Calories in and calories out
At first glance, calculating weight loss seems pretty easy. Just take the number of calories you burn, subtract the number of calories you take in from food, and you have your calorie deficit. Your body will have to cover this deficit by burning its fat reserves, which means you can estimate the weight that will be lost.
We can look at a hypothetical example of this calorie math. Your calorie expenditure is made up of 3 main parts:
- Resting metabolic rate (BMR) – The number of calories your body burns just to stay alive, such as breathing, pumping blood, etc.
- Thermic effect of food – The calories your body uses to digest, absorb, and metabolize food.
- Thermic effect of activity – The calories you use during exercise and non-exercise activities such as gardening, cleaning the house or fidgeting.
Let’s say you’re a man with a typical BMR of 1,800 kcal. The thermic effect of food usually accounts for roughly 10% of BMR, so that would be 180 kcal. You have a sedentary job but go cycling every day. Your thermic effect of activity may come up to about 700 kcal. This means your total daily calorie expenditure is 2680 kcal.
If you decide to take in 2130 kcal from food, you would be left with a 550-kcal deficit. To lose 1 kg of body fat, you need to burn roughly 7700 kcal. At a pace of 550 kcal per day, it would take about 2 weeks to lose 1 kg of body fat and about 1 month to lose 2 kg. Losing 2 kg in January with a very modest calorie deficit of 500 kcal sounds pretty nice. Unfortunately, the math isn’t so simple.
Estimating weight loss is hard
The first problem is that measuring calories is not easy. You can be off by hundreds of calories even if you try really hard. Check out our article series about calorie counting if you want to know about all of the possible problems.
On top of calorie counting, there are other factors that affect how we lose weight. Here are the most significant ones.
- Gender – Men tend to lose weight slightly faster than women, on average.
- Age – Weight loss gets increasingly difficult as we get older.
- Starting weight – An obese person can lose much more weight per month than someone who just carries a few extra kilos.
- Sleep – People with worse sleep have a much harder time losing weight.
- Medication and medical conditions – For example, antidepressants can stall weight loss and even promote weight gain.
- Repeated dieting – People that go through yo-yo dieting phases losing and gaining weight significantly have a harder time losing weight with every attempt.
Losing weight too fast is unhealthy
Almost everyone wanting to lose weight would prefer to do so quickly. Unfortunately, rapid weight loss carries some serious risks. Here are the most common side effects of weight loss that occurs way too fast.
- Muscle loss
- Dehydration, malnutrition
- Headaches, irritability, fatigue
- Hair loss
- Menstrual irregularities
Aim for 1% of body weight per month
So, what is a safe rate of weight loss? It depends on where you currently are on your weight loss journey. Weight loss typically happens faster in the first few weeks and then slows down. In general, a safe range of weight loss is somewhere between 0,5 –1,4 kg or about 1% of your body weight per week.
Before you start comparing your numbers to this range, keep in mind that weight loss is not a linear process. A successful weight loss journey is made up of weeks where you lose no weight and weeks where you lose more than you thought. That is completely healthy and to be expected. Research suggests that people that keep food diaries and measure their weight regularly are more likely to stay on track and reach their goal.