What does underweight mean?

Being underweight means that your weight is below the healthy weight range — the weight that research has shown you need to be if your body is to work properly. Being outside this range, whether overweight or underweight, can affect your immunity, fertility, mental wellbeing and other aspects of health.

How do I know if I am underweight?

A good measure of whether you are a healthy weight is the body mass index (BMI). This is calculated using your weight (in kilograms) and your height (in meters squared). For most adults, a healthy weight range is a BMI of 18.5kg/m2 to 24.9kg/m2. If your BMI is under 18.5kg/m2 then you would be considered underweight.

Note that BMI calculations do not apply in the same way to all people. For example, athletes, people of some ethnic backgrounds, older people and pregnant females should not rely on their BMI to calculate if they are underweight. If you think you might be in one of these groups, ask your doctor for advice.

Children’s weight is usually measured using growth charts, rather than BMI. Growth charts check how a child is growing against what would be expected, based on other children of the same sex and age.

What are the symptoms of being underweight?

If you are underweight you might find you get sick often, or feel very tired. That may be because you are not getting the right nutrients you need from your diet. You may also find your hair gets thinner or falls out, your skin gets very dry and your teeth are affected.

Children who are underweight may not grow as expected for their age.

Being underweight can lead to health problems including:

• osteoporosis
• infections, which are harder to fight off
• anaemia
• irregular periods in females, or menstruation may stop altogether
• pregnancy problems, such as difficulty getting pregnant or a higher chance of preterm labour
• heart problems

Being underweight can be especially dangerous for older people since it increases the risk of breaking a bone and becoming ill.

What causes being underweight?

You may become underweight for many different reasons. You may be born naturally small and your low BMI can be due to your genes. You may have a very high metabolism and find it hard to put on weight, even if you eat foods that have a lot of calories.

You may not follow a healthy, balanced diet because you forget to eat or cannot afford nutritious foods. You might find it hard to eat properly because you are sick, or your medicines make you feel nauseous. If you do a lot of physical activity you may burn up more calories than you can eat, leading to you being underweight.

There are also some physical conditions can cause weight loss, leading to underweight. These include:

• stomach problems, such as nausea, vomiting or diarrhoea
• an infection
• cancer, HIV or lung disease
• diabetes
• thyroid problems
• Crohn's disease

Mental health problems can also cause someone to be underweight. These include:

• anxiety and stress
• eating disorders such as bulimia and anorexia nervosa
• dementia

People who have been ill, in hospital or who are older are at an increased risk of being underweight.

What are the risks of being underweight?

If you are underweight, you may be at greater risk of certain health conditions, including malnutrition, osteoporosis, decreased muscle strength, hypothermia and lowered immunity. Underweight is also known to shorten people’s lives, due to early death.

Underweight females have less chance of becoming pregnant than those who are a healthy weight.

When should I see a doctor?

If your BMI is under the healthy weight range, you are losing weight without trying to lose weight or are having difficulty gaining weight, you should see your doctor.

If you are worried that you or someone you care for may have an eating disorder, talk to your doctor — it is important to seek treatment from a health professional.

How can I gain weight safely?

If you are underweight, it is important to eat a variety of foods that give you the nutrition you need. You should make sure you eat foods with enough energy to help you gain weight, protein to repair your body and build your muscles, and vitamins and minerals to keep your body cells functioning well.

The aim is to gain weight gradually by eating healthy foods. Even if you are underweight, try to avoid foods with a lot of added sugar, fat and salt, like cakes, takeaway foods and sugary drinks. The Australian dietary guidelines can help you learn about the amount and kinds of foods that you need to eat for health and wellbeing. By following the dietary patterns recommended in the guidelines, you will get enough of the nutrients essential for good health, and also help reduce your risk of long-term health problems.

You can put on weight by eating small meals frequently throughout the day. Try to snack on healthy, high-energy foods like cheese, nuts, milk-based smoothies and dried fruit.

More tips for gaining weight safely include:

• using full cream milk in coffee, tea or with meals
• using healthy fats like olive oil or avocado in your meals
• sprinkling some grated cheese on cooked food
• adding milk powder to soups, stews and drinks
• adding protein powder to milkshakes
• replacing black tea or coffee with milky drinks
• doing some light exercise to increase your appetite

If you have difficulty finding the time to prepare meals on a daily basis, it can be useful to have meals delivered or prepare a few meals in advance.

Resources and support

If you or someone you care for has an eating disorder, talk to your doctor — it is important to seek treatment from a health professional.

To find a dietitian near you, or after-hours medical service, use the healthdirect Service Finder tool.

The Butterfly Foundation can also provide information and support. Call them on 1800 33 4673, 8am – midnight AEST, 7 days a week, chat online or email them.

Call healthdirect on 1800 022 222 at any time to speak to a registered nurse (known as NURSE-ON-CALL in Victoria) for more information and advice.