In conversations about health, weight often takes centre stage, but mostly at one end of the scale. While obesity has long been seen as the major concern, being underweight can be equally, if not more, dangerous. Defined as having a Body Mass Index (BMI) below 18.5 kg/m², being underweight can signify a nutritional imbalance and lack of body reserves that can compromise nearly every system in the body. Being too thin may, in some cases, increase the risk of premature death more than being overweight. A 2025 Danish study by the European Association for the Study of Diabetes concluded that being too thin can be deadlier than being overweight. Hence, it’s time we shift focus from just ‘losing weight’ to maintaining balance.

Understanding Underweight

BMI, calculated as weight (kg) divided by height (m²), helps estimate whether your weight is in a healthy range.

• Underweight: < 18.5 kg/m²
• Normal weight: 18.5 to 24.9 kg/m²

While BMI has been shown to have limitations, a persistently low value often points to undernutrition, chronic illness, or malabsorption. Being underweight can be due to inadequate food intake, metabolic disorders, excessive exercise, or stress-related loss of appetite.

Health Risks of Being Underweight

1. Weak Immunity and Nutrient Deficiencies

Low weight often means low reserves of key nutrients such as protein, iron, zinc, and vitamins A and D. This weakens immunity, making the body prone to infections and slow recovery. Frequent colds, hair loss, and fatigue may all be early signs of nutritional deficiencies.

2. Bone Loss and Muscle Weakness

Being underweight increases the risk of osteoporosis and fractures due to reduced bone density. Low muscle mass also lowers strength and balance, raising the chance of injuries. Women with a BMI below 18.5 are at particularly high risk for bone-related conditions later in life.

3. Heart and Recovery Complications

Contrary to belief, underweight individuals are not immune to heart problems. A 2022 study, published in the Journal of the American Heart Association, shows that they may face a higher risk of heart failure due to poor nutrition and muscle loss. Additionally, those with low BMI tend to recover more slowly from surgeries and illnesses because of limited energy reserves.

4. Fertility and Hormonal Imbalance

A very low body weight can disrupt hormones that regulate menstruation and fertility. Women may experience irregular or missed periods, while men may face reduced testosterone levels and low sperm quality. For pregnant women, being underweight increases the chances of premature birth and low-birth-weight babies.

Why It Happens

The dangers of being underweight come from reduced nutrient stores, hormonal disruptions, and poor energy availability. When the body doesn’t get enough fuel, it begins to break down its own muscle for energy. Low fat levels also impair hormone production, impacting reproductive and bone health.

When to Seek Help

It’s important to get professional advice if:

• Your BMI is consistently below 18.5
• You experience unexplained weight loss
• You often feel tired or fall sick easily
• You have hair thinning, brittle nails, or irregular periods

Persistent underweight could be a symptom of deeper medical or psychological issues, from thyroid disorders to eating disorders, and requires a full assessment.

Healthy Weight Gain Tips

1. Focus on nutrient-dense foods

Include healthy fats (nuts, seeds, avocado), complex carbs (oats, whole grains), and lean proteins (eggs, paneer, fish). Avoid processed, sugar-heavy foods that add fat without nutrition.

2. Eat frequently

Small, balanced meals every 3 to 4 hours can help increase calorie intake without feeling overly full.

3. Add strength training

Building muscle improves metabolism and bone density. Aim for resistance exercises two to three times a week. Sign up for a virtual class today!

4. Track progress safely

Work with a registered dietitian to ensure gradual, sustainable gain, around 0.5 to 1 kg per month is ideal.

5. Check underlying causes

Conditions like hyperthyroidism, gut disorders, or nutrient malabsorption can prevent healthy weight gain and need medical attention.

Being underweight is not a badge of good health; it’s a warning sign. From reduced immunity to fertility and bone issues, it poses risks that can quietly erode long-term well-being. Maintaining a healthy BMI isn’t just about aesthetics; it’s about ensuring your body has enough fuel, nutrients, and strength to function optimally.

If your BMI falls below 18.5 or you’ve struggled with unintentional weight loss, don’t ignore the signs.