There’s a lot of focus in the medical world on the health effects of being overweight, but what about the effects of being underweight? There are certain health risks associated with being underweight or having poor nutrition.

These risks include:

  • malnutrition, vitamin deficiencies, or anemia
  • osteoporosis from too little vitamin D and calcium
  • decreased immune function
  • increased risk for complications from surgery
  • fertility issues caused by irregular menstrual cycles
  • growth and development issues, especially in children and teenagers

Keep reading to learn more about these risks of being underweight, plus how to identify if you are underweight, what symptoms you may experience, and how you can find help.

Your body mass index (BMI) can help you and your doctor determine if you’re underweight. BMI is an estimate of your body fat based on your height and weight.

BMI rangeWeight status
below 18.5underweight
30 or aboveobese

There are some limitations to determining your health using BMI alone.

  • Athletes can have muscular builds. Since muscle weighs more than fat, BMI may overestimate body fat for these individuals.
  • Older adults may have lost muscle. In this case, BMI may underestimate body fat.

If you’re underweight, you may not be eating enough healthy foods with key nutrients to fuel your body. That can cause malnutrition. Over time, malnutrition can affect your health in a number of different ways that may be noticeable to you or those around you.

Your symptoms might include:

  • feeling tired or drained of energy
  • getting sick often or having trouble fighting off illness
  • having irregular or skipped periods in females
  • experiencing hair thinning or loss, dry skin, or teeth issues

A study from Japan compared dietary habits of underweight women with a desire to be thin vs. underweight women without this desire. They found that the underweight women with a desire to be thin had less healthy eating habits than underweight women who did not have this desire.

If you’re underweight, you may be more likely to also be malnourished if your low BMI is caused by an unbalanced diet or an underlying disease that affects nutrient absorption. Malnutrition can also lead to anemia or a deficiency in essential vitamins. Anemia can also be caused by malabsorption of nutrients.

A recent review of studies found a connection between increased infections and being underweight. The researchers noted their difficulty in determining if this is a result of being underweight or if it has more to do the underlying causes for being underweight. For example, malnourishment may lead to decreased immune function and also cause people to be underweight. More research is needed to fully understand the connection between weight and immune function.

One study found evidence that underweight people who had total knee replacement surgery were more likely to develop infections following the surgery than people who were not underweight. While they could not determine the reasons for this, they believe underweight people aren’t able to heal wounds as well as people with a normal BMI. They also found that the underweight group had low preoperative hemoglobin. While more research is needed, the findings suggest that being underweight can affect your ability to heal wounds.

Another study found increased complications in underweight people who had total hip replacement surgery compared with people of normal weight. Complications following coronary bypass surgery and lung transplants also seem to be higher for people who are underweight. Researchers have also linked low BMI to increased incidences of postoperative deaths within the first year following a lower extremity bypass surgery.

Low body weight may increase your risk for low bone mineral density (BMD) and osteoporosis. One study looked at BMD in 1,767 premenopausal women, and found that 24 percent of women with a BMI of 18.5 or lower had a low BMD. Only 9.4 percent of participants with a BMI higher than 18.5 had low BMD. The study results suggest that being underweight increases risk for osteoporosis.

Women with low BMIs are at increased risk for amenorrhea, which is an absence of menses, and other menstrual cycle dysfunctions. Irregular or missed menstrual cycles may be an indicator of anovulation, or that you aren’t ovulating. Chronic anovulation may cause infertility.

If you’re trying to conceive and are underweight, talk to your doctor. They can do a simple blood test to see if you’re regularly ovulating. They can also test for other signs of infertility.

Your doctor may recommend reaching a healthy weight before becoming pregnant. Being underweight while pregnant can pose risks for your baby. That’s why it’s important to maintain a healthy weight during pregnancy.

Learn more: Maintaining a healthy pregnancy »

Developmental delays can be seen in underweight children, especially children under the age of 3 when the brain is rapidly developing. The brain needs nutrients to properly develop. Underweight children may be missing key nutrients due to malnutrition and malabsorption. That can impact the development of the brain and lead to delays in developmental milestones.

Your child’s pediatrician will chart your child’s growth at well-visit appointments. They will use these measurements to see how your child compares with average growth for other children their age, and how your child’s percentages change over time. If your child’s growth percentage decreases, that can be a warning sign that they are not gaining weight at the expected rate. For example, if your child is in the 45th percentile at their 12-month appointment and in the 35th percentile at their 15-month appointment, their doctor may be concerned about their weight gain.

Your child’s pediatrician will also ask about developmental milestones during regular visits. Remember that not all children hit milestones at the same time. Instead, doctors look to see if your child is hitting them within a certain time range. For example, some children take their first steps when they are under a year old, whereas others don’t start walking until they are several months into their first year. Learning to walk or talk later won’t signal a problem unless your child is also late with other milestones.

If you suspect you’re underweight, make an appointment with your primary care doctor or a dietitian. Your doctor can examine your medical history and help identify any issues that may be leading to poor nutrition or weight loss.

Before your appointment, you may want to ask yourself:

  • Have I felt sick lately? What other symptoms have I experienced?
  • Am I skipping any meals or eating mostly small snacks?
  • Have I been stressed or depressed, making me lose my appetite?
  • Am I currently trying to lose weight?
  • Does not eating give me better feelings of control?

Share the answers to these questions with your doctor. If your doctor rules out any serious underlying medical issues, you may then identify a goal weight. From there, you may come up with a plan to help you reach that weight through healthy eating and other appropriate treatments.

With help from your doctor, you may be able to attain a normal BMI through lifestyle changes and healthy eating. Your doctor can also help you navigate solutions for limited access to nutrient dense foods, psychological issues, underlying health conditions, medication side effects, and other situations that contribute to being underweight or malnourished.

By making a few tweaks to your diet and lifestyle, you can gain healthy weight and avoid the negative health effects of being underweight.

  • Try eating smaller, more frequent meals. Add more snacks into your routine as well.
  • Stick with foods that are rich in nutrients, like whole grains, fruits, vegetables, dairy products, nuts and seeds, and lean proteins.
  • Pay more attention to what and when you’re drinking. Smoothies are a better choice than diet soda, coffee, and other beverages. You can fill them with fruits, vegetables, and dairy products.
  • If drinks decrease your appetite, consider saving them for 30 minutes after you eat a meal.
  • Get more calories in your meals by adding things like cheese, nuts, and seeds as toppings to main dishes.
  • Start exercising. You can gain weight by adding muscle to your body. Working out may also help to stimulate your appetite.