When you want to lose weight, it only makes sense to do your research to see what the most popular diets are right now. But, while there are plenty of trendy eating plans floating around, there’s an old standby that’s suddenly become popular again: the kilojoule (calorie) deficit diet.
The diet is simple at baseline: It doesn’t tell you which foods you can and can’t eat. Instead, it just focuses on kilojoules, which are the amount of energy that’s released when your body breaks down food. The more kilojoules food contains, the more energy it can give your body, explains Jessica Cording, dietitian and author of The Little Book of Game-Changers.
Your body needs a certain amount of kilojoules to function normally, but when you take in more kilojoules that you need, the excess amount is stored in your body as fat, Cording says.
That’s where the kilojoule deficit diet comes in. But how does the kilojoule deficit diet work and how can you know if it’s right for you? The experts break it all down.
So, what is a kilojoule deficit diet?
The concept behind the kilojoule deficit diet is pretty simple: You try to eat fewer kilojoules than you burn. “A kilojoule deficit is when we consume fewer kilojoules than our bodies burn in a day, both in terms of our natural resting energy expenditure—your natural metabolic rate—and any exercise or physical activity we engage in on top of that,” says dietitian Dana Ellis Hunnes PhD.
Can a kilojoule deficit diet help you lose weight?
If you calculate things right, a kilojoule deficit diet should help you lose weight. “Kilojoules are one of the primary factors in weight gain or weight loss,” Cording says. “If somebody is consuming fewer kilojoules than they burn, that can result in weight loss.”
So, if you typically eat 9000kJ a day, on a deficit diet, you should lose weight if you strive to cut that back to 8300 or so kilojoules a day.
There’s a reason why this happens. “When we take in fewer kilojoules than we burn, our body has to get the energy from somewhere to continue its natural functioning,” Hunnes explains. “This additional energy to maintain bodily functions comes from stored energy within our own bodies.” Your body will usually burn fat first, followed by glycogen, which is what carbs are turned into in your body after you eat them, Hunnes says. And, when that fat is burned off, you end up losing weight.
How to do a kilojoule deficit diet
There are a few ways to go about this. “The gold standard would be to find out how many kilojoules you burn in a day,” Hunnes says. That can include having a test done known as indirect calorimetry, which measures the amount of carbon dioxide you exhale, combined with some complicated calculations to figure out how much energy you burn at rest (i.e. when you’re just sitting there, not doing anything). There’s also something called a Bod Pod that can measure how much energy you burn at rest, but both methods aren't easily accessible to everyone.
So, you can use online equations like Eat for Health Calculators to try to figure out what your daily energy intake should be in order to help you maintain or reach a certain weight goal. To use it, you enter in basic information like your current weight, age, and activity level. From there, the planner will give you an estimate of how many kilojoules you are eating. If the aim is to loose weight you use your ideal weight to calculate the your requirements.
But this isn’t an exact science and it doesn’t take into account things like how much muscle mass you have (muscle tissue burns more kilojoules than fat) and your individual metabolism, Cording says. “There are so many factors that go into energy expenditure,” she points out. Still, it can give you a rough estimate to work with.
If you want to make it easy on yourself and you know you want to lose about 500 grams a week, you can try to slash 2000kJ from your existing dietary intake, says Keri Gans, dietitian and author of The Small Change Diet. Plus a goal of 0.5kg weight loss per week is consider safe and sustainable. And for the average woman that means reducing your kilojoule intake from 8900kJ to 7500kJ, (or roughly 1500 cals) per day.
But being able to cut that many calories and feeling comfortable with it depends on what you were eating before, Cording says. If you had plenty of high-calorie processed foods and sweets in your diet, swapping them out in favour of plenty of fruits and vegetables, lean proteins, and healthy fats may not feel like a huge deal in terms of your hunger levels, she says. If you were already eating pretty healthy to begin with, though, you may feel more comfortable cutting out 520kJ to 1050kJ a day instead, with the understanding that it will likely take a little longer for you to lose the weight. “I’ve seen people do really well with that,” Cording adds.
How much of a kilojoule deficit is too much?
Reducing your daily intake to between 6200 kJ and 7000kJ (1500cals to 1700cals) can create a kilojoule deficit that may assist with weight loss. However, it’s important to understand that studies suggest that reducing your intake too much below that may actually have the opposite effect by slowing metabolism. Some tell-tale symptoms you’re going to fast include fatigue, nausea, dizziness, and constipation.
In general, Hunnes says, “I strongly encourage not cutting out more than 15 to 20% of calories.”
What’s the healthiest way to do a kilojoule deficit diet?
In addition to not cutting out too many kilojoules from your daily diet, experts say there are some other tips that can help you do a deficit diet in a healthy way:
- Focus on what you’re adding to your diet. “If you are adding more fruits and veggies to your diet, you might naturally wind up eating less foods with lots of added sugar and fat,” Gans says. “Therefore, you will lose weight.”
- Strive for nutrient-rich foods. Those include fruits, vegetables, 100% whole grains, low-fat dairy, seafood, poultry, lean meats, and healthy fats, Gans says.
- Minimise processed foods. “I recommend incorporating the most natural forms of foods into your diet as possible— single-ingredient, as close to nature as can be found foods,” Hunnes says. “The fewer things out of a package the better.”
- Watch your portion sizes. While you could calculate exact kilojoules at every meal, that can get stressful and time-consuming. “Keeping portion sizes in check is most likely one of the best tools to help,” Gans says.
- Pay attention to your macronutrients. Macronutrients, aka “macros,” are the main components of food that you need to be healthy, Cording explains. “Make sure you’re getting enough protein, healthy fats, and calories that feels good to you,” she says.
- Eat in. “Eating at home gives you greater control over what is in your food,” Hunnes says.
If you’re interested in doing a kilojoule deficit diet and aren’t sure where to start, check in with your doctor or a registered dietitian—they should be able to help offer personalised guidance.
Newsletter
