How many calories do I need to consume per day?

There is no precise number of calories that a human has to consume per day because each person has different needs. When we talk about calories, we are talking about the energy a person needs: energy for the body to continue functioning, for our hearts to beat, for us to be able to breathe, for the brain to be active, to be able to carry out daily tasks etc. For everything in our body to function properly, we need a minimum calorie intake per day. The correct term is kilocalories (kcal), but we have standardized them as calories – a measurement of the energy a person needs.

As such, the answer to your question is that you need to consume as many calories as your body requires for energy. And energy needs are different in each individual, depending on several factors. The first is age, because energy needs vary as we get older. It also depends on gender, and this is related to the body area that each person has. Body composition is also a factor: fat, muscle mass, weight, what you eat and the amount of activity you do – both daily activity and whether or not you also perform additional physical activity.

What do exist are standards according to formulas that take into account these variables, and which tell us the amount of energy people need according to age bracket: from 2 to 5 years old; from 5 to 8 years old; from 8 to 11 years old; from 12 years old onwards – differentiating between girls and boys – and so on. Additionally, specific life situations in which energy needs are increased are also taken into account. These mainly occur during pregnancy and breastfeeding, or when very intense physical activity is carried out, for example by athletes.

It is also necessary to consider the purpose of calorie consumption. If what I want is simply to be healthy, there are specific recommendations. If I want to lose or gain weight, I will have to adapt my energy intake. But again, there is no precise amount. The World Health Organization states that a healthy adult individual should consume between 2,000 and 2,400 calories a day: women toward the 2,000-calorie end of that range and men toward 2,400. However, we cannot lose sight of the androcentrism of science, which identifies an adult individual as a male of about 40 years of age.

So far, so simple, but there is another issue to consider: each person has a different energy balance that depends on their genetics and what we call basal energy expenditure, which represents 75% of calories required per day. People who are lucky enough to have a high basal energy expenditure – for example, someone who burns 1,200 calories per day but only consumes 1,100 – are genetically predisposed to be thin. If it transpires that your basal energy expenditure, which cannot be influenced, is very low, you will gain weight easily.

Basal energy expenditure is related to age and as age advances, basal expenditure decreases. It has been estimated that, from the age of 45-50 years, for each decade we add to our age, basal energy expenditure decreases by 10%. Therefore, there is a point after reaching that age when many people ask: if I’m eating the same food as when I was 30 years old, and doing a similar amount of physical activity, why am I putting on weight?

Fortunately, total energy expenditure does not only depend on that basal energy expenditure, which is genetically conditioned, but also depends on other factors such as physical activity.

There are several formulas for calculating energy expenditure but one of the most simple – although it must be taken into account that other components such as age, height or activity must be added – is for women to use the factor of 0.9 kcal per kilogram of body weight and men 1.0 kcal per kilogram, then multiply that value by 24. This gives us an estimate, an approximation. According to the result of that calculation and what a person intends to achieve with energy intake – to maintain, lose or gain weight – we have an idea of the number of calories that we should ingest. If a person wants to lose weight, has a low basal expenditure and eats a lot, they will have to increase their physical activity in time, intensity or frequency.

There is a technique to determine the exact number of calories we need: indirect calorimetry. This involves a piece of equipment that is not commonly available and that is generally used in ICUs, where measurements have to be very precise, so patients are not at risk of life-threatening malnutrition.

María José Castro Alija is a medical specialist in nutrition and a professor and researcher at the University of Valladolid. She is part of the research group on Assessment and Multidisciplinary Intervention in Health Care and Sustainable Lifestyles (VIMAS+)

Question sent via email by Soledad Ferrol

Coordination and writing: Victoria Toro

We Respond is a weekly scientific consultation, sponsored by the Dr. Antoni Esteve Foundation and the L’Oréal-Unesco ‘For Women in Science’ program, which answers readers’ questions about science and technology. The scientists and technologists who answer these questions are members of AMIT (Association of Women Researchers and Technologists).

The advice offered in this article is of a general nature and should not replace medical consultation. If you have any questions about a specific problem, contact your doctor or pharmacist.

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