While public health discussions often focus on being overweight, a body weight that is too low also carries health consequences. The most common tool for assessing weight is the Body Mass Index (BMI), which compares a person’s weight to their height. A BMI below 18.5 for an adult is classified as underweight, a condition associated with several health problems.
Causes of Being Underweight
A person might be underweight for various reasons spanning medical, psychological, and lifestyle categories. Certain medical conditions can lead to unintentional weight loss. An overactive thyroid (hyperthyroidism) can speed up metabolism, while digestive disorders like celiac disease and Crohn’s disease can impair nutrient absorption. Other chronic illnesses such as cancer, diabetes, and infections like HIV/AIDS can also be contributing factors.
Psychological factors play a large role in body weight regulation. Conditions such as depression, anxiety, and stress can decrease appetite, leading to a lower food intake. Eating disorders, including anorexia nervosa and bulimia, are mental health conditions characterized by an unhealthy preoccupation with weight and food, which directly results in being underweight. These conditions often involve a distorted body image.
Genetics and lifestyle can also predispose a person to being underweight. Some individuals have a naturally high metabolism, a trait that can run in families, making it difficult to gain weight. Athletes and those who engage in frequent, high-intensity physical activity burn a substantial number of calories, which can result in a low body weight. Inadequate access to nutritious food can also prevent individuals from meeting their caloric needs.
Risks of Nutritional Deficiencies
Being underweight is often an indicator of malnutrition, meaning the body isn’t receiving the energy, vitamins, and minerals needed to function properly. A primary consequence is a compromised immune system. Without sufficient fuel from food, the body cannot produce an adequate number of immune cells, leaving an individual more vulnerable to infections and illnesses.
A common issue linked to being underweight is iron-deficiency anemia. Iron is a fundamental component of hemoglobin, the protein in red blood cells that transports oxygen. When iron intake is insufficient, the body can’t produce enough healthy red blood cells. This leads to symptoms like fatigue, dizziness, and shortness of breath because the body’s tissues are not receiving enough oxygen.
The effects of nutrient shortfalls are also visible externally. Deficiencies in certain vitamins are linked to problems with skin, hair, and teeth. For example, a lack of vitamins C and E can result in dry skin, while insufficient protein can cause hair to become brittle or fall out. Dental health can also suffer, with an increased risk of gum disease when the body lacks nutrients for maintenance and repair.
Effects on Bone and Muscle Integrity
Low body weight is a risk factor for osteoporosis, a condition that causes bones to become weak, brittle, and more susceptible to fractures. This occurs from an insufficient intake of bone-building nutrients like calcium and vitamin D, and a lack of physical stress on the bones. Body weight itself provides a mechanical load that stimulates bone to grow stronger; without it, bone mineral density can decline.
In addition to bone loss, underweight individuals are at risk for sarcopenia, the progressive loss of muscle mass and function. This condition is driven by an inadequate intake of protein and calories necessary for muscle repair. As muscle tissue diminishes, it leads to generalized weakness, frailty, and a reduced capacity to perform everyday tasks. This can create a cycle where decreased strength leads to less activity.
The combination of weakened bones and reduced muscle mass creates a heightened risk of injury. With less muscle to support the skeleton and bones that are less dense, even a minor fall can result in a serious fracture. This is especially dangerous for older adults, as fractures can lead to hospitalization and a decline in mobility and independence.
Complications in Reproduction and Development
Being underweight can have effects on the reproductive system and developmental processes. In women, a low percentage of body fat can disrupt the hormonal balance that regulates the menstrual cycle. This can lead to irregular periods or amenorrhea, the complete absence of menstruation. Without regular ovulation, which is the monthly release of an egg, it becomes difficult to conceive, leading to infertility.
For underweight women who do become pregnant, there are increased risks for complications. These women have a higher likelihood of experiencing preterm birth, which is delivering a baby before 37 weeks of gestation. They are also more likely to have a baby with a low birth weight, defined as less than 5.5 pounds, which can face numerous health challenges.
During childhood and adolescence, being underweight can interfere with growth and development milestones. The body requires significant energy and nutrients to support bone growth, muscle development, and puberty. When these resources are lacking due to an inadequate diet, growth can be stunted. Puberty may also be delayed because the body has not reached the necessary weight to initiate these hormonal changes.