According to the Centers for Disease Control and Prevention, 39.8 percent of U.S. adults in 2015 had obesity, so public focus tends to center on the negative health consequences of excess weight. The health risks associated with low BMI, or body mass index, though, are discussed less frequently.
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A BMI of less than 18.5 is considered underweight. A low BMI is associated with health problems such as malnutrition, menstrual cycle irregularity and a heightened risk of depression and osteoporosis.
Understand the BMI Measurement
The American Heart Association notes that BMI indicates the amount of body fat for most people. It is often used as a screening tool to identify whether or not an adult is at a healthy weight. BMI uses height and weight to assign a numerical value.
The following equation can be used to determine BMI for adults over the age of 20: BMI = weight in pounds / (height in inches x height in inches) x 703.
A BMI between 18.5 and 24.9 indicates an adult is at a normal weight, while a BMI of less than 18.5 is considered underweight. A BMI between 25 and 29.9 is considered overweight, and a BMI of 30 or higher indicates that an adult has obesity.
If you prefer to calculate BMI with a BMI calculator in kg/m2 (the standard BMI measurement unit), the National Heart, Lung and Blood Institute offers a tool for this task.
Read more: What Should I Weigh for My Age & Height
Low BMI and Poor Health
The Office of Women's Health (OWH) suggests that those with low BMI run the risk of malnutrition, meaning that their bodies are not getting enough vitamins and minerals to fully function. Malnutrition can cause serious health problems like a weaker immune system and anemia.
According to OWH, anemia occurs when a person's blood cannot carry enough oxygen through your body because it lacks iron. If someone has anemia, he or she may experience dizziness, lightheadedness, weakness or fatigue.
Additional potential problems include:
Menstrual cycle irregularity: Having a regular period is a sign of good health and, often, a BMI within a normal range. The dangers of being an underweight female are an irregular period or even one that stops completely, which is called amenorrhea. If your period stops completely, it is possible that your body fat has dropped too low, meaning that you have stopped ovulating each month, or releasing an egg from an ovary.
Read more: Can Running Cause Your Period to Come Late?
Difficulty getting pregnant: OWH states that menstrual cycle problems make it more difficult to get pregnant, especially if your period completely stops. If you stop having your period, then you are likely not ovulating each month, or releasing an egg from an ovary.
Higher risk of osteoporosis: According to OWH, osteoporosis is a condition in which bones become weak and break easily. It typically affects people later in life, but prevention starts earlier. A low BMI increases the risk of osteoporosis later in life.
Increased danger of depression: In a December 2017 study published in the British Journal of Psychiatry, being underweight was shown to raise the risk of depression in men and women. The study showed that the risk of depression is 16 percent higher for people with low BMI measurements than those in the normal range.
Understand the Limitations of BMI
BMI is often thought of as an absolute way to measure healthy weight. But, according to the Mayo Clinic, some experts think it is misleading. They say the index offers a false sense of good health to people in the normal range. Instead, some doctors recommend measuring central obesity, or excess belly fat.
Central obesity increases the risk of high blood pressure, diabetes and heart disease. To get a full understanding of your health in relation to your weight, both BMI and waist circumference should be taken into account.
- Centers for Disease Control and Prevention: "Prevalence of Obesity Among Adults and Youth: United States, 2015–2016"
- American Heart Association: "Body Mass Index (BMI) in Adults"
- Office of Women's Health: "Underweight"
- Office of Women's Health: "Weight, Fertility, and Pregnancy"
- Office of Women's Health:"Osteoporosis"
- NCBI: Medicine (Baltimore): "Relationship Between Obesity and Depression in Korean Adults: Korea National Health and Nutrition Examination Survey 2014"
- Mayo Clinic: "Mayo Clinic Minute: The Problem With BMI"
- National Heart, Lung, and Blood Institute: "Calculate Your Body Mass Index"