What are the risks of being underweight?

When body mass index (BMI) is discussed, the focus is often on the adverse effects of having an excessively high BMI. A high BMI can indeed put you at risk for various obesity-related diseases, such as diabetes and certain cancers. It is equally true that a low BMI, below 18.5, can put you at risk of developing potentially critical complications associated with underweight and malnutrition. If you have a BMI under 16, there is, for example, a risk of anorexia.


It can be challenging to maintain a healthy weight. Both being overweight or underweight can be detrimental to your health, so it is essential to maintain a healthy average weight.

Below is a brief description of the risks associated with underweight and the health hazards or benefits that it can lead.


Many people have an unhealthy low BMI. That may occur because individuals are rationing their caloric intake or restricting the types of foods they consume. These behaviors then reduce the amount and kind of nutrients that become available for absorption in the body. That restriction in food intake can lead to malnutrition over time.

Underweight produces a lack of energy

Underweight, the body does not have the energy supplies needed for an active life. You may experience weakness and fatigue. That feeling of tiredness can be the body’s way of conserving energy to maintain the most critical bodily functions. The more underweight a person has, the clearer this fatigue will appear.


Lack of proper nutrition can deplete the body of vitamins and minerals, which you need to fight various illnesses. For example, deficits in vitamin A, which is commonly ingested through the diet, can weaken a thin defensive layer of tissue in the respiratory tract as well as the gastrointestinal tract. This deficit can then cause infections. Iron, zinc, vitamin D, and vitamin E are also nutrients that, when insufficient, can lead to a higher risk of ailment caused by malnutrition.

Bone structure

Keeping a healthy bone structure is another BMI related problem. This is due to an increasingly clear link between low BMI and osteoporosis, especially among women. Osteoporosis is a medical condition that develops at low bone mineral density. The lower the mineral density, the more brittle and breakable the bone becomes. Osteoporosis develops due to a lack of essential nutrients such as vitamin D and calcium.


In some cases, a low BMI can result in infertility problems. In general, infertility is more commonly seen in women with low BMI. These can be caused by an intense exercise program or a very restrictive diet. Too small levels of fat affect the body’s ability to store estrogen, which can then inhibit ovulation and affect the menstrual cycle.

Underweight affects the heart

Just as a low BMI can cause a reduction in bone density and a reduction in body fat, it can also result in loss of heart muscle mass. Lower heart mass results in thinner and weakened heart walls, which increases the risk of heart disease and heart attack. Low blood pressure can also be the result as heart muscle strength decreases.

Loss of weight is healthy for many people. However, be aware that an unhealthy obsession with weight loss can counteract the health benefits that a reduction in weight can bring. Consult your physician or other healthcare professional to determine the perfect BMI level for your unique situation to maintain a healthy and active life. If weight gain is achieved, the BMI may be classified as normal, which is a level higher than underweight.