Underweight health risks: Causes, symptoms, and treatment

Could loss of weight mean something more?

There’s a world of difference between people who lose weight because they’re actively trying to become more healthy (congratulations!) and losing weight without meaning to.

The most common reason for this that I see in my practice is overactive thyroid gland. However, a host of other conditions, ranging from digestive problems such as Crohn’s disease and inflammatory bowel disease to more serious conditions like cancer, could be to blame.

If you have lost weight without meaning to, please don’t panic — cancer is not the most likely cause — but do see your GP to get it looked into.

Some women are naturally thinner than others. But certain health problems, certain medicines, or other serious problems can lead to chronic (long-term) underweight or sudden weight loss. These include:

  • Health problems that affect metabolism, such as overactive thyroid or diabetes
  • Health problems that affect the digestive system, such as celiac disease or Crohn’s disease
  • Other health problems, such as viral hepatitis, cancer, COPD, or Parkinson’s disease
  • A lack of appetite due to stress, illness, or substance use
  • Medicines that may cause nausea or lack of appetite
  • Eating disorders, such as anorexia
  • Over-exercising, such as for athletic training
  • Age. Underweight can especially be a problem for older women who may have loss of appetite, problems chewing, or a health problem.1″{amp}gt;1
  • Genes. Underweight can run in families.

Also, there may be a link between underweight in childhood and developing an eating disorder in adolescence.2″{amp}gt;2

Maybe. People who are underweight due to an eating disorder should not exercise unless their doctor tells them to. Physical activity is important for your health, muscle strength, balance, and flexibility.8″{amp}gt;

If you normally do high-intensity aerobic workouts, your doctor or nurse may talk to you about more moderate or less vigorous aerobic, strength training, and flexibility exercises.

Being underweight isn’t good for you. It could cause:

  • Nutritional deficiencies: if you’re underweight, it’s likely that you’re not consuming a healthy, balanced diet, which can lead to you lacking nutrients that your body needs to work properly. Calcium, for example, is important for the maintenance of strong and healthy bones. If you don’t get enough calcium, you risk developing osteoporosis (fragile bone disease) in later life. If you’re not consuming enough iron, you may develop anaemia, which can leave you feeling drained and tired.
  • Weakened immune system: your immune system isn’t 100% when you’re underweight, so you’re more likely to catch a cold, the flu or other infections.
  • Fertility problems: women who are underweight can find that their periods stop.

What’s the difference between underweight and malnourished?

Your body mass index, or BMI, is the relationship between your weight and your height. A BMI of 20-25 is ideal; 25-30 is overweight and over 30 is obese. If your BMI is under 18.5, you’re considered underweight. If your BMI is 18.5-20, you’re a bit underweight and can’t afford to lose more.

Some people naturally find it hard to put on weight. If your weight is constant and you have no long-term medical problems and a good diet, you probably don’t need to worry. If you’re malnourished, on the other hand, you definitely need to do something about it.

Scarily, over three million people in the UK are malnourished or at risk of malnutrition. Most of them are older and have long-term health problems — one in three people going into hospital or care homes have malnutrition.

You don’t have to be underweight to be malnourished — if, for instance, you’ve stopped eating healthily because of illness, you can lose weight and become malnourished even if your BMI isn’t in the underweight range.

You can use the body mass index (BMI) to find out whether your weight is in a healthy or unhealthy range. BMI is a tool to estimate body fat. Find your BMI by typing your height and weight into this BMI calculator from the Centers for Disease Control and Prevention.

Women with a BMI of less than 18.5 are considered underweight. The average woman’s height is 5 feet, 4 inches. If you weigh 107 pounds or less at this height, you are considered underweight with a BMI of 18.4. A healthy weight range for that woman would be 108 to 145 pounds.

BMI is just one way to measure healthy weight. Some women have a low, but still healthy weight. Talk to your doctor or nurse about what is a healthy weight for you.

In the United States, about 2% of women have underweight.3″{amp}gt;3

Underweight raises your risk for serious health problems. Some women have a low, but still healthy weight. But if you have experienced sudden weight loss or are not eating enough to keep your body working, you may develop serious health problems, including:

  • Problems with your menstrual cycle. A regular period is a sign of good health. Losing too much weight can cause periods to be less regular or stop completely. This can happen if your body fat drops so low that you stop ovulating, or releasing an egg from an ovary each month. This is especially true if you are losing weight because you are not eating enough or because you are exercising too much, which may be signs of an eating disorder like anorexia nervosa.
  • Problems getting pregnant. Problems with your menstrual cycle can make it harder to get pregnant, especially if your period stops completely. If you do not get a period, then you are probably not ovulating, or releasing an egg from an ovary each month.
  • Osteoporosis. Underweight increases your risk of osteoporosis later in life. Osteoporosis is a condition that causes bones to become weak and break easily.
  • Malnutrition. Malnutrition means your body is not getting enough vitamins and minerals to do what it needs to do. This can cause serious health problems, such as a weaker immune system and anemia. Anemia happens when your blood cannot carry enough oxygen to your body because of a lack of iron. If you have anemia, you may feel dizzy, lightheaded, weak, or tired.
  • Depression. Studies show that depression is more common in women who are underweight than women who are at a healthy weight.4,5″{amp}gt;4,5

Women who are underweight may also be more likely to die early than people of normal weight.6″{amp}gt;6 Women who are underweight earlier in adulthood may also experience menopause sooner than women who stayed a normal weight.7″{amp}gt;7

If you are worried about your weight, talk to your doctor or nurse.

Your weight can affect your health. Your weight can also make it harder to get pregnant. Talk to your doctor or nurse if you are underweight and have period problems or symptoms of malnutrition, such as extreme tiredness, headaches, or hair loss.

Your doctor or nurse will ask you questions and may do tests to learn more about what is causing your symptoms or weight loss. Your doctor or nurse may suggest making changes in your eating and exercise habits, depending on the cause of your weight loss.

For more information about underweight, call the OWH Helpline at 1-800-994-9662 or check out the following resources from other organizations:

If our healthy weight calculator has told you that you may be underweight, think about why this might be:

  • Have you felt unwell? There might be an underlying medical cause for your low weight, such as an overactive thyroid.
  • Have you been finding it difficult to make time to have a healthy, balanced diet with regular meals?
  • Have you lost your appetite, perhaps because you’re worried or stressed?
  • Have you been trying to lose weight?

Eating less and unintentional weight loss can occur in older people. But getting older doesn’t mean that losing weight is inevitable. Find out what to do if you are over 60 and underweight.

Eating disorders

If you feel anxious or worried when you think about food, or feel that stress or low self-esteem are affecting the way you eat, you may have an eating disorder.

If you think you may have an eating disorder, talk to someone you trust and consider speaking to your GP, because help is available.

If you’re concerned about someone else, find out how you can support them.

Exercise for good health

We all know exercise can help you lose weight, so you should avoid it if you’re underweight, right? Wrong! Regular exercise can counter the side effects of malnutrition and help you build muscle tone.

  • Weight-bearing exercise guards against thinning of the bones. Swimming is great gentle exercise for your heart and muscles, but doesn’t help your bones.
  • Yoga and Pilates can help you to tone up and keep supple without burning too many calories. They also help if you have joint problems like arthritis.
  • Many councils run courses for older or less fit people, so you can start gently.
  • Exercise improves balance and muscle strength. This cuts the risk of falls, which are a major problem in older patients and can lead to a loss of confidence and independence even if you don’t suffer a major injury.

With thanks to ‘My Weekly’ magazine where this article was originally published.

Things to consider

If you’ve been ill and not eating properly, or if you’re underweight, ask your doctor about screening you with a simple questionnaire called the MUST tool. In my practice, I often find relatives of elderly loved ones are the people who flag this up as a possible issue.

Possible problems include:

  • Getting more infections.
  • Taking longer to recover from illness.
  • Slow wound healing (eg, after surgery).
  • Thinning of the bones.
  • Irregular heart rhythms.
  • Higher risk of heart attack.
  • Lack of periods, and infertility.

Many people who are malnourished because of illness have simply been unable to eat enough calories. Here’s the good news! You may be able to take up all those unhealthy foods, like full-fat milk and cream, on doctor’s orders! Full-fat foods provide more calories in less volume.

  • Ask your doctor for a referral to a dietician to assess what you need.
  • Eat small quantities frequently, especially if you get full quickly.
  • Avoid filling up on fluids before a meal.
  • Don’t fill up on ’empty’ calories like sugary sweets — if you’re not eating much, getting enough vitamins and minerals is crucial.

If all a ‘food first’ approach doesn’t work, oral nutritional supplements (ONS) may be an option. These are carefully designed to provide the protein, vitamins and minerals you need in small volumes. They come in a variety of sweet and savoury flavours.

The National Institute for Health and Care Excellence (NICE) has recommended they should be considered for anyone who’s malnourished, as they may help recovery more quickly and avoid hospital admissions.

ONS are usually only available for people who have had a full assessment and have been found to be malnourished. If this is the case, you should be referred to a dietician who can advise on whether ONS would be appropriate for you. You may only need them for a few weeks.

Talk to your doctor or nurse about an eating plan that can help you gain weight in a healthy way. To gain weight in a healthy way, you should focus on getting enough nutrients — calories, vitamins, and minerals — for your body to work correctly.

To reach a healthy weight, you should eat foods from all of the food groups with a calorie amount that is healthy for your current weight. Your doctor or nurse can tell you how quickly to gain weight in a safe and healthy way. Gaining weight suddenly, or by eating a lot of sweet or fatty foods, is not healthy.

If you need to gain weight because of an eating disorder, work with your doctor or nurse to gain weight safely and treat the eating disorder. If the eating disorder is not treated, it may come back or continue and cause health problems.

Because many Americans are overweight, there are many resources geared toward losing weight. But some of these resources can also provide guidance for you to gain weight in a healthy way. See the list of resources at the end of this page to learn more.

If diet is the cause of your low weight, changing to a healthy, balanced diet that provides the right amount of calories for your age, height and how active you are can help you achieve a healthy weight.

Aim to gain weight gradually until you reach a healthy weight.

Try to avoid relying on high-calorie foods full of saturated fat and sugar – such as chocolate, cakes and sugary drinks – to gain weight.

These foods can increase body fat instead of lean body mass and increase your risk of developing high levels of cholesterol in your blood.

Instead, aim for regular meals and occasional snacks, and base your diet on the Eatwell Guide. This means:

  • Eating at least five portions of a variety of fruit and vegetables every day.
  • Basing meals on potatoes, bread, rice, pasta or other starchy carbohydrates. Choose wholegrain where possible.
  • Having some dairy or dairy alternatives (such as soya drinks and yoghurts). Have whole (full-fat) milk until you build your weight back up.
  • Eating some beans, pulses, fish, eggs, meat and other protein. Aim for two portions of fish every week – one of which should be oily, such as salmon or mackerel.
  • Choosing unsaturated oils and spreads, such as sunflower or rapeseed, and eating them in small amounts.
  • Drinking plenty of fluids. The government recommends 6-8 cups/glasses a day. But try not to have drinks just before meals to avoid feeling too full to eat.

If you’re having foods and drinks that are high in fat, salt and sugar, have these less often and in small amounts.

Try to choose a variety of different foods from the five main food groups. Learn more about these food groups and how they form part of a healthy diet.

However, it’s important to remember that the Eatwell Guide is aimed at the general population. For those who need more specialised nutrition advice, consult your GP or a registered dietitian.

If you don’t eat meat, find out how to have a healthy vegetarian diet.

If you’re trying to gain weight, eat foods that are not only healthy but also high in energy. Try the following:

  • For breakfast, porridge made with whole (full-fat) milk with chopped fruit or raisins sprinkled on top; or eggs on toast.
  • Milkshakes are a great snack (make them at home and take them to work or college). Fortify them with milk powder for extra protein and calories.
  • For a healthier lunch, try a jacket potato with baked beans or tuna on top, which contains energy-giving starchy carbohydrates and protein.
  • Peanut butter on toast makes a great high-energy snack.
  • Yoghurts and milky puddings, such as rice pudding, are high in energy. 
  • Unsalted nuts.

Although fruit and vegetable juices and smoothies count towards your 5 A Day, remember to limit these to no more than a combined total of 150ml a day.

Fat has a bad reputation for causing people to be overweight and obese. However, not all fat is bad. In fact, breaking down and storing energy (calories) as fat is good. It’s just one of the many ways the body uses food to function, heal, and grow.

Stored energy from fat helps you get through a strenuous job or workout. It plays a key role in brain development, and in preventing inflammation (swelling) and blood clots. Fat contributes to healthy hair and skin, as well.

You can determine whether you are underweight by using a Body Mass Index (BMI) calculator. This online tool considers your weight, height, age, and gender to calculate a score. If your BMI is less than 18.

Your doctor may put you on a weight gain program if you are underweight. However, this is not permission to go crazy with junk food. Healthy weight gain requires a balanced approach, just like a weight loss program.

Eating junk food may result in weight gain. However, it will not satisfy the nutrition your body needs. Even if the fat, sugar, and salt in junk food doesn’t result as extra weight, it can still harm your body. For a healthy weight gain, the following tips can help:

  • Add healthy calories. You don’t need to drastically change your diet. You can increase calories by adding nut or seed toppings, cheese, and healthy side dishes. Try almonds, sunflower seeds, fruit, or whole-grain, wheat toast.
  • Go nutrient dense. Instead of eating empty calories and junk food, eat foods that are rich in nutrients. Consider high-protein meats, which can help you to build muscle. Also, choose nutritious carbohydrates, such as brown rice and other whole grains. This helps ensure your body is receiving as much nourishment as possible, even if you’re dealing with a reduced appetite.
  • Snack away. Enjoy snacks that contain plenty of protein and healthy carbohydrates. Consider options like trail mix, protein bars or drinks, and crackers with hummus or peanut butter. Also, enjoy snacks that contain “good fats,” which are important for a healthy heart. Examples include nuts and avocados.
  • Eat mini-meals. If you’re struggling with a poor appetite, due to medical or emotional issues, eating large amounts of food may not seem appealing. Consider eating smaller meals throughout the day to increase your calorie intake.
  • Bulk up. While too much aerobic exercise will burn calories and work against your weight goal, strength training can help. This includes weightlifting or yoga. You gain weight by building muscle.

Before beginning a weight gain program, talk to your doctor. Being underweight may be due to an underlying health problem. It won’t be corrected by diet changes. Your doctor will be able to help you track your progress. He or she will make sure that healthy changes are taking place.

People who are underweight typically are not getting enough calories to fuel their bodies. Often, they are also suffering from malnutrition. Malnutrition means you are not taking in enough vitamins and minerals from your food. If you’re underweight, you may be at risk for the following health issues:

  • Delayed growth and development. This is especially true in children and teens, whose bodies need plenty of nutrients to grow and stay healthy.
  • Fragile bones. A deficiency in vitamin D and calcium, along with low body weight, can lead to weak bones and osteoporosis.
  • Weakened immune system. When you don’t get enough nutrients, your body cannot store energy. This makes it difficult to fight illness. It may also be difficult for your immune system to recover after being sick.
  • Anemia. This condition can be caused by not having enough of the vitamins iron, folate, and B12. This can cause dizziness, fatigue, and headaches.
  • Fertility issues. In women, low body weight can lead to irregular periods, lack of periods, and infertility.
  • Hair loss. Low body weight can cause hair to thin and fall out easily. It also can cause dry, thin skin and health issues with teeth and gums.

Many underweight people are physically healthy. Low body weight is due to a variety of causes, including:

  • Genetics. If you’ve been thin since high school and it runs in your family, it’s likely that you were born with a higher-than-usual metabolism. You also may have a naturally small appetite.
  • High physical activity. If you’re an athlete, you probably know that frequent workouts can affect your body weight. However, high physical activity also can be a part of an active job or an energetic personality. If you’re on your feet a lot, you may burn more calories than people who are more sedentary (inactive).
  • Illness. Being sick can affect your appetite and your body’s ability to use and store food. If you’ve recently lost a lot of weight without trying, it may be a sign of disease, such as thyroid problems, diabetes, digestive diseases, or even cancer. Talk to your doctor about sudden weight loss.
  • Medicines. Certain prescription medicines can cause nausea and weight loss. Some treatments, such as chemotherapy, can reduce appetite and worsen weight loss from illness.
  • Psychological issues. Our mental well being affects every part of our lives. Things like stress and depression can disrupt healthy eating habits. Severe body image fears and distortions can lead to eating disorders. If you’re suffering from damaging emotional issues, talk to your doctor. He or she can help you get the care, assistance, or counseling you may need.

Questions to ask your doctor

  • How do I gain weight if I’m not hungry?
  • Is being underweight more serious for babies?
  • What are some affordable, healthy foods to help me gain weight?
  • Should I stop exercising if I am underweight?
  • Should I stop taking my prescription medicine if I am underweight?


  1. Stajkovic, S., Aitken, E.M., Holroyd-Leduc, J. (2011). Unintentional weight loss in older adults. CMAJ; 183(4): 443-449.
  2. Cimino, S., Cerniglia, L., Almenara, C.A., Jezek, S., Erriu, M., Tambelli, R. (2016). Developmental trajectories of body mass index and emotional-behavioral functioning of underweight children: A longitudinal study.Scientific Reports; 6: 20211.
  3. Centers for Disease Control and Prevention (CDC) National Center for Health Statistics (NCHS). (2018). Prevalence of Underweight Among Adults Aged 20 and Over: United States, 1960–1962 Through 2015–2016. Hyattsville, MD.
  4. Mukherjee, S. (2013). Comparing Adult Males and Females in the United States to Examine the Association between Body Mass Index and Frequent Mental Distress: An Analysis of Data from BRFSS 2011Psychiatry Journal; 2013: 230928. doi:10.1155/2013/230928.
  5. de Wit, L.M., van Straten, A., van Herten, M., Penninx, B.W., Cuijpers, P. (2009). Depression and body mass index, a u-shaped association. BMC Public Health; 9:14. doi: 10.1186/1471-2458-9-14.
  6. Flegal, K.M., Graubard, B., Williamson, D., Gail, M.H. (2005). Excess Deaths Associated With Underweight, Overweight, and ObesityJAMA: The Journal of the American Medical Association; 293(15): 1861-1867.
  7. Szegda, K.L., Whitcomb, B.W., Purdue-Smithe, A.C., et al. (2017). Adult adiposity and risk of early menopauseHuman Reproduction.
  8. U.S. Department of Health and Human Services. (2018). Physical Activity Guidelines for Americans (PDF, 14.2 MB).

This content is provided by the Office on Women’s Health.

The MUST score

  1. What’s your BMI? (over 20 scores 0, 18.5-20 scores 1 and under 18.5 scores 2).
  2. Have you lost weight without meaning to in the last 3-6 months? (under 5% weight loss scores 0, 5-10% scores 1, over 10% scores 2).
  3. Have you been seriously unwell so that you haven’t eaten any proper food for at least five days? (This rarely happens unless you’re unwell enough to be in hospital, but scores 2 if the answer is yes.)
  4. If your total score from steps 1-3 is 0, you’re at low risk of malnutrition; if it’s 1, you’re at medium risk; 2 or more means high risk.
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