How many kJ per day should I burn to lose weight?

Find out what role our kJ intake and the 'starvation mode' has on our weight loss efforts.
Published 21 February 2021

In order to achieve weight loss an individual needs to consume less energy (kilojoules) than the will body burn. For example if an individual needs 8,700 kilojoules per day to maintain weight, reducing daily intake to 6,600 kilojoules (assuming exercise stays the same), should provide around 500g per week weight loss.

How many kilojoules should I eat per day?

The average adult needs about 8,700 kilojoules (kJ) a day to maintain a healthy weight. This number is a average and varies based on many factors including how active we are, our age, sex, height and weight. Our body then uses this energy to keep our body functioning.

Starvation and weight loss

Your metabolism can slow down during kilojoule restriction. It’s true, as you start to restrict your kilojoule intake, your metabolism will reduce. As your body starts to lose weight it becomes more efficient, requiring fewer kilojoules to perform the necessary daily functions for survival. Consequently, this can slow (but not stop) the anticipated rate of weight loss.

For example, if an individual needs 8,700 kilojoules per day to maintain weight, reducing intake to 6,600 kilojoules, assuming exercise stays the same, should provide a 500g per week weight loss (note: 500g of weight is equivalent to about 14,700 kilojoules). Furthermore, reducing to 4,500 kilojoules should result in a weight loss of 1kg per week and going down to 2,400 kilojoules a day should result in a weight loss of 1.5kg per week. However, if an individual actually reduces their intake to 2,100 kilojoules, the weight loss would not likely be a steady 1.5kg per week because of the reduced metabolic rate. It would likely be around 1kg. This 'lower than expected' rate of weight loss is a lot different to 'no' weight loss as the 'starvation mode' notion proposes.

What is the starvation myth?

The ‘starvation mode’ theory is when a person trying to lose weight reduces their kilojoules intake enough they actually slow down their metabolism, which prevent further weight loss from happening.

Does starvation work for weight loss?

While there is no ‘biological’ evidence to support the ‘starvation mode’ myth, there may be behavioural reasons why weight loss stops when kilojoules are severely reduced. Science shows over-restricting food or depriving ourselves from food actually makes us more likely to overeat these foods. The reason for this is that we experience cravings for the food and when the opportunity eventually arises to eat this food, we haven’t created a plan for how to eat it in moderation. Instead of heavily restricting the foods you eat, learn how to incorporate them into your diet in a healthier way.

Another common reason for a weight plateau is being less diligent with your eating and activity behaviours. Often when we start a weight loss program we’re highly motivated, following the program religiously, tracking everything we eat and moving more each day. But as the weeks or months go by our motivation can start to waiver and old unhealthy habits can start to slip back into our routine. We often avoid keeping ourselves accountable when this happens, and sometimes we don’t even realise our portion sizes have crept up or we’re reaching for a second serving. If you think you may have fallen into this trap, the best thing is to go back to basics and start following the program like you were when you first started.

The good news is once you’ve achieved your weight loss goals and your weight has stabilised, it does not appear that the dip in metabolism is permanent. Several studies showed that metabolism goes back to expected levels with sustained weight loss, discounting the theory that a lowered metabolism helps to explain the common phenomenon of weight regain following weight loss.

Slow and steady is most effective for long term sustainable weight loss

Australian Dietary guidelines as well as WW’s program guidelines, recommend a safe weight loss rate of 500g-1 kg a week. You may lose more in the first few weeks, but this should even out over time. Losing weight too quickly isn’t good for your health, and can actually make it harder for you to sustain the weight loss long term. Losing weight too quickly can also pose a risk to your overall health including developing gall stones, irregular heart beat and excessive loss of lean muscle mass. Following a ‘quick-fix’ approach to weight loss also doesn’t help you work on the root cause of your weight gain in the first place. Weight loss isn’t just about what you eat and the activity you do – it’s about your mindset and the way you think too. Shifting your mindset takes time, that’s why a slow and steady approach will turn about to be the most effective in the long run.