The Impact of Low BMI on Patient Health
Low Body Mass Index (BMI), often classified as underweight, can significantly affect overall health. It is linked to weakened immunity, reduced muscle mass, and increased risk of fractures, as well as mental health challenges like depression and anxiety.
Understanding the causes and consequences of low BMI is crucial for developing effective prevention and intervention strategies to improve patient outcomes.
Body mass index (BMI): What Is It?
People’s weight is categorized using their BMI. Instead of evaluating individual health, BMI charts are mostly used to evaluate community health.
People at the extremes (having a high or low BMI) will always exist within a population.
A high or low BMI could be a sign of high stress, inconsistent activity habits, or a bad diet. A person is not necessarily healthy just because their BMI is “normal.”
Body composition, such as muscle, fat, and bone density, is not taken into consideration by BMI. An erroneous reading might also result from sex and other variables that affect your weight. Therefore, for some people, such as children and young people under the age of 18, pregnant women, and athletes, a BMI calculation is not a useful metric.
Normal BMI Ranges:

A BMI of less than 18.5 is considered low and puts an adult in danger if they are 20 years of age or older. A BMI of 18.5 to 24.9 is regarded as normal; a BMI of 25 to 29.9 is deemed overweight; and a BMI of 30 or over is deemed obese. There are, nevertheless, certain exceptions to each of these ranges. Someone incapable of walking, for instance, may have a low BMI due to muscle atrophy, although this does not always indicate underweight.
BMI for kids and teenagers is calculated using percentiles derived from the Centers for Disease Control and Prevention’s growth charts. A girl or boy in the fifth percentile is deemed underweight; being overweight or underweight is based on comparison with people of the same sex, age, height, and weight. Children and teenagers who need to measure precisely within one-quarter pound and one-eighth of an inch can use the Centers for Disease Control’s online percentile calculator.
Reasons for Low BMI:
Your BMI may have decreased as a result of a prolonged sickness or medical treatment that caused you to lose weight. Overwhelming stress, like losing a loved one, might also make you lose weight, which will lower your BMI. Other problematic factors include being elderly or abusing drugs or alcohol. To find out if you have an overactive thyroid or an underlying condition like celiac disease, a doctor may examine you.
Moreover, eating disorders may cause a dangerously low body mass index. One may have anorexia nervosa if their BMI is 16.5 or lower. According to one study, patients with severe anorexia who were admitted to Japanese hospitals had an average BMI of 13.1. In 2013, the findings were released in the journal Eating and Weight Disorders.
What signs of underweight are present?
Being underweight might lead to severe fatigue or recurrent illness. That can be the result of your diet not providing you with the proper nutrients. It also causes your skin to become very dry, your hair to thin or fall out, and your teeth to get impacted.
Underweight kids could not develop normally for their age.
Underweight can result in several health issues, such as:
- cardiac problems
- Pregnancy issues, such as trouble becoming pregnant or an increased risk of preterm labor
- osteoporosis
- infections, which are more difficult to treat
- irregular periods in women; or menstruation may completely stop
For older adults, being underweight can be particularly risky since it raises the possibility of bone fractures and illness.
Risks Associated with Low BMI:
Because it suggests you are probably not consuming a healthy, balanced diet rich in fruits, vegetables, lean protein, dairy, and whole grains, a low BMI is harmful. This puts you at risk for deficits in vital nutrients that could affect your health in the long run.
Vitamin A deficiency can cause visual issues, while vitamin C deficiency can lead to immunological issues and chronic illnesses. Underweight women may have irregular menstrual cycles and reproductive issues.
Your overall mortality is also affected by a low BMI. Despite receiving medical treatment, study participants with a BMI of 11 or below who were part of the Japanese anorexia study passed away while in the hospital.
According to a 12-year study of Korean men and women, those who were overweight or underweight were more likely to die than those who were average weight. Compared to study participants with a normal BMI, individuals with a low BMI were more likely to die from respiratory disease.
A subsequent study’s findings demonstrated that individuals with diabetes who had a low BMI were more likely to develop cardiovascular disease than those with normal or higher BMIs. The findings were reported by researchers at the 2013 Congress of the European Society of Cardiologists.
How to Deal with Low BMI:
You might think about speaking with a dietician if your BMI is low. To reach a “normal” BMI range, you will need to modify your diet to gain weight. Try to consume 250–500 extra calories per day than you typically consume, but avoid empty calories from soda and junk food because they don’t offer adequate nourishment.
Eating a little more at each meal or spreading out the extra calories over a few smaller meals is beneficial. Nutrient-dense, high-calorie foods like nuts and nut kinds of butter, seeds, dried fruit, whole grains, avocado, dairy, olives, and olive oil, as well as protein-rich foods, will help you reach your weight-gain objectives. A low BMI may be addressed by combining muscle-building strength exercises with nutritional changes.
FAQs
What is considered a very underweight BMI?
Since a BMI of less than 16.0 is known to be linked to a significantly higher risk of illness, poor physical performance, fatigue, and even mortality, this cut-off point is a legitimate extreme limit.
What is the BMI danger zone?
Normal BMI Ranges
A BMI of less than 18.5 is considered low and puts an adult in danger if they are 20 years of age or older. An individual is considered normal if their BMI is between 18.5 and 24.9, overweight if their BMI is between 25 and 29.9, and obese if their BMI is 30 or above.
Is a BMI of 16.5 considered underweight?
A BMI below 16.5 indicates severe underweight, while a BMI between 16.5 and 18.5 indicates underweight. Whenever an individual has a BMI between 25 and 30, they are considered overweight. Class I obesity is defined as having a BMI between 30 and 35.
What BMI is problematic?
A “healthy” level is between 18.5 and 24.9 points. The 25–29.9 range is regarded as overweight. Obese people fall between the ages of 30 and 39.9.
Can heart attacks occur in slim people?
According to a recent study, adults who lead sedentary lives and maintain a healthy weight may be just as susceptible to heart attacks and strokes as those who are overweight.
What is the first place you lose weight?
Males typically lose weight in their upper body first, whereas women typically lose weight in their legs. In general, women have more body fat than males, particularly in the thighs and hips, and they will retain it there for a longer period.
References
- Martinac, P., & Jonathandowney/iStock/GettyImages. (2011, May 4). What is a dangerously low BMI? Livestrong.com. https://www.livestrong.com/article/434699-what-is-a-dangerously-low-bmi/
- NHS inform. (2024, July 12). Body mass index (BMI) | NHS inform. NHS Inform. https://www.nhsinform.scot/healthy-living/food-and-nutrition/healthy-eating-and-weight-management/body-mass-index-bmi/
- Healthdirect Australia. (n.d.). What to do if you are underweight. Healthdirect. https://www.healthdirect.gov.au/what-to-do-if-you-are-underweight