Underweight vs Overweight: Risks Health Insights And New Study

The Dangers of Underweight vs. Overweight: New Study Insights

BMI chart illustration showing health risks of underweight vs overweight categories

Being too thin may be deadlier than mild overweight, new study finds.

We often hear that overweight is a serious health risk—but recent research suggests that being underweight, or even being at the lower end of a “healthy” weight range, might be more dangerous than we thought. A large Danish study published in September 2025 shows that people who are thin may face higher mortality risks than those who are mildly overweight. Let's dive into what this means, what the study found, and how to think about weight, health, and longevity in a nuanced way.

Understanding BMI and Health: Basic Concepts

Body Mass Index (BMI) is a widely used measure to classify weight status. It is calculated by dividing a person's weight in kilograms by the square of height in meters (kg/m²). According to standard definitions:

Underweight: BMI < 18.5
Healthy (or “normal”) weight: BMI 18.5–24.9
Overweight: BMI 25–29.9
Obese: BMI ≥ 30

These categories are used globally, including by institutions like the World Health Organization. However, BMI has limitations: it does not account for fat distribution (e.g., visceral vs. subcutaneous), muscle mass, age, sex, or other health conditions.

What the New Danish Study Found

A study conducted in Denmark, involving about 85,761 participants (most of them older adults; median age ~66.4 years), tracked mortality over a five-year period. Researchers looked at different BMI categories and compared death rates, adjusting for sex, comorbidities, and education level. The study was presented at the European Association for the Study of Diabetes (EASD). According to the report via ScienceDaily, key findings include:

People who were **underweight** (BMI < 18.5) were about **2.73 times more likely** to die than those in the reference group (BMI between 22.5–<25 li="">
Even individuals in the lower healthy weight ranges (BMI 18.5 to <20) and mid-healthy ranges (20 to <22.5) showed **higher mortality risks** compared to the upper part of the normal weight range (22.5 to <25). For example, those with BMI 18.5–20 had about **twice the risk**, and those with BMI 20–22.5 about **27% higher risk**.
People in the overweight category (BMI 25 to <30), and even some in the lower obesity category (BMI 30 to <35), did **not** have a significantly higher risk of death compared to the reference group. This is sometimes called “metabolically healthy obesity” or the “fat but fit” phenomenon.
But severe obesity (e.g. BMI ≥ 40) did show significantly increased mortality risk (over twice as likely). Also BMI 35–<40 23="" about="" elevated="" higher="" li="" risk="" showed="">

Possible Explanations & Nuances

These findings challenge simplistic views of weight and health. Some of the complexities include:

Reverse Causation

Some individuals may lose weight because of underlying diseases (e.g. cancer, chronic conditions) before those conditions are diagnosed. In such cases, low weight is a symptom rather than a cause of elevated risk. Adjusting for this is difficult in observational studies.

Age, Sex, and Health Status

Since the Danish study participants were older (median ~66), results may differ in younger populations. Also, sex differences or other demographic variables (education, comorbidity) may influence associations.

Fat Distribution and Metabolic Health

Where fat is stored matters. Visceral fat (deep in the abdomen, around organs) is more metabolically active and harmful compared to fat stored in other areas (hips, thighs). Someone with a higher BMI but favorable body fat distribution and good metabolic markers (e.g. healthy blood pressure, glucose, lipids) might fare better than someone with “normal” BMI but poor metabolic health.

What We Know from Other Research

This new study aligns with, contradicts, or refines prior research. Some examples:

BMI and All-Cause Mortality Reviews: A large BMJ systematic review found that overweight and obesity typically associate with increased risk of death, but the lowest risk point (nadir) lies somewhere around BMI 23-24 among never smokers.
Metabolically Healthy Obesity Studies: Research has examined subpopulations who have obesity but maintain good metabolic health; these studies support that excess weight alone doesn’t always equal poor outcomes.

Implications for Individuals & Health Guidance

So what does all this mean for someone reading? Here are some practical takeaways:

Don’t assume lower weight is always better. Being underweight or losing weight unintentionally can signal risk. If you’re thin, but feeling unwell or losing weight, it’s worth checking in with a healthcare provider.
Focus on metabolic health: things like blood sugar, cholesterol, blood pressure, inflammatory markers, fitness, and physical activity are important, possibly more so than BMI alone.
Healthy weight targets may be individual. Age, body composition, fat distribution, genetics, lifestyle all matter. What is “ideal” for one person may differ for another.
Avoid stigma. Judging solely by weight can lead to harmful practices or unhealthy mindsets (e.g. extreme diets, or ignoring deeper signs of health). Mental health, self-esteem, and access to care are part of the equation.

Limitations & What We Still Don't Know

This study and similar ones leave some open questions:

Shorter follow-up period. Five years is helpful, but longer term studies might show different patterns.
Generalizability. Older age, certain populations, and specific health-care settings may not represent younger or more diverse groups.
Role of weight change. It’s unclear how losing or gaining weight over time influences risk (especially weight loss due to illness vs deliberate weight loss).
Other health behaviors. Diet quality, physical activity, sleep, stress, social determinants likely affect outcomes significantly but are harder to measure comprehensively.

How This Should Shape Public Health Messaging

Public health bodies and clinicians may need to adjust messages about weight, health, and risk in ways that:

Encourage a holistic view of health—beyond just weight or BMI.
Warn against both extremes: underweight and severe obesity each carry risk.
Promote metabolic health, fitness, and healthy body composition.
Personalize guidelines. One-size-fits-all recommendations may mislead or exclude people with different risk profiles.

In sum, the Danish study provides strong evidence that being underweight—or even being at the lower end of what is considered “healthy weight”—can carry serious mortality risk. Being mildly overweight or having moderate obesity, if coupled with good metabolic health, may not be as risky as previously assumed. That said, severe obesity still increases risk significantly. The bigger lesson: health is about more than the scale. It involves metabolic markers, fitness, lifestyle, fat distribution, and individual variation.

FAQ

Q: Is BMI a reliable indicator of individual health?

A: It can be useful as a rough guide, but BMI does not capture everything. It does not measure fat distribution, muscle vs fat, age, sex, or underlying health conditions. Other measurements (waist circumference, blood tests, fitness levels) often provide important additional information.

Q: Should everyone aim for a BMI between 22.5 and 25?

A: Not necessarily. While that range was used as the reference (lowest mortality) in the Danish study, ideal BMI may differ based on individual factors. What’s more important is achieving and maintaining metabolic health (good glucose control, healthy lipids, blood pressure, etc.) rather than strictly matching numbers.

Q: Does being overweight but fit really exist?

A: Evidence supports the concept of “metabolically healthy obesity,” which refers to individuals with higher BMI who nonetheless have good metabolic profiles. However, metabolic health can change over time, and being overweight can still carry risk over the long term if other unhealthy behaviors or conditions are present.

Q: What risks does being underweight bring?

A: Being underweight can be associated with malnutrition, weakened immune function, increased vulnerability to infection, loss of muscle mass, osteoporosis, and in many cases, being a signal of underlying disease. The Danish study found significantly higher mortality for underweight individuals.

Q: How should one interpret fat distribution vs weight?

A: Fat around the abdomen (visceral fat) is more harmful in many studies. It tends to impact metabolic processes, increasing risk for type 2 diabetes, heart disease, certain cancers. Fat stored elsewhere (e.g. hips, thighs) tends to carry less risk. Measurements like waist circumference may help capture this risk better than BMI alone.

Q: What steps can someone take to improve health beyond worrying about BMI?

A: Focus on balanced nutrition (adequate protein, micronutrients), maintain or build muscle strength, ensure regular physical activity, manage stress, get quality sleep, avoid smoking, monitor blood markers (cholesterol, blood sugar, blood pressure), and consult healthcare providers particularly if weight changes unexpectedly.

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