Updated 05 August 2013

Causes of underweight

Desperate to gain weight? In this article, DietDoc takes a closer look at underweight and factors that may be causing it.


More and more Health24 readers are "desperate to gain weight".

Because of this upsurge in interest in people who do not want or need to lose weight, but in contrast struggle to gain and retain weight, we take an in-depth look at this nutritional problem.

In this article, we consider how underweight is defined and what may be causing it.

Definition of underweight

Anyone who is 15 to 20% below the normal weight for age and height is classified as underweight, according to Krause (2000).

In real terms, this means that if you are an average woman between the ages of 25 and 50, and you are about 1.63 metres tall, your weight should be approximately 59 kg if you are not underweight.

As soon as your body weight falls to 50 kg (-9 kg or -15%), or even 47 kg (-12 kg or -20%), you are underweight. Expressed in terms of body mass index (BMI), our standard woman weighing 59 kg would have a BMI of 22. As her weight drops, her BMI would reduce to 18,8 (-15% of body weight), which is just still within the normal range, or 17,7 (-20% of body weight), which is below the normal range.

For a man aged 25 to 50 years, with an average height of 1,76 metres, a normal body weight with a healthy BMI of 23 would be 71 kg. Reductions of -15% and -20% of body weight would reduce our standard man's weight to 60 kg and 57 kg, with BMIs of 19,4 and 18,4 respectively.


Underweight can be caused by a variety of factors, some of which are psychological, while others are physical.

a. Physical causes of underweight

Many illnesses can cause temporary weight loss. If you have had a bad dose of flu, accompanied by a high temperature, you may lose a few kilos, but you will probably gain back those kilos once you are well again.

There are five major medical reasons for pronounced weight loss and underweight:

1. Hyperthyroidism

The thyroid gland, which produces the hormone thyroxine, is often regarded as the 'conductor' of the 'orchestra of the body'. If your thyroid hormone production is normal, all will be well with your health, metabolism and weight.

If, however, the function of your thyroid gland is disturbed so that it produces too much thyroid hormone, your metabolism and many body functions such a heartbeat, body temperature, etc., will be deranged and you will experience pronounced weight loss.

This condition, which is called hyperthyroidism, thyrotoxicosis, or overactive thyroid, is associated with symptoms such as: enlarged thyroid gland causing a 'goitre' or bulging at the base of the neck, nervousness, tiredness, loss of weight despite adequate intakes of food, increased body temperature with excessive sweating, increased heart rate, and in some patients, exophthalmos (bulging eyeballs).

If you experience sudden weight loss accompanied by any of the above-mentioned symptoms such as goitre, excessive sweating and tiredness, you need to have a medical examination and thyroid function tests.

2. Cancer

Malignancy or cancer can also be associated with rapid weight loss, tiredness, lack of appetite, nausea, and inability to gain weight.

Because malignancies are so serious, it is important to have your doctor give you a thorough examination and have blood and other tests done to determine if you could be suffering from conditions such as leukemia, or other types of cancer.

3. Tuberculosis

Tuberculosis, or TB, is very common in South Africa. This is a wasting disease that is accompanied by rapid weight loss, coughing, night sweats, tiredness and malaise.

Once again, you urgently need to find out if the weight loss you experience is due to TB, so that you can receive the appropriate treatment.

4. Diabetes

Although many patients with insulin resistance and diabetes tend to suffer from overweight, excessive weight loss, tiredness, excessive thirst and urination, are also symptoms of diabetes.

Patients displaying these symptoms need to have their blood glucose and insulin levels tested to see if they suffer from diabetes. This is particularly significant in families with a hereditary tendency to diabetes.

5. HIV/Aids

Probably the main cause of wasting or excessive weight loss in South African patients is HIV/Aids. This condition, also initially called 'slim disease', causes pronounced weight loss and hampers weight gain.

If you suspect that you may be living with HIV/Aids and your weight keeps on dropping even when you eat large quantities of food, you should have an HIV test as soon as possible.

With the correct medication and diet, you will be able to halt the progression of this disease and improve your survival rate and quality of life.

b. Psychological causes of underweight

Besides anorexia and/or bulimia (which are not the subjects of our discussion today), one of the most important causes of underweight is depression.

Patients suffering from depression not only often have a reduced appetite, but they may actively lose weight at an alarming rate. If you have any reason to believe that you may be suffering from depression, it is essential to get help as fast as possible from clinical psychologists or psychiatrists to stop your weight loss.

Other symptoms that may point towards depression are moodiness, tiredness, an overpowering urge to sleep all day, inability to do simple daily tasks, a feeling that 'life is not worth living' and ideas of suicide.

c. Other factors

There are also a number of other factors that could be causing weight loss or hampering weight gain, namely:

  • Medication – many medicines either suppress appetite or actually cause weight loss. Check with your pharmacist if you have recently started a new medication and now are losing weight without wanting to.
  • Excessive exercise – you may well be overdoing your exercise sessions to such an extent that your diet cannot keep up with the increased demands.
  • Excessive stress – individuals who are very stressed may stop eating due to a lack of appetite, nausea or being preoccupied with their worries.
  • Enzyme deficiencies – deficiencies of digestive enzymes and/or stomach acid may also hamper digestion and absorption of food thus causing weight loss.
  • Other medical conditions such as coeliac disease (gluten allergy), or cystic fibrosis are associated with inability to gain weight.

If you struggle to gain weight or suddenly experience unexplained weight loss, consult a medical doctor for a checkup. The doctor will test you for overactive thyroid, malignancies, TB, diabetes mellitus, HIV/Aids, enzyme deficiencies, coeliac disease, cystic fibrosis and any other physical condition that could be hampering weight gain.

In addition, your doctor should be able to pick up if you are depressed or excessively anxious or stressed, and send you for appropriate psychotherapy, or prescribe antidepressants or anxiolytics.

Medical treatment of these illnesses in conjunction with the appropriate diet will assist with weight gain. – (Dr Ingrid van Heerden, DietDoc)

(Krause's Food, Nutrition & Diet Therapy, 10th Ed. WB Saunders Co, USA)

Dr Ingrid van Heerden is a registered dietician and holds a doctoral degree in Nutrition and Biochemistry. She believes that "we are what we eat" and offers free nutrition and weight loss advice via her DietDoc service on Read more of her articles.


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