How many calories you need a day and how to work them out

Every weight-loss plan starts from one number, the calories you burn in a day. No device at home can measure it. There are, however, equations that estimate it well enough to get started, and ways to correct it as you go.

Step 1: your resting metabolic rate

Your resting metabolic rate is the energy you burn just to keep your body running, lying down and rested. For most people it is the largest part of the energy they burn in a day.

The equation MesogeaFit uses is Mifflin-St Jeor, from 1990:

The equation came from measurements in 498 healthy adults aged 19 to 78, with and without obesity. A 2005 review found it the most reliable of the commonly used equations. In a 2013 study of 337 adults, it came within ±10% of the measured value for 82% of people, that is 87% of those without obesity and 75% of those with obesity. So it is fairly certain that it gives a good starting point, but for one person in five the error is larger.

Step 2: how much you move

Your resting metabolic rate is multiplied by a factor for your daily activity. This gives your maintenance calories, the amount you need to stay at the same weight.

The FAO and the World Health Organization give three groups: sedentary or light activity 1.40 to 1.69, active 1.70 to 1.99, and vigorous 2.00 to 2.40. For example, an office day corresponds to about 1.53. The European Food Safety Authority uses 1.4, 1.6, 1.8 and 2.0.

MesogeaFit uses low factors, from 1.35 for a very sedentary life up to 1.75. The value of 1.35 is just below 1.40, the lowest figure the FAO gives for adults. We chose it so that the starting point leans towards fewer calories. If you need more, your weigh-ins will show it. Exercise that you enter separately is counted at half its value at most.

Try it with your own details:

Three examples

With a ±10% error, the first woman's resting metabolic rate could be anywhere from 1,255 to 1,535 kcal. That is a difference of 280 kcal, about the size of a snack.

Why the number needs correcting

The equation gives a starting point. Your real energy use shows in what your weight does when you know what you eat. A 2015 study showed that a mathematical model, using only a person's details and weigh-ins, estimated changes in how much the participants ate with an error of 40 kcal a day for the group. For each person on their own, the error was larger.

That is why MesogeaFit starts from the equation and then adjusts your calories every 2 weeks. It compares the food you log with the average of your weigh-ins. Adjustments are made in small steps, up to 150 kcal at a time.

The ‘7,700 calories = 1 kg’ rule

You come across it often. A kilo of fat holds about 7,700 kcal, so eating 500 kcal less every day gives half a kilo a week. The rule overestimates weight loss, and the gap grows over time, for two reasons:

• Energy use falls as you lose weight. A smaller body needs less energy, about 24 kcal less a day for every kilo lost, according to the model published in the Lancet in 2011.
• Appetite goes up. In a study of 153 people with type 2 diabetes taking a medicine that removes calories through the urine, appetite rose by about 100 kcal a day for every kilo they lost. It concerns one specific group, so the figure is of low certainty.

In 7 controlled weight-loss studies, participants lost 9.1 kg on average, while the rule predicted 12.5. That was 27% less, already within 1 to 3 months.

One point works the other way. The 7,700 kcal figure roughly fits people with a lot of body fat, more than 30 kg. People with less fat also lose more of other tissue, which holds less energy per kilo, so each kilo needs a smaller deficit.

So the rule gives only a rough idea. That is why the app shows your goal date as a range of months and recalculates it every week.

From maintenance to weight loss

To lose weight, you eat less than your maintenance calories. In the MesogeaFit app:

• The rate is 0.25% to 1% of your body weight a week, with 0.5% as the default. Never more than 1 kg a week. See the article on the rate of weight loss.
• Calories never drop below a floor, set at the higher of resting metabolic rate × 0.8 or 1,200 kcal for women and 1,500 for men.
• The weight-loss plan is only created after you answer the safety questions.

For the 30-year-old woman in the example, resting metabolic rate × 0.8 is 1,031 kcal, so her floor is 1,200.

Who needs a different calculation

• Under 18. Growing bodies have different needs and the equation is not meant for teenagers. The app does not create a weight-loss plan for minors.
• In pregnancy needs are higher, and you should not follow a weight-loss diet.
• During breastfeeding needs also go up. The app allows a small deficit only from the 8th week after giving birth and if body mass index (BMI) is 25 or more. It never goes below 1,800 kcal, and it stops the deficit if milk supply drops.
• Older people, athletes, very muscular people and people with obesity show larger errors with the equation. Correcting from weigh-ins matters even more for them.
• People with a condition that changes metabolism, such as thyroid disease, heart failure or cancer treatment. The equation has not been tested in these cases. Ask your doctor.