Causes of Underweight and How to Deal with It

When Weight Doesn't Match - What Does Underweight Mean and How to Recognize It?

In a time when there is frequent discussion about the obesity epidemic and the health risks associated with being overweight, underweight is often overlooked. Yet it can be equally dangerous—and sometimes even more insidious, as it hides behind an unassuming appearance and is often socially perceived as the "ideal of beauty." But how do you distinguish between healthy slimness and unhealthy underweight? And why is it important to know this boundary?

What is Underweight and How to Determine It?

Underweight is a condition where a person's body weight falls below the level considered healthy with respect to their height, gender, and age. The most common indicator for a preliminary assessment is the BMI – Body Mass Index. The calculation is simple: weight in kilograms is divided by the square of height in meters. The result then falls into one of the basic categories:

• BMI below 18.5 – underweight
• 18.5–24.9 – normal weight
• 25–29.9 – overweight
• 30 and above – obesity

For example, a person measuring 170 cm and weighing 50 kg has a BMI of 17.3 – which already indicates underweight. This simple BMI underweight test is available on several online calculators and can serve as a first step. However, it's important to keep in mind that it is not a perfect tool. It does not consider muscle mass distribution or body composition. Therefore, in case of suspicion, it is advisable to consult the condition with a doctor or nutrition specialist.

Underweight and Its Health Consequences

While overweight often draws attention to the problem itself—with joint pain, shortness of breath, high blood pressure—underweight may initially appear inconspicuous. People with it may feel fine and even be athletically active. However, the problem arises if low weight is accompanied by fatigue, weakened immunity, hair loss, or menstrual disorders in women.

The body, in the case of long-term underweight, begins to exact its toll. Muscle mass is lost, metabolism slows down, and resistance to infections decreases. In children and adolescents, underweight can slow growth and development. In older individuals, it increases the risk of falls, osteoporosis, and overall deterioration in quality of life. Doctors also often highlight the link between underweight and psychological problems—such as anxiety, depression, or eating disorders.

One specific story is the case of a young student who, after moving abroad, began to avoid food due to stress. Initially, her weight loss was praised by those around her—she looked "fit." It wasn't until she started fainting and stopped menstruating that it became apparent her BMI had dropped below 16. It required intervention from doctors and psychotherapists to bring her back into balance.

Why Does Underweight Occur? Causes Are Not Just About Food

Underweight is definitely not just a matter of eating less, although at first glance it may seem that way. Sure, for some people, insufficient food intake is the main cause—whether it's intentional calorie restriction due to diets, or situations where something simply "gets in the way" of eating. There may be a lack of appetite, time, energy, or even money. But that's far from the whole story. Some people may burn far more than they realize—increased energy expenditure is not reserved only for top athletes.

It also concerns those who toil on a construction site or run around on a night shift in a hospital. And then there are health problems—some diseases literally "leave a mark" on weight—whether it's autoimmune diseases like celiac disease, thyroid disorders, Crohn's disease, type 1 diabetes, or even cancer. And what about the psyche—it can really mess with one's appetite: stress, anxiety, depression, or eating disorders such as anorexia or bulimia often leave not only internal disruption but also numbers on the scale that are dropping.

And we mustn't forget about medications—there are groups of drugs that can affect appetite, slow digestion, or disrupt the absorption of important substances. So when someone struggles with underweight, it's definitely not just about raiding the fridge—the causes can be numerous and often hide where one wouldn't expect them.

It's clear that the cause of underweight is not always simple and sometimes requires a comprehensive diagnosis. If BMI remains below 18.5 for a long time, it's time to take notice, even if the person otherwise feels fine.

Underweight vs. Overweight

Interestingly, underweight and overweight have more in common than it might initially seem. Both extremes represent a deviation from ideal weight and both can have serious health consequences. In both cases, prevention, early recognition, and resolution are important. And this is where awareness and understanding one's bodily needs play a key role.

While social pressure often pushes towards slimness to the point of extreme thinness, reality shows that healthy weight is individual. One person may feel great with a BMI of 22, while another needs to have a bit more on their bones to be in balance. It is important to monitor not just numbers, but also body signals—fatigue, moods, digestion, or sleep.

And the opposite is true too—people who cross into the overweight category don't need to panic immediately. BMI is an indicator, not a judgment. Much more important than the chart is the overall lifestyle—diet, exercise, sleep, stress levels, and mental health.

How to Gain Weight Healthily? It's Not Just About Caloric Surplus

If someone is in the underweight range and wants to improve their situation, it's not enough to just "eat more." It is important to eat smartly and balanced. The focus should be on quality and nutritionally rich foods, not empty calories. What does this mean in practice?

• Increase the intake of healthy fats—such as nuts, seeds, avocados, olive oil, or fatty fish.
• Add complex carbohydrates—whole grain bread, quinoa, potatoes, sweet potatoes, oatmeal.
• Support protein intake—legumes, tofu, eggs, Greek yogurt, possibly dietary supplements.
• Eat regularly and with pleasure—avoid long fasting, learn to prepare simple and tasty meals.

It is also advisable to include light strength activity, which will help build muscle mass. And most importantly—work on your relationship with your body. Underweight is often associated with a negative self-image, with a feeling that one is "not good enough." Yet self-love and sensitivity to one's own body are key to healing.

"The body is not an aesthetic object, but a tool that carries us through life. It deserves care, not judgment," reminds psychologist and therapist Jana Malíková in an interview for Respekt magazine.

In a world where the ideal of beauty changes with each season, it is important to find your own balance. Whether someone is dealing with overweight or underweight, the first step always leads to understanding the connections—between the body, mind, and the environment in which we live.

Fortunately, today there are many tools available to monitor and manage your weight—from simple BMI calculations to underweight tests and nutritional analyses to professional help. Whether you start at the mirror, on the scale, or in your feelings—every step towards a healthy body is a step in the right direction.