What Does It Mean to Be Underweight?

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For years, society has discussed weight and its relation to individual health. Stigmas have been created, beauty standards impacted, and opinions shaped based on the information we've all received. But what is a healthy weight? Though the messaging about weight and health is starting to shift, you may only associate having extra body weight with poor health. That said, being underweight can negatively affect your health, too.

You’re not alone if it’s unclear to you what a healthy weight means, especially in a society where being thin is considered ideal. But it's important to understand that you can't see a healthy weight. In fact, a true healthy weight varies from person-to-person and depends on a variety of factors. 

Defining Weight

How do you know if you’re at a healthy weight? Many healthcare professionals use mathematical equations like the body mass index (BMI) as a tool.

The BMI assesses weight status by comparing your weight to your height. The calculation estimates body fatness, which health professionals use as a screening tool to determine disease risk. However, the BMI isn’t meant to serve as a diagnostic tool for body fatness or a singular measure of overall health.

A BMI that falls between 18.5 and 24.9 is considered a normal or healthy weight range, while a BMI less than 18.5 falls within the underweight range.

BMI is a dated and biased measuring tool for weight and health that doesn’t account for several factors that influence both, such as body composition, ethnicity, race, gender, and age.

Despite being a flawed measure, BMI is widely used in the medical community because it’s an inexpensive and quick method for analyzing potential health status and outcomes.

Measuring your body composition may provide a more accurate picture of your weight and health. Body composition compares how much of your total weight comes from lean body mass and how much from fat. The percentage that comes from body fat is used to assess health.

  • Healthy body fat percentage for men: 17.6 to 25.3%
  • Healthy body fat percentage for women: 28.8 to 35.7%

Men with less than 17.6% body fat and women with less than 28.8% body fat are below normal, or underweight, according to a 2018 study published in the Journal of Exercise Rehabilitation.

Fit individuals and athletes, however, may have lower body fat percentages and be at a healthy weight.

Health Risks of Being Underweight

When it comes to weight and risk of health problems, we tend to focus on issues associated with being overweight. But being underweight can be just as detrimental to your health.

Vitamin deficiencies, osteoporosis, lowered immune function, and infertility are some of the health risks you may face as an underweight individual.

Vitamin Deficiencies

Vitamins and minerals are essential nutrients your body needs to function properly. When you’re underweight, you may not get enough energy and nutrients to maintain a healthy weight. This means you may not get an adequate supply of vitamins and minerals.

You’re more likely to have vitamin deficiencies if you’re underweight, according to a 2017 report published in Nutrients. Not having an adequate supply of essential nutrients affects how your body functions. 

These nutritional deficiencies may cause health problems like anemia from lack of iron, folate, or vitamin B12; poor wound healing from deficiencies in vitamin C; or night blindness because of inadequate intake of vitamin A.

Osteoporosis

Osteoporosis is a serious bone disease that causes porous, brittle bones that are more susceptible to fractures. Anyone can develop osteoporosis. However, poor nutrition increases your risk.

You need an adequate supply of calcium and vitamin D to build bone mass during childhood, and maintain bone mass as an adult. If you’re underweight and not meeting your nutrient needs, it’s likely you’re not getting enough calcium or vitamin D. In fact, vitamin D is a nutrient many underweight individuals fail to get enough of.

Decreased Immune Function

Your immune system helps you fight off infections and diseases. Though no single food or supplement can boost your immune health, you need to eat a balanced diet filled with an adequate amount of essential nutrients in order for your immune system to function at its best.

If you’re underweight because you’re not eating enough, you may not get all the nutrients needed for a strong immune system. This may decrease immune function and put you at risk of catching every cold that circles your workplace.

Fertility Obstacles

Your body needs a certain amount of fat to perform essential functions, and there’s a correlation between low body fat and being underweight. Though most people think having too much fat is bad for health, not having enough isn't good either.

If you’re underweight and have a low body fat percentage, you may have problems with fertility. Not having enough body fat affects hormone production, leading to fertility problems in women and men.

What to Do if You’re Underweight

If you’re underweight and looking for a solution to improve your weight, consult with a primary care provider. They can accurately assess your weight and how it might affect your health, and provide guidelines that fit your unique needs.

Though nutrition affects weight, there are other factors that influence your body size such as genetics, physical activity, and medication. Ask a medical professional about these factors and how they may apply to you.

You should also ask a doctor what you can do to gain weight and the correct exercise regimen for your situation.

Weight Bias

Though thinness is often considered “ideal,” there is weight bias towards smaller individuals. Many people feel that it's acceptable to make comments about someone being too thin and how they need to eat more to gain weight. However, health and weight aren’t synonymous.

Being small, or even underweight according to the numbers, doesn’t mean you need to gain weight. A healthy weight isn’t necessarily “seen” and not a sign that something is wrong or that a person has an eating disorder. 

Tips for Healthy Weight Gain

For some individuals, gaining weight is as hard as losing weight. Eating foods high in calories may pack on a few pounds, but it won’t always provide your body with the nutrition it needs. For healthy weight gain, you need to eat a balanced diet filled with a variety of nutrient-rich foods.

Some tips for healthy weight gain include:

  • Eating five to six small meals a day
  • Drinking nutrient-dense beverages in between your meals (milk, juice, smoothies)
  • Snacking on nuts, seeds, dried fruit, or cheese
  • Topping salads and soups with grated cheese, nuts, or beans
  • Spreading nut butters on bread, crackers, or muffins
  • Mixing in nonfat dried milk powder into cereal, yogurt, or mashed potatoes
  • Adding strength-training to your exercise routine

When gaining weight, you want to add a healthy mix of both muscle and body fat. Strength-training can help increase muscle mass.

Make sure you get enough protein in your diet when working out to gain more muscle mass. Meat, poultry, seafood, beans, nuts, seeds, and dairy products are good sources of protein. Include a protein-rich food at each meal to meet your needs. 

A review study published in 2018 in The British Journal of Sports Medicine recommends aiming for 0.7 grams of protein per pound of body weight when working out to gain weight. So, if you weigh 120 pounds, you need 84 grams of protein a day.

Schedule regular follow-ups with a health care provider or request a referral to a registered dietitian for guidance and support. 

A Word From Verywell

Your weight isn’t always the best indicator of health. A healthy weight varies from person-to-person and isn’t something you can really “see.” Being underweight because you're not eating enough affects your health now and in the future.

If you have concerns about your weight and how it’s affecting your health, schedule a consultation with a primary care provider. 

Frequently Asked Questions

  • What does being underweight feel like?

    You may feel tired or find yourself getting sick more often when you’re underweight. These symptoms often occur because of an inadequate supply of energy, vitamins, and minerals.  


    Not eating a balanced diet to support a healthy weight may also affect your hair and skin. Your hair may thin and your skin may dry and crack.

  • How long does it take to gain weight?

    How long it takes you to gain weight depends on many factors, including your nutritional needs, exercise routine, and genetics. When strength-training and eating a nutrient-rich diet with an adequate supply of energy and protein, you may gain about 2 pounds a month.

  • What is malnutrition?

    According to the World Health Organization (WHO), malnutrition is when a person fails to get an adequate amount of energy and nutrients. This includes deficiencies and excesses. So, not getting enough nutrients or getting too much.


    The WHO further classifies malnutrition into subgroups that includes undernutrition; micronutrient; malnutrition; and overweight, obesity, and diet-related diseases like type 2 diabetes and heart disease.


    Being underweight may put you at risk of undernutrition or micronutrient malnutrition.

16 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Assessing your weight.

  2. Branco BHM, Bernuci MP, Marques DC, et al. Proposal of a normative table for body fat percentages of Brazilian young adults through bioimpedanciometry. J Exerc Rehabil. 2018;14(6):974-979. doi:10.12965/jer.1836400.200

  3. American Council on Exercise. Percent Body Fat Calculator: Skinfold Method.


  4. Centers for Disease Control and Prevention. Assessing Your Weight

  5. Bird JK, Murphy RA, Ciappio ED, McBurney MI. Risk of deficiency in multiple concurrent micronutrients in children and adults in the United States. Nutrients. 2017;9(7):655. doi:10.3390/nu9070655

  6. National Institutes of Health. NIH Osteoporosis and Related Bone Diseases National Resource Center. Osteoporosis overview.

  7. Klemm S. The Academy of Nutrition and Dietetics. Support your health with nutrition.

  8. Dobner J, Kaser S. Body mass index and the risk of infection - from underweight to obesity. Clin Microbiol Infect. 2018;24(1):24-28. doi:10.1016/j.cmi.2017.02.013

  9. Boutari C, Pappas PD, Mintziori G, et al. The effect of underweight on female and male reproduction. Metabolism. 2020;107:154229. doi:10.1016/j.metabol.2020.154229

  10. American Academy of Family Physicians. Healthy ways to gain weight if you’re underweight.

  11. Alberga AS, Russell-Mayhew S, von Ranson KM. et al. Weight bias: a call to action. J Eat Disord 4, 34 (2016). doi.10.1186/s40337-016-0112-4

  12. Ellis E. The Academy of Nutrition and Dietetics. Healthy weight gain.

  13. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults [published correction appears in Br J Sports Med. 2020 Oct;54(19):e7]. Br J Sports Med. 2018;52(6):376-384. doi:10.1136/bjsports-2017-097608

  14. Health Direct. What to do if you’re underweight.

  15. Slater GJ, Dieter BP, Marsh DJ, Helms ER, Shaw G, Iraki J. Is an energy surplus required to maximize skeletal muscle hypertrophy associated with resistance training. Front Nutr. 2019;6:131.doi:10.3389/fnut.2019.00131

  16. World Health Organization. Malnutrition.

By Jill Corleone, RD
Jill is a registered dietitian who's been learning and writing about nutrition for more than 20 years.