Disadvantages Of Having A Low BMI That Aren't Often Discussed

Body mass index (BMI) is a tool that is used to estimate whether a person's weight is healthy based on the ratio of their weight in kilograms relative to their height in meters (you can use pounds and inches, too). The World Health Organization defines a healthy BMI as lying between 18.5 and 25, and much attention is given to the negative health effects associated with having a high BMI, labeled as overweight, obese, or morbidly obese. However, having a low BMI can also have its disadvantages for your health, though they are not discussed as often.

One of the most notable issues that can arise from being underweight is a lack of energy, which can be two-fold. Firstly, an underweight person may feel low energy in their day-to-day lives, particularly if one of the causes of the low BMI is lack of calories. Adults need between 1,600 and 3,000 calories per day for the body to have enough energy to function normally, and a calorie deficit can cause feelings of fatigue and exhaustion.

On a cellular level, lack of energy can have more serious repercussions. Cells need energy for essential processes, and in an energy-poor situation, the mechanisms may become less effective. This can impact the function of the immune system, and coupled with a potential lack of nutrients from diet, can mean that illness and injury can take longer to recover from than would usually be expected.

It is reasonable to assume that underweight individuals weigh less because of a lack of fat or muscle, but bone mass can also be a factor in the equation. Our bones are constantly renewing, with fresh bone being built to replace the old. Good dietary nutrition is required to build strong bones, particularly calcium, vitamin D, and protein. If one of the causes of underweight is low calorie intake, there may not be enough of these crucial nutrients for bone growth, leading to loss of bone mass and potentially osteoporosis.

Hormones like estrogen and testosterone also play a vital role in maintaining strong and healthy bones. Having a low BMI can often be correlated with hormonal imbalances, further increasing the risk of bone issues developing, particularly in older age as the risk of osteoporosis increases.

Imbalance of sex hormones caused by a low BMI can also have an impact on fertility. A 2021 study in BMC Pregnancy and Childbirth found that in frozen–thawed embryo transfer in vitro fertilization cycles, being underweight reduced implantation rates, clinical pregnancy rates, and ongoing pregnancy rates, compared to those with a BMI in the normal range.  For women who are underweight due to anorexia nervosa, up to 84% experience lack of menstruation, known as amenorrhea. This does not mean that everyone with a BMI below 18.5 will have hormonal imbalances, fertility issues, or risk of bone disease, but if low weight is connected to poor nutrition, these unseen issues could be playing out silently in the background.

While serious issues such as osteoporosis and infertility may not be apparent for many years, there are several issues that being underweight can be associated with that are more obvious, and can be important markers for overall health. In a 2023 study in Nutrients, researchers collated the results of nine separate blood tests — including blood count and lactate dehydrogenase, which tests for tissue damage — into one "risk score" to represent overall health. The results showed that in general, as BMI decreased, the more abnormal the blood results became, suggesting a higher risk of poor health.

Low BMI can often be associated with lack of nutrition, and common indicators include issues with skin, teeth, and hair. A lack of crucial vitamins and minerals such as vitamins A, C, and E, zinc, and omega-3 fatty acids can cause dry skin lacking in elasticity, brittle hair and nails, and gum disease.

A low BMI can have many causes, and while it is often correlated to insufficient calorie intake, there can also be underlying conditions to blame. An overactive thyroid can speed up the metabolism and cause sudden or ongoing weight loss, and can cause serious issues if left untreated. Poor appetite and lowered caloric intake can also be a symptom of mental health issues such as depression or anxiety, potentially leading to a drop in weight over time. Anyone experiencing unexplained or ongoing weight loss should seek medical advice to help lower the risk of health issues associated with being clinically underweight.