What does a calorie deficit mean in practice?
Your body weight is determined by energy balance, meaning the relationship between how many calories you consume and how many calories your body burns. When energy intake is lower than energy expenditure, you are in a calorie deficit. The body's total daily energy expenditure is divided into three main components:
- Basal metabolic rate (BMR): The energy your organs require to function at complete rest. This accounts for around 60 to 70 per cent of your daily expenditure and is influenced by factors such as age, sex, weight and body composition.
- Thermic effect of food (TEF): The energy required to break down and digest the food you eat. This accounts for around 10 per cent of energy expenditure.
- Physical activity: All movement you perform. This includes both planned exercise and all spontaneous everyday movement (NEAT), such as standing up, taking the stairs or gesturing.
A healthy and sustainable target for most people is an energy deficit of around 300 to 500 kilocalories per day, which usually results in steady weight loss of around 0.5 kilograms per week. To create this calorie deficit, your food choices are the decisive factor. Although physical activity is important for preserving your muscle mass and improving your health, it is almost impossible to exercise away a calorie surplus through exercise alone because the body often compensates through increased hunger.
Instead, the primary purpose of exercise is to preserve muscle mass as you lose weight. An excessively large calorie deficit should usually be avoided, as it increases the risk of the body breaking down valuable muscle mass instead of fat and makes it very difficult to meet the body's basic needs for nutrients, vitamins and minerals.
Why does the body's energy expenditure decrease when you lose weight?
When you reduce your calorie intake, the body responds by lowering your energy expenditure, a survival mechanism known as metabolic adaptation. The body has a built-in thermostat, a so-called "set point", that strives to keep body weight stable. For every kilogram you lose, the body's energy expenditure decreases by around 20 to 30 kilocalories per day.
This reduction is due to several factors. A smaller body requires less energy to function and move. The body also often unconsciously reduces spontaneous everyday movement (NEAT) when energy levels fall. At the same time, hormonal changes occur. Levels of the satiety hormone leptin decrease, while levels of the hunger hormone ghrelin increase. Research shows that for every kilogram lost, the biological drive to eat increases by the equivalent of 100 kilocalories per day. It is this increased hunger, combined with the reduced calorie expenditure, that usually causes weight loss to stop after a while.
The role of exercise when you want to lose weight and support your energy expenditure
Strength training is crucial during a calorie deficit because it signals to the body to preserve muscle mass. During weight loss, you normally lose both fat tissue and muscle mass. If the loss of muscle mass becomes too great, your basal metabolic rate decreases, making it more difficult to continue losing weight and then maintain the weight loss.
Strength training two to three times a week can help limit the loss of muscle mass during weight loss while also strengthening the muscles. Muscle uses more energy at rest than fat tissue, which means that preserving muscle mass helps preserve energy expenditure. Cardio exercise is also an excellent complement for strengthening cardiovascular health and increasing total calorie expenditure during the day.
Foods that keep you full during a calorie deficit
Successfully maintaining a calorie deficit over time is about maximising satiety and nutrient intake with fewer calories. When you restrict your calorie intake, it is essential that your food is nutrient-dense and provides plenty of volume for the calories.
To manage hunger and maintain muscle mass, you should prioritise a high protein intake. Protein is the nutrient that signals the greatest feeling of fullness to the brain. A recommended intake during weight loss is around 1.5 to 2 grams of protein per kilogram of body weight, calculated using your target weight. Protein sources such as fish, chicken, eggs, pulses and quark are excellent choices.
Combine protein with plenty of fibre-rich carbohydrates, such as vegetables and root vegetables. Fibre binds fluid in the stomach and prolongs gastric emptying, which contributes to a more stable blood sugar curve and longer-lasting satiety. Eating slow-release carbohydrates and healthy fats, such as those found in olive oil and oily fish, is also important for keeping energy levels stable and providing the body with the nutrients it needs.
Medical treatment to support a calorie deficit
Modern weight-loss medicines (GLP-1 analogues) make it possible to maintain a calorie deficit by reducing the strong biological hunger signals that the body would otherwise activate during weight loss. Obesity is a chronic disease in which the body's natural systems for hunger and satiety are often out of balance.
The medicines mimic the satiety hormones that the gut naturally releases after a meal. They work by slowing gastric emptying while also binding to receptors in the brain's appetite centre. This reduces appetite, increases satiety and reduces constant cravings and thoughts about food. The treatment does not change the need for a calorie deficit for weight loss to occur, but it reduces the biological burden on the patient so that healthy dietary and lifestyle advice can actually be maintained over time.
Frequently asked questions about calorie deficit and weight loss (FAQ)
How large should a calorie deficit be?
A healthy calorie deficit is generally around 300 to 500 kilocalories per day. This provides sustainable weight loss of around 0.5 kilograms per week and reduces the risk of muscle loss, fatigue and nutritional deficiencies. A deficit of around 500 kcal per day is a good target that many people can follow to lose weight at a reasonable rate.
How many calories per day do I need to be in deficit to lose half a kilogram per week?
To achieve steady and sustainable weight loss of 0.5 kilograms per week, you generally need an energy deficit of around 500 kilocalories per day. Physiologically, this is because one kilogram of body fat corresponds to approximately 7,000 to 7,700 kilocalories. By reducing your intake by 500 kilocalories per day, you therefore create a total deficit of around 3,500 kilocalories over seven days, resulting in approximately half a kilogram of weight loss. Maintaining a moderate deficit of 300 to 500 kilocalories per day is a good starting point, as losing weight too quickly increases the risk of significant fatigue and the body breaking down more muscle mass instead of fat.
Why does my weight loss stop despite a calorie deficit?
The body adapts to a lower weight through metabolic adaptation. Energy expenditure decreases as you weigh less, which means that your previous calorie deficit has now become your new energy balance. To continue losing weight, your intake needs to be adjusted. It is therefore important to regularly recalculate your calorie needs and adjust your calorie intake according to your current weight and activity level.
Can you build muscle while in a calorie deficit?
Yes, this is called body recomposition. This is particularly possible for people who have a higher starting weight or are new to strength training. It requires strength training combined with a sufficiently high protein intake.
Is simply eating less enough to lose weight?
You can eat in a calorie deficit and still, technically, remain weight stable. However, if the deficit is created through nutrient-poor foods or extreme diets, the risk of losing muscle mass, developing nutritional deficiencies, becoming extremely hungry and regaining weight when the diet ends increases. It is therefore important to eat healthy and nutritious food even when you are eating fewer calories, so that you feel well and can successfully maintain healthy weight loss in the long term.
Is it dangerous to have too large a calorie deficit?
An excessively large and prolonged calorie deficit (extreme calorie restriction) carries medical risks. It leads to loss of cardiac and skeletal muscle, impaired immune function, hormonal disturbances, hair loss and nutritional deficiencies. It also increases the risk of gallstones and the development of binge eating. You should therefore never maintain a deficit that is greater than you can sustain in a healthy way, and it is important to adjust your intake according to your age, activity level and individual needs.
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