What is a BMI less than 18 called? Defining the "Underweight" Category
A BMI below 18.5 is classified as underweight for adults. While the threshold is 18.5, a BMI under 18 falls firmly within this category. BMI is a simple screening tool that compares a person's weight to their height, providing a general estimate of body fat. It is not a diagnostic tool but serves as an indicator for potential health issues related to body weight. Several factors can contribute to a low BMI, from genetics and high metabolism to underlying medical conditions or mental health issues.
The Health Implications of a Low BMI
Being underweight, particularly when the BMI drops significantly below 18, is linked to a range of health issues. It is often a signal that the body is not receiving sufficient nutrients, leading to deficiencies. For instance, a lack of adequate calcium can contribute to weakened bones and osteoporosis. A low BMI can also weaken the immune system, making individuals more susceptible to infections and illnesses. While it's possible for some people to be naturally thin and healthy, a consistently low BMI warrants attention from a healthcare professional.
Potential health risks associated with a low BMI include:
- Malnutrition and Nutrient Deficiencies: Insufficient intake of calories and vital nutrients like vitamins and minerals can cause a variety of deficiencies affecting overall health.
- Weakened Immune System: A compromised immune system can make the body more vulnerable to frequent and severe infections.
- Osteoporosis: Inadequate calcium and vitamin D intake can lead to low bone density and a higher risk of fractures.
- Anemia: Being underweight increases the risk of low blood counts, leading to fatigue, headaches, and dizziness.
- Fertility Issues: For women, a low BMI can cause irregular periods or even stop menstruation, which may lead to infertility.
- Growth and Development Delays: In younger individuals, being underweight can hinder proper growth and development.
- Higher Mortality Risk: Research has linked being underweight to an increased risk for premature death compared to those with a normal BMI.
Causes Behind a Low BMI
Many factors can contribute to an underweight status. A thorough medical evaluation is necessary to determine the root cause, which can range from genetic predisposition to chronic illness.
- Genetics: Some individuals have a naturally high metabolism and low body fat, and a low BMI may simply be a result of their natural build.
- High Metabolism: People with a very high metabolic rate can burn calories more quickly, making it difficult to gain weight even with a high-calorie diet.
- Underlying Illnesses: Various medical conditions can cause weight loss. Examples include an overactive thyroid, diabetes, digestive diseases like Crohn's, or chronic infections.
- Mental Health Conditions: Depression, anxiety, and obsessive-compulsive disorder can affect appetite and lead to weight loss. Eating disorders such as anorexia nervosa are also a critical cause of low BMI and require specialized treatment.
- Frequent Physical Activity: High-level athletes or those with a very active lifestyle may burn more calories than they consume, leading to a low BMI.
Comparison: Dangers of Underweight vs. Overweight
While public health messaging often focuses on the risks of overweight and obesity, being underweight also presents significant health dangers. The severity and nature of the risks differ, as outlined in the table below.
| Feature | Underweight (BMI < 18.5) | Overweight/Obese (BMI > 25) |
|---|---|---|
| Associated Risks | Malnutrition, weakened immune system, osteoporosis, anemia, fertility issues, higher risk of mortality. | Heart disease, stroke, type 2 diabetes, high blood pressure, sleep apnea, certain types of cancer. |
| Underlying Causes | Genetics, high metabolism, chronic illness, mental health issues, inadequate nutrition. | Excess calorie intake, sedentary lifestyle, genetic factors, certain medical conditions. |
| Nutritional Concern | Deficiency of essential nutrients (vitamins, minerals, protein, calories). | Excess intake of calories, unhealthy fats, and sugars; often associated with poor diet quality. |
| Psychological Impact | Eating disorders (anorexia, bulimia), depression, anxiety. | Body image issues, depression, low self-esteem. |
| Prevalence | Less common in many developed countries but a significant global issue. | High prevalence worldwide, considered an epidemic in many regions. |
Management and Outlook
If you are concerned about a low BMI, the first step is to consult a healthcare professional. They can help identify any underlying causes and recommend a tailored plan. For those who are medically healthy but looking to gain weight, the goal is to do so healthily by focusing on nutrient-dense foods and strength training. Weight gain should be gradual and should not be achieved by consuming excessive amounts of unhealthy, sugary foods. A dietitian can provide personalized dietary advice, including tips like eating more frequently and incorporating healthy fats.
The outlook for an individual with a low BMI depends on the cause and their overall health. Addressing any underlying medical or mental health conditions is critical. As noted in research, gaining weight to a healthy range can be beneficial for overall health and mortality. The key is a multi-faceted approach involving medical evaluation, nutritional guidance, and potentially mental health support. Seeking professional help is essential for a safe and effective path to a healthy weight. For more detailed nutritional guidance, reliable sources like Healthdirect offer further reading on what to do if you are underweight.
Conclusion
A BMI of less than 18.5 is officially known as underweight. While some individuals may be naturally thin, this classification can indicate significant health risks, including malnutrition, a weakened immune system, and bone density issues. It is a screening tool, not a diagnosis, and a consultation with a healthcare provider is recommended for anyone concerned about their weight. A healthy, gradual weight gain achieved through nutrient-dense foods and exercise is the best approach, supported by professional guidance. Awareness of the causes and associated risks is crucial for prioritizing health and taking appropriate action.
