What Is a Dangerously Low BMI?

Your body mass index (BMI) depends on your weight and height. It can give a general idea of whether you’re at risk of a weight-related health condition. BMI helps doctors quickly screen for serious issues with body weight.

If your BMI becomes dangerously low, such as less than 16, you are at greater risk of malnutrition and premature death. Speak with your doctor to discuss whether your BMI presents a health risk.

Standard BMI Ranges

BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters.

According to the Centers for Disease Control and Prevention (CDC), for adults 20 years and older, standard BMI ranges include:

• Underweight: less than 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obesity: 30 and higher

It’s possible for your BMI to be in the underweight category but you’re still healthy. However, because it comes with some risks, including malnutrition, Cleveland Clinic advises talking to your doctor if you have a BMI less than 18.5.

A BMI of less than 16 is considered dangerously low and is associated with a markedly increased risk of poor health, malnutrition, and early death, according to the World Health Organization (WHO).

Causes of Low BMI

It’s possible that you may have had a prolonged illness or a medical treatment that caused your weight to drop, which reduced your BMI.

Excessive stress, such as the death of a loved one, can also lead to weight loss and cause your BMI to drop. Substance use, alcohol misuse, advanced age, and eating disorders can also result in a dangerously low BMI.

A doctor might examine you to determine whether you have an underlying condition, like celiac disease or an overactive thyroid, that causes weight loss. Losing more than 5 percent of your weight during a 6- to 12-month period without trying typically indicates a problem, according to Cleveland Clinic.

Dangers of a Low BMI

A low BMI is dangerous because it indicates you are likely not eating a balanced, nutritious diet full of fruit, vegetables, lean protein, dairy, and whole grains. This puts you at risk of essential nutrient deficiencies that can have long-term consequences for your health.

For example, inadequate calcium intake can lead to weakened bones and osteoporosis, in which your bones fracture easily, reports Mayo Clinic. Too little vitamin C can result in compromised immune function. A lack of vitamin A can bring on vision problems.

Other complications of being underweight include:

• Weakened immune system
• Anemia
• Pregnancy complications
• Infertility
• Giving birth to babies who are underweight

A low BMI also has implications for your overall mortality. One study including nearly 2 million people found having a BMI in the underweight category was associated with a life expectancy about 4.5 years fewer than people with a BMI in the healthy category.

Addressing Low BMI

If you have a low BMI, consider consulting a dietitian. They can help you adjust your diet to gain weight safely.

Consider the following tips from Mayo Clinic when trying to gain weight in a healthy way:

• Focus on nutrient dense food. Foods such as nuts, nut butters, seeds, whole grains, avocado, dairy, and olive oil are nutrient-dense, high-calorie foods that can help you meet your weight gain goals. Try to limit or avoid foods low in nutrients, like fast food and sodas.
• Eat more frequently. Rather than eating larger amounts at each meal, it may be easier to distribute the extra calories over the course of several smaller meals throughout the day.
• Try smoothies or shakes. Adding high-calorie, nutritious ingredients, such as Greek yogurt, full-fat dairy, nut butters, avocado, dates, and bananas, to a shake or smoothie can help lead to weight gain.
• Exercise. Combining dietary measures with strength training to build muscle can help address a low BMI.

The Takeaway

• A BMI less than 18.5 for adults 20 years and older is associated with a number of health risks. Severe concerns emerge if BMI drops below 16, including increased risk of malnutrition and early death.
• Causes of a dangerously low BMI may vary from eating disorders and underlying medical conditions to high levels of stress and severe dietary restrictions.
• Low BMI is associated with several health risks, including nutrient deficiencies, weakened immune system, osteoporosis, and adverse outcomes, like infertility and pregnancy complications.
• If you suspect your BMI is very low, talk to your doctor. They may suggest a dietary plan to safely increase your weight with nutrient-dense foods and strength training exercises.