conjunction with following the 5 Australian Dietary Guidelines, to assist in keeping within nutritional requirements.

Will I put all the weight back on when I start to eat my normal intake again? 

There is no escaping the laws of thermodynamics: if calorie intake is less than energy expenditure, weight loss occurs. If calorie intake is greater than energy expenditure, weight gain occurs. However, this does not necessarily mean that all weight will be regained or that any regained weight will be all body fat. It is likely that if exercise frequency or training intensity is maintained or increased, with a gradual increase in calorie intake, this can contribute to improved performance and thus increased lean muscle mass (rather than fat).

The introduction of more calories after a calorie deficit – particularly after an extended duration – should be at a slow pace to avoid a faster rate of fat regain. Also, keep in mind that the muscles in the body are often quite low in glycogen stores during a prolonged deficit, so it can be easy to confuse the refilling of these stores with ‘fat’ when consuming higher amounts of calories.

Are there any negative consequences of being in a caloric deficit? 

Some of the repercussions from being in a calorie deficit for extended durations of time include:

  • Decreased BMR due to metabolic adaptations
  • Increased fatigue and potential nutritional deficiencies
  • Disruption to menstrual cycle and fertility in females (in cases where calories are greatly restricted or body fat percentage is quite low)
  • Lowered immune system
  • Changes in social behaviour and/or disordered eating due to dietary control

To ensure optimal health and maintainability in applying a calorie deficit, it is recommended to consult with an Accredited Practising Dietitian who can safely tailor a plan based on your requirements.

If I am successfully losing weight in a calorie deficit, do I still need to exercise? 

It is recommended to implement or continue with an exercise regime during a calorie deficit to not only assist with calorie expenditure, but also to maintain lean muscle mass during the deficit phase. If exercise is not performed during a calorie deficit, it can cause metabolic adaptations to occur at a faster rate, thus slowing down body composition changes. Lack of exercise during a calorie deficit also runs the risk of muscle atrophy, due to the body using lean muscle mass as a source from which to extract energy. If the body composition goal is to lose fat and build/maintain muscle mass while in a calorie deficit, lifting things is a must!

If you’ve learnt everything there is to know about calorie deficits and it all just seems like too much effort – that’s fine! Focus on exercise, eating well (predominantly whole foods and fresh produce) and listening to your hunger cues and you will be well on your way to a healthier, happier body.

If fitness is your passion – check out our many Personal Training Course options here.

Lacey Blackman

Lacey Blackman

Online Coach
Lacey Blackman is an Educator for the Australian Institute of Fitness and a Personal Trainer with over 9 years of experience in the fitness industry. After graduating from AIF in 2014 with a Certificate IV in Fitness (Master Trainer Level 1), Lacey opened her personal training and online coaching business “Lean Like Lacey” which specialises in women’s health and fitness. In that time, Lacey has gained hands-on experience within various fitness facilities around Brisbane - from privately-owned studios to commercial gyms. Lacey’s passion for health and fitness has since expanded into nutrition and business, and subsequently undertook a Certificate in Applied Sports Nutrition and Diploma of Fitness to continue growing her knowledge in these areas. Using her professional knowledge as a Personal Trainer and AIF Educator, and personal experiences as a professional natural bodybuilder, Lacey is determined to inspire and empower future generations of PTs in the ongoing fight against SEDS.

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