The BMI is often used as the criteria to determine if a person is within the normal weight range.


Your Body Mass Index is the relationship between your weight and height. Anyone who is 20% below the normal weight for age and height is classified as underweight.

The ideal BMI is 18.5-25, if your BMI is under 18.4 you are considered underweight.

Some people naturally find it hard to put on weight. If your weight is constant and you have no long-term medical problems and a good diet, you probably don’t need to worry. If you are malnourished, on the other hand, you definitely need to do something about it.

Underweight causes

A person may be underweight due to genetics, metabolism, lack of food (poverty), or illness. It can be caused by a variety of factors, including anorexia, hyperthyroidism, cancer, diabetes, psychological problems, inflammatory bowel disease and infections.

Adults with gastrointestinal or liver problems may be unable to absorb nutrients adequately. And people with anorexia nervosa become underweight due to self-starvation and/or excessive exercise.

Main causes

  • Hyperthyroidism: When the function of your thyroid gland is disturbed, it can produces too much thyroid hormone. Your metabolism and many body functions such as body temperature or heartbeat will be deranged and you will experience weight loss.

  • Cancer: Cancer is also associated with rapid weight loss, tiredness, lack of appetite, nausea, and inability to gain weight.

  • Tuberculosis: A very common disease in South Africa, is always accompanied by rapid weight loss, coughing, night sweats, tiredness and malaise.

  • Diabetes: Excessive weight loss, tiredness, thirst and excessive urination are symptoms of diabetes. Have your blood glucose and insulin levels tested, particularly if your family have a hereditary tendency to diabetes.

  • HIV / Aids: Initially calle “slim disease”, HIV can cause pronounced weight loss and hampers weight gain.

  • Psychological causes: Anorexia, Bulimia and Depression are the most important causes of underweight.

  • Other factors: Medication, excessive exercise, excessive stress, enzyme deficiencies, gluten allergy or cystic fibrosis.

Understanding the causes of underweight is crucial. Explore further resources on addressing underweight issues and related conditions such as anorexia, hyperthyroidism, cancer, and diabetes for comprehensive information.

Underweight problems

People who are underweight may have an increased risk of anemia, heart irregularities and osteoporosis.

One of the better known conditions associated with low body weight is anorexia. People who are underweight can find it extremely difficult to put on weight.

If you are underweight (BMI less than 18.4), you can develop health problems, especially if you are also inactive:

  • Lowered Immune System: Immune system needs fuel to take on invading infections, and unless underweight adults is eating very carefully, they could easily starve this disease defense guard. The get more infections.
  • Heart Disease: Irregular heart rhythms and higher risk of heart attack.
  • Anemia: A deficiency of red blood cells, they are responsible for transporting oxygen to the organs. Anemia is caused by nutritional deficiencies of iron, B12 and folate.
  • Lack of periods and infertility: A Woman's menstrual cycle often stops or becomes irregular when they are too skinny. And it’s also harder for them to sustain the pregnancy. Men are 20 times more at risk for sexual dysfunction such as erectile dysfunction or inability to ejaculate.
  • Osteoporosis: Body fat produces estrogen, needed for healthy bones.
  • Taking longer to recover from illness
  • Respiratory disease
  • Digestive diseases
  • Diabetes
  • Cancer

Underweight treatment

Being underweight can be a concern if it’s the result of poor nutrition or if you are pregnant. So if you are underweight, see your doctor or dietitian for an evaluation to plan how to meet your goal weight.

Healthy ways to gain weight:

  • Eat more frequently: Eat five to six smaller meals during the day rather than two or three large meals.
  • Choose nutrient-rich foods: Choose whole-grain breads, pastas, fruits, cereals, vegetables, dairy products, nuts and seed.
  • Try shakes: Drink smoothies or healthy shakes made with milk and fresh juice.
  • Add toppings: Increase your calorie intake with each meal by adding nut or seed toppings, and cheese. Try almonds, sunflower seeds, fruit or whole-grain wheat toast.
  • Exercise: Exercise can help you gain weight by building up your muscles. Exercise can also stimulate your appetite.
  • Snacks: Have a bedtime snack, such as peanut butter and jelly sandwich.

Please, before beginning any weight gain plan, be sure to consult your doctor. Being underweight can indicate an underlying health issue which won’t be corrected by diet changes.

For professional guidance on addressing underweight concerns and developing a tailored treatment plan, consult with a qualified doctor or dietitian specialized in underweight management.

Am I underweight?

You can calculate your body mass index to know if you are underweight or within the normal weight range.