What is Underweight For a 16 Year Old?

At sixteen, teenagers make choices that affect the rest of their lives. In regards to physical health, the teen years mark an important time for building strong bones, maturing sexually, developing full stature and establishing strong organs and body systems. A low body weight can signal that a teenager is not meeting the energy demands of these processes, which impacts current and future health. With these concerns in mind, what is considered underweight for a sixteen year old?

While not a perfect indicator, a BMI that falls below the 5th percentile on a BMI-for-age growth chart is categorized as underweight. For a 16-year old girl at a height of 64 inches, underweight is anything lower than 98 pounds (lbs) (44.5 kg). For a 16-year old boy at a height of 68 inches, underweight is anything lower than 113 lbs (51 kg). Other heights and ages would be classified differently. The CDC provides a free BMI calculator at this website for an estimate: https://www.cdc.gov/healthyweight/bmi/calculator.html

These specific weight numbers only refer to sixteen year olds of the listed heights and genders. Teenagers of different statures will have different cut off weights for the BMI percentile categories. Furthermore, BMI lacks important information that affects weight readings such as body composition, genetics, medical conditions, and other contributing factors. Health experts use other tools to provide a more complete assessment of health. Those with concerns about weight should reach out to a healthcare professional.  

Continue reading for more information about consequences of being underweight and how to gain necessary weight. 

Why Would a Sixteen Year Old be Underweight? 

Poor energy availability leads to a low weight. Weight gain and growth require adequate energy. Therefore, understanding the energy balance equation will provide insight into the cause and recovery for underweight individuals. 

The energy balance equation is energy availability equals energy input minus energy output. When energy input is less than energy output, the body does not have the energy required for normal growth or function.

Energy input mainly comes through consuming adequate calories. Teenagers who skip meals, eliminate food groups, follow rigid food rules, show disordered eating behaviors, have picky eating tendencies or with malabsorption conditions are at risk of poor nutrition.   

Energy output is made up of physical activity, resting metabolic rate, thermic effect of food and non exercise activity thermogenesis (NEAT-energy of small movements, like fidgeting). Teenagers who participate in large amounts of physical activity, under-consume per amount of exercise or have a medical condition that increases metabolic rate may experience a higher energy output than what their input meets, causing poor growth.

The energy balance equation is just a tool and does not account for all the contributors to weight status. Genetics, ethnicity, socioeconomic status, stress, medications, sleep habits and environment also factor into how much a person weighs. 

What Consequences Come From Being Underweight?

An unhealthy low weight signals that the body is not receiving what it needs to function and develop properly. Usually in consequence of one of the before mentioned causes. A malnourished state leads to various acute and long term negative consequences.

Vitamin and Mineral Deficiencies

Low weight from malabsorption or inadequate intake results in vitamin and mineral deficiencies. These deficiencies bring on a host of additional health related issues.


A lack of a period/menstruation results from decreased hormone levels. An imbalance in these hormones affects the bones, heart and fertility of teenagers. The sides that it is normal for athletes to not have a period is false and those suffering from amenorrhea should seek professional help.

Low Bone Mineral Density

Most bone growth occurs during the teenage years. Missing this time window for building bones can significantly harm bone health. Possible poor calorie and calcium intake along with hormone disruption puts underweight teenagers at risk for weak bones and osteoporosis as they age. 

Brittle Skin, Hair, and Nails 

Underweight teenagers may notice dry and weak skin, nails and hair due to lack of protein, nutrients and energy. The body conserves nutrients for vital organs and functions.

Stunted Growth and Development

Without needed calories and nutrients, the body cannot support full growth and development.

Muscle Wasting

Malnourishment leads to a catabolic state where the body turns to alternative forms of energy to function. This adaption results in breakdown of muscle and fat storage for use as energy.


Hunger and fatigue make it hard to feel content. Under-nourished teenagers will anger quickly and feel little desire to participate in other activities.

Cold Intolerance 

Lack of body fat makes it difficult for underweight individuals to tolerate the cold.

Cloudy Mind

The brain does not receive the nutrients it needs and malnutrition creates an inability to focus or think clearly. 

Impaired Immune System

Teenagers with poor nutrition experience increased sickness and injury as their immune system shuts down.

Digestive Issues

Without adequate energy, digestive organs atrophy causing digestive problems and bloating.

Decreased Organ Health

The lack of available energy affects all organs, including the heart and brain. Untreated low energy availability can eventually result in death as vital organs fail.

Can a Teenager at a Healthy Weight Be Malnourished?

Underweight is a symptom, not a cause of malnutrition. However, poor nutrition occurs at any weight. Any teenager participating in dieting, food restriction or large amounts of exercise may find themselves experiencing the negative heath consequences of low energy availability.

Weight loss during the teenage years should only occur under the direction of a healthcare professional. Weight loss comes from creating an energy deficit. This energy imbalance during these important years of growth and development can incur severe and lasting health consequences.

How Can a Teenager Gain Weight the Healthy Weight?

A teenager hoping to gain weight will need to adjust either energy input, output or both.  Weight gain often calls for increasing calories, eating more balanced meals and participating in less physical activity. Counseling helps with underlying causes such as disordered eating, stress or an unhealthy relationship with exercise or food. Those with unexplained weight loss should talk to a healthcare professional to determine any medical conditions.


A great way to increase calories comes through power packing meals and snacks. This technique means using higher calorie, but lower volume foods to increase the calorie or nutrition quality of meals. 

Examples of Power Packing Include:

  • Adding nut butter to oatmeal or smoothies
  • Cooking food choices in plant oils
  • Sprinkling flax, chia, or hemp seeds on cereals and salads
  • Spreading avocado on sandwiches
  • Melting cheese on vegetables
  • Use sauces and dressings with meals
  • Consuming full fat versions of foods (whole milk, whole yogurt)

Discover more ways to power pack at https://intermountainhealthcare.org/ckr-ext/Dcmnt?ncid=529719016

Finding ways to improve nutrition through food will provide the best form of nourishment. However, some teenagers may find it necessary to consume oral supplements such as Ensure or Boost. Talk to a doctor or registered dietitian to see what approach works best for your unique needs.

Try to add in more nutrient dense foods rather than poor nutrient quality food. Nutrient dense foods will provide more vitamins, minerals, antioxidants and other beneficial components to promote long lasting health. These foods include whole grains, fruit, vegetables, seafood, legumes, meat, nuts, seeds, eggs, unsweetened full-fat dairy, plant oils, avocado, olives.

Lower nutrient dense foods can help with weight gain, but do not provide a good source of essential nutrients. Filling up on these foods will not meet full nutrition needs. Continue to enjoy these types of foods in moderation or on special occasions. These foods include fried food, sweets, baked goods, chips, sweetened beverages, sugar cereal and ultra processed foods.

A teenager eating more food than they are used to, may experience uncomfortable GI issues. Eating more frequently with smaller meals and snacks can help ease discomfort. 

Physical Activity

In order to gain necessary weight, some teenagers may need to temporarily stop exercising completely. Others will find that they can continue to work out but for shorter time periods or with less intensity. Healthcare professionals can help teenagers know what to do regarding physical activity in order to best protect their health.

An essential component to gaining weight is matching calorie output with input. In other words, teenagers need to eat enough food to meet the additional calories burned  through physical activity. Some individuals find it helpful to consume a small balanced meal or snack before and after a workout to ensure they are eating enough. 

Ask For Help

Those suffering from eating disorders require professional help in order to address the barriers impeding healthy weight behaviors. Also, stress or mental illness cause weight loss. These conditions also necessitate expert guidance. Weight gain can sometimes be as simple as eating more and moving less, but commonly involves treatment of underlying factors. 

The National Eating Disorders Association offers information and other resources for those struggling with disordered eating.


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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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