What is Underweight for an 18 Year Old?

On the verge of adulthood, eighteen year olds look to the future with excitement and some anxiousness. This year in life brings many life altering events and decisions such as graduating high school, moving out of the home and choosing where to work or study. In order to confidently face these significant life adjustments, eighteen year olds should focus on their health as well. As a commonly used tool for assessing health, teenagers often consider their weight status. Some may wonder what is underweight for an eighteen-year old?

An underweight classification for an eighteen year old comes from using a BMI-for-age growth chart, which is based on age, sex, height, and weight. A teenager is classified as underweight when their BMI falls below the 5th percentile.

For example, an 18-year old girl at 64 inches tall (5 foot 4 inches or 162 cm) would be considered underweight below 102 pounds (46 kg). An 18-year old boy at 69 inches tall (5 foot 9 inches or 175 cm) would be considered underweight below 119 pounds (54 kg). You can use an online BMI calculator at this website: https://www.cdc.gov/healthyweight/bmi/calculator.html

This measurement is only one of the many tools used to determine health status of a teenager. BMI fails to account for many important factors such as body composition, genetics and other conditions. An underweight teenager will usually experience other malnutrition symptoms. Teenagers or parents with weight concerns should speak to a health care professional.

Continue reading to discover more information regarding underweight symptoms, causes and ways to gain weight.

How Do I Know if My 18-Year Old Teenager is Underweight?

You should check in with your child’s pediatrician or doctor at a yearly physical and check up where weight will be plotted on a growth chart. The healthcare professional should inform you of weight status, whether underweight, normal, overweight, or obese. Keep in mind that growth spurts affect everyone differently and sometimes a teenager gains weight in preparation to gain height. Growth trends are more important than a single point on a growth chart.

You can also use an online BMI calculator to estimate weight status. Using age, sex, weight, and height, you can use an online BMI calculator at this website:  https://www.cdc.gov/healthyweight/bmi/calculator.html

Using the CDC’s growth charts and BMI calculator, an 18-year old girl at 64 inches tall (162 cm) would be considered underweight below 102 pounds (46 kg). An 18-year old boy at 69 inches tall (175 cm) would be considered underweight below 119 pounds (54 kg).

Why is My 18-Year Old Underweight?

A below normal weight occurs for a variety of reasons. However, in most cases the low weight results from an energy imbalance where teenagers fail to meet energy needs. In other words, energy input does not match energy output, creating low energy availability. Without adequate energy, the body cannot grow or develop properly.

Energy Balance=Energy InputEnergy Output
Calorie intakeEnergy used for rest, physical activity, digestion, and other movement

The balance equation simplifies this concept. Energy balance = energy input – energy output. Energy input refers to calorie intake and energy output refers to resting metabolic rate (energy used at rest), physical activity (exercise, walking and other large movement), thermogenic effect of food (energy used to digest food) and non exercise activity thermogenesis (small movements like fidgeting). 

Disordered eating, dieting, picky eating, dietary restrictions, malabsorption and digestive pain can all contribute to insufficient energy intake. Elevated exercise levels, an unhealthy obsession with burning calories and medical conditions that increase energy use put a teenager at risk for excessive energy output. 

Any of these factors or a combination of them contribute to poor energy availability. When energy input falls below energy output, the body does not receive the energy it needs to grow and develop properly.

See also:

Should a Teenager Be Concerned About Being Underweight?

A medical expert will consider more than just one weight measurement to determine health status. However, a low weight that falls into the underweight classification is usually a sign of malnutrition or other health condition and should be addressed. A malnourished body cannot meet all of the demands of daily living. Instead it focuses available energy on survival, meaning that other areas of health suffer.

The following list describes some of the negative consequences an underweight teenager might face:

  • Vitamin and mineral deficiencies
  • Amenorrhea (lack of menstruation)
  • Low bone mineral density
  • Brittle skin, hair and nails 
  • Stunted growth and development
  • Muscle wasting
  • Irritability
  • Cold intolerance 
  • Cloudy mind
  • Impaired immune system
  • Weight loss 
  • Fatigue
  • Digestive issues
  • Decreased organ health

If your body isn’t getting the energy and nutrients it needs, it eventually won’t be able to function anymore. Chronic (long term) malnourishment may even lead to death as vital body systems shut down. 

Tips to Gain Weight for Underweight 18-Year Olds

Increasing calories through food and decreasing physical activity will adjust the energy equation to favor weight gain. Other lifestyle choices can contribute to weight goals as well such as getting adequate sleep, managing stress, addressing mental barriers and treating underlying health conditions. 


Underweight teenagers will want to heighten calorie intake in order to see weight gain. While this practice appears simple at first glance, mental and physical obstacles make gaining weight difficult. Many teenagers will find it helpful to meet with a registered dietitian, therapist or healthcare professional during this journey.

Some teenagers will obtain a majority of their calories from low nutrient dense foods in weight gain efforts. However, excessive amounts of low nutrient dense foods such as desserts, chips and soda will not translate to improved health, despite weight gain. An individual can still experience malnutrition with adequate calorie intake when foods lack important nutrients. Teenagers  should continue to focus on balance, variety and including nutrient dense foods to establish a health supportive and weight restorative eating pattern.

In particular, using more healthy fats at meals can increase calorie, vitamin, mineral and antioxidants content. These types of fats are found in plant oils, nuts, nut butter, seeds, avocado, olives and sea food.

Continue eating fruits and vegetables as they are an important source of nutrients and fiber, but ensure balance by leaving room for more calorie dense foods as well. Roasting vegetables with olive oil, melting cheese on cooked veggies, dipping fruit/vegetables in dips or sauces and adding nut butter to sliced fruit increases caloric content of favorite fruits and vegetables.

As the building block for the body, teenagers with weight gain goals should ensure they consume adequate protein. With meat, seafood and dairy foods, try to choose higher fat versions. Plant based protein such as nuts and seeds offer unsaturated, heart healthy fats as well. Other good protein rich foods include legumes (beans, soy, lentils), cottage cheese and eggs. 

Grains provide energizing carbohydrates and teenagers should always include carbohydrate rich foods in their day. Whole grains offer more nutrients than refined grains but both can find a place in a balanced eating pattern. Try to limit those with high amounts of sugar. 

Of course, don’t be afraid to try new foods, break previous food rules and enjoy snacks and treats. Balance means avoiding restriction and practicing moderation by allowing for pleasure with food.

For those who find it particularly difficult to gain weight, smoothies and other liquids make fast and easy calorie additions. Also, while it is best to focus on weight gain through food, supplements such as Ensure and Boost can help. Eating more frequently, with at least 3 meals and 2-3 snacks, allows for smaller and more manageable meal quantities. 

Finally, if the volume of food needed to gain weight feels overwhelming, look to add calories through power packing. Power packing means using higher calorie, but lower volume foods to increase the calorie or nutrition quality of meals. Examples include:

  • Cooking foods with extra oil
  • Melting cheese on eggs, vegetables, sandwiches
  • Nut butters with smoothies, fruit, baked goods
  • Chia, flax or hemp seed sprinkled on salads and hot cereal

Additional ideas are found at https://intermountainhealthcare.org/ckr-ext/Dcmnt?ncid=529719016

Physical Activity 

Some teenagers may find themselves unable to meet the energy demands of their current level of physical activity. Teenagers who reduce time and intensity of workouts can experience desired weight gain. Even switching from cardio to more weight bearing or flexibility focused movement can assist with weight gain. 

Others need to completely stop extra physical activity  to return to a healthy weight. Also, some vital signs and symptoms may not normalize, despite weight gain, without complete cessation of exercise. The unique circumstances of each teenager emphasizes the importance of seeking expert guidance. 

Professional Help

Weight issues generally coincide with other physical, mental and emotional concerns. Focusing solely on the calorie component of weight gain reduces the likelihood of lasting results. A holistic approach where the teenager receives necessary mental, emotional and physical assistance will set them up for success. Parents and teenagers should seek the support of licensed professionals in their weight gain journey.

Does Being Underweight Mean a Teenager Has an Eating Disorder?

As a common cause of weight abnormalities, teenagers and parents may want to consider the possibility of an eating disorder with an underweight BMI. On the other hand, some underweight individuals may not have an eating disorder. Other causes of a low weight could be an undiagnosed disease or medical condition, picky eating, disordered eating (not a full fledged eating disorder), exercise habits or genetics. 

Furthermore, eating disorders affect teenagers of all sizes, gender and race. A teenager in a larger body can be just as likely to have an eating disorder as one in a smaller body. An eating disorder is a serious concern. Instead of using only weight as a qualification to seek help, parents and teenagers should look for other concerning behaviors and symptoms. 

Signs of an eating disorder:

  • Significant weight fluctuations up or down
  • Rigid food rules
  • Secrecy around eating
  • Low energy
  • Insomnia or poor sleep
  • Brain fog
  • Increased irritability
  • Dry or brittle hair, skin and nails
  • Loss of menses (amenorrhea)
  • Restrictive mentality
  • Preoccupation with food
  • Purging (vomiting,exercise, laxatives)
  • Eating large amounts of food at one time

The National Eating Disorder Association (NEDA) offers a screening tool, helpline as well as other resources. https://www.nationaleatingdisorders.org.

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Katherine Harmer, RDN

I'm a Registered Dietitian Nutritionist with a love for coaching others to success in their health goals, especially teenage athletes. Tennis was my sport of choice in high school. Now I'm a little bit older, a little bit smarter, and a little bit worse at tennis.

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