I only eat 1000 kcal per day, but I don’t lose weight

There’s one thing we can all agree on: everyone’s body needs energy to exist. The question of how much energy it needs is a little more complicated. A widespread fallacy is that there are a huge range of differences in people’s metabolic rates. The amount of energy we need is influenced by various factors, but the main ones are body mass, and what that mass is made up of.

A studio apartment requires less energy to heat than a four-bedroom house. The same is true of smaller and bigger people. A person with less body mass needs less energy to supply it. Furthermore, muscle mass requires more energy to maintain it than fat mass. This can be compared to the furnishings of a home: muscle mass is like the electronic devices that use up energy when they are on, but also use up some energy when they are in stand-by mode. Fat mass, on the other hand, just needs to be kept warm and supplied with blood.

Because of the biology of the human body, women naturally have a higher proportion of fat and a lower proportion of muscle mass than men. This explains why a man and a woman of the same size and weight will still have different energy requirements.

So, when we look at a scale of the human body’s energy requirements we will see at one end the lowest requirement — that of a small woman whose body weight is low (= low body mass) and who is extremely inactive (= little muscle mass, and little energy consumption due to movement) — and at the other end of the scale we see the highest requirement — a very big, heavy man (= high body mass) who leads a very active life ( = a lot of muscle mass, and high energy consumption due to movement).

To put some numbers into this equation, a forty-year-old woman who is 150 cm tall and weighs 45 kg, and who leads an inactive life, will have a daily energy requirement of about 1200 kcal when doing nothing but lying in bed all day. At the other end of the scale, a twenty-year-old male body builder with a height of 200 cm and a weight of 100 kg will require 2500 kcal a day even if he doesn’t move at all — that’s how much it takes just to keep so much more body mass supplied with energy.

If we look at the normal daily routines of these two people (the inactive woman has a sedentary job in an office and spends her free time at home on the couch, while the body builder has a physical job that keeps him on his feet all day, and spends his evenings pumping iron at the gym), the difference is even greater. The woman will ultimately require around 1400 kcal a day, while the muscle man might use up more than 4000 kcal daily.

As mentioned before, these are extreme examples. Most people are somewhere between 150 cm and 200 cm tall, weigh more than 45 kg, and are neither completely inactive, nor top-level athletes. In a study carried out in 2004, Donahoo et al. (short for et alii — Latin meaning, more or less, ‘and team’) investigated the energy consumption of people while at rest — how much energy their bodies required if they were to just stay in bed all day. Around 96 per cent of participants were found to have a resting energy requirement within a 600 kcal range of difference, and 66 per cent had a resting energy within a 200 kcal range. This means that most people’s resting energy requirements were neither extremely high nor extremely low, but within a relatively average range.

In 2005, Johnstone et al. studied 150 people and found that even those people with the lowest resting energy requirements still burned more than 1000 kcal in a day. This means that if such a person’s calorie intake was restricted to 1000 kcal per day, they would necessarily lose weight. Even for our petite inactive woman, if she ate only 1000 kcal per day she would be eating 400 kcal less than she burned on a normal day. One kilo of fat mass is equivalent to about 7000 kcal, meaning that our small woman would shed about 2 kg a month on this diet.

It’s even more dramatic if the woman in question is not petite, but overweight. The greater the body’s weight, the more energy it consumes. For one thing, the body needs more muscle mass to transport 100 kg in weight from A to B than 50 kg, and for another thing, that greater body mass needs to be heated, as mentioned above. Thus, an overweight person has a higher basic energy requirement than someone of the same height but normal weight.

So, if the 150-cm-tall woman weighs 100 kg rather than 45 kg, her resting energy requirement will no longer be just 1200 kcal, but 1750 kcal per day (and if she engages in light physical activity, as much as 2000 kcal per day). If she consumes only 1000 kcal a day, she will already lose a whole kilo in weight per week. With a daily intake of 1000 kcal or less, weight loss would be inevitable in her case. For most people, a reduction in their daily calorie intake to below 1500 is enough for them to consistently shed several kilograms a month.

A person’s energy consumption can be calculated relatively precisely using certain formulae. The only information you need is height, weight, sex, and approximate information about your daily activity levels. You can find plenty of online calculators on the internet; just search ‘basal metabolic rate calculator’ or ‘BMR calculator’ (your basal metabolic rate is the number of calories you would require if you were resting all day). Of course, these calculators are not 100 per cent accurate, but they can give you an estimation of your daily energy requirements with an accuracy of about 95 per cent. There’s a very high probability that your BMR will lie somewhere between 1400 and 2000 kcal per day — unless, as mentioned above, you happen to fall into one of the two extremes of very high or very low body mass.

The bottom line is that most people consume far more than 1500 kcal per day, but even people with extremely low consumption still need significantly more energy than 1000 kcal. Which means it’s practically impossible not to lose weight on a daily calorie intake of 1000 kcal.