Being underweight is just as serious a health concern as being overweight. In Canada, many people focus on losing weight — but having too little body weight can lead to serious problems, including weakened bones, a weakened immune system, and even fertility issues. This article explains what it means to be underweight, what causes it, and what you can do about it safely.

What Does Being Underweight Mean?

Doctors use a tool called the Body Mass Index (BMI) to measure whether your weight is healthy for your height. BMI is calculated by dividing your weight in kilograms by your height in metres squared (kg/m²).

Generally, a BMI below 18.5 is considered underweight. However, BMI is not perfect. For example, a competitive athlete may have a higher BMI without being overweight. Always talk to your family doctor to understand what your BMI means for you personally.

You can learn more about healthy weight ranges through Health Canada’s nutrition and healthy eating resources.

Common Causes of Being Underweight

There are many reasons a person may be underweight. Some causes are medical, while others are related to lifestyle or mental health. Understanding the root cause is the first step toward getting the right help.

Medical Conditions

Several health conditions can make it hard for the body to absorb nutrients or maintain a healthy weight. These include:

Eating disorders — such as anorexia nervosa or bulimia nervosa
Type 1 diabetes — the body cannot process sugars properly without enough insulin
Hyperthyroidism — an overactive thyroid speeds up metabolism
Celiac disease — the body cannot absorb nutrients from gluten-containing foods
Inflammatory bowel disease (IBD) — including Crohn’s disease and ulcerative colitis
Cancer and chemotherapy — both the disease and treatment can cause significant weight loss
HIV/AIDS — affects the immune system and nutrient absorption
Tuberculosis and other chronic infections
Chronic diarrhoea or intestinal parasites

Medications

Some medications reduce appetite as a side effect. These include certain stimulants, antidepressants, blood pressure medications, and osteoporosis treatments. In addition, aspirin and other anti-inflammatory drugs can cause stomach upset, which may reduce how much you eat.

Depression and anxiety can cause a person to lose interest in eating. Chronic stress can also interfere with appetite and digestion. These are real medical issues — not simply a matter of willpower.

Metabolic and Genetic Factors

Some people naturally have a faster metabolism, meaning their body burns calories more quickly. Others may have been born with fewer fat cells, making it harder to store energy. In these cases, a high-calorie diet and lifestyle changes can still help.

Other Contributing Causes

Poor eating habits or skipping meals
Intense physical training or competitive sport
Chronic sleep problems
Exposure to environmental pollutants
Dental or mouth problems that make eating painful

Health Risks Linked to Being Underweight

Being underweight puts significant strain on the body. Without enough nutrients, many systems begin to suffer. Here are the most common health risks.

Weakened Immune System

Your body needs adequate nutrition to fight off illness. People who are underweight tend to get sick more often and take longer to recover. They are more vulnerable to infections, colds, and other illnesses.

Bone Loss and Osteoporosis

Being underweight is closely linked to lower bone density. Over time, this can lead to osteoporosis — a condition where bones become fragile and break more easily. This is especially concerning for women after menopause, but it can affect anyone.

Anaemia

Anaemia happens when your blood does not carry enough oxygen. It is common in people who are underweight due to a lack of iron, vitamins, or protein in their diet. Symptoms include tiredness, shortness of breath, and chest discomfort.

Therefore, eating iron-rich foods like spinach, broccoli, and lean meat is important. You can find more detail on anaemia through the Mayo Clinic’s guide to anaemia and nutrition.

Fertility Problems

Being underweight can disrupt hormone levels in women. This may lead to irregular or absent menstrual periods — a condition called amenorrhoea. As a result, ovulation may stop altogether, making it difficult to become pregnant.

In many cases, reaching a healthy weight through a balanced diet can restore normal menstrual function and improve fertility.

Type 1 Diabetes

Sudden, unexplained weight loss is one of the early warning signs of Type 1 diabetes. In this condition, the body does not produce enough insulin to process sugar. Without insulin, the body begins breaking down fat and muscle for energy, which leads to weight loss.

Growth and Development Problems

In children and teenagers, being underweight can interfere with normal growth and development. It can also affect concentration, mood, and energy levels — making it harder to do well at school or stay active.

Symptoms to Watch For

Being underweight does not always feel obvious at first. However, there are signs that may suggest your body is not getting enough nutrition. These include:

Constant tiredness or low energy
Getting sick frequently
Feeling cold more often than others
Hair loss or brittle nails
Irregular or missed periods
Difficulty concentrating
Irritability or mood changes
Noticeable or rapid weight loss

Furthermore, some people feel drowsy or mentally foggy when they are not eating enough. These symptoms can affect daily life, work, and relationships.

How to Gain Weight Safely

If you are underweight, the goal is not simply to eat more junk food. The focus should be on adding nutrient-dense calories that support your body’s needs. Here are some practical, evidence-based tips.

Eat More Frequently

Instead of three large meals, try eating five or six smaller meals throughout the day. This approach is often easier on digestion and helps increase total calorie intake. Add healthy snacks like nuts, seeds, nut butter, and cheese between meals.

Choose Nutrient-Rich Foods

Focus on foods that are high in healthy calories and nutrients. Good choices include:

Whole grains — oats, brown rice, whole grain bread
Healthy fats — avocado, olive oil, nuts, seeds
Lean proteins — eggs, chicken, fish, legumes, dairy
Fruits and vegetables — especially raw or lightly cooked to preserve enzymes

Add Fibre and Support Gut Health

A diet rich in fibre supports healthy digestion. Good sources of fibre include vegetables, fruits, whole grains, and legumes. In addition, eating yoghurt with live bacterial cultures supports healthy gut bacteria, which improves nutrient absorption.

Omega-3 fatty acids — found in fatty fish, flaxseeds, and walnuts — also help support digestion and reduce inflammation in the gut.

Consider Vitamins and Minerals

A daily multivitamin may help fill nutritional gaps, especially if your diet has been poor for some time. However, supplements should complement a healthy diet — not replace it. Talk to your family doctor or pharmacist before starting any supplement.

Strength Training

Light resistance exercise, such as weight training, can help build healthy muscle mass rather than just adding fat. This approach supports overall health and improves body composition. Always start slowly and consider working with a certified fitness professional.

For more guidance on healthy weight gain, Healthline’s evidence-based nutrition resources offer practical advice backed by research.

When to See a Doctor

If you are concerned about being underweight, do not wait. Speak with your family doctor as a first step. They can run blood tests, check your BMI, and help identify any underlying medical conditions.

If you do not have a family doctor, visit a walk-in clinic. Most provincial health plans in Canada cover assessments related to nutrition and unexplained weight loss. Your doctor may also refer you to a registered dietitian — a service often covered under provincial programmes.

Seek care sooner rather than later if you notice rapid weight loss, missed periods, extreme fatigue, or signs of an eating disorder. Early treatment leads to much better outcomes. As always, please consult your healthcare provider before making significant changes to your diet or lifestyle — this article is for informational purposes only and does not replace professional medical advice.

Frequently Asked Questions

What is considered being underweight in Canada?

In Canada, being underweight is generally defined as having a BMI below 18.5. However, BMI is just one tool — your family doctor will consider your full health picture, including age, muscle mass, and medical history, before making any assessment.

What are the dangers of being underweight?

Being underweight can lead to serious health problems, including anaemia, osteoporosis, weakened immunity, fertility issues, and poor concentration. In severe cases, long-term malnutrition can be life-threatening. Getting medical advice early is important.

How can I gain weight if I am underweight?

The safest way to gain weight when underweight is to eat more nutrient-rich foods, increase meal frequency, and add healthy fats and proteins to your diet. A registered dietitian can create a personalised plan — ask your family doctor for a referral.

Can being underweight affect my menstrual cycle?

Yes. Being underweight can disrupt hormone levels, leading to irregular or absent periods — known as amenorrhoea. In many cases, reaching a healthier weight through improved nutrition can help restore a regular cycle and improve fertility.

Is being underweight a sign of an eating disorder?

Being underweight can sometimes be linked to eating disorders such as anorexia nervosa or bulimia nervosa, but it is not always the cause. There are many medical, genetic, and lifestyle reasons a person may be underweight. A doctor can help identify the cause.

Should I take supplements if I am underweight?

Supplements like multivitamins, Omega-3 fatty acids, and minerals may help if your diet is lacking in key nutrients. However, supplements should support a healthy diet — not replace it. Always speak with your doctor or pharmacist before starting any new supplement.

Key Takeaways

Being underweight is a serious health concern, not just a cosmetic issue.
A BMI below 18.5 is generally considered underweight, but your doctor can give you a full picture.
Common causes include eating disorders, thyroid problems, diabetes, digestive conditions, and psychological factors like anxiety or depression.
Health risks include anaemia, osteoporosis, weakened immunity, fertility problems, and growth issues.
Safe weight gain focuses on nutrient-dense foods, increased meal frequency, gut health, and gentle strength training.
Talk to your family doctor or visit a walk-in clinic if you are concerned — most provincial health plans cover nutrition-related assessments.
Early action leads to better outcomes. Do not ignore the signs.