Weight loss that is meaningful clinically (in relation to your disease) is the loss of more than 5% of usual body weight over a six-month period. This arbitrary cut-off needs to be considered carefully as this does not tell about whether the given weight loss is due to loss of body fat (which is desirable outcome) or due to loss of muscle mass relative to body fat, which is bad especially in elderly or ill person.

Why does weight loss matter?

Weight loss occurs when there is an inadequate energy intake or excess energy expenditure. Progressive involuntary weight loss is a matter of serious concern as it mostly suggests serious medical or psychiatric illness. Sometimes voluntary weight loss of unusual severity in a non-obese person may be a concern due to an underlying psychiatric illness. However voluntary weight loss in an overweight person may be harmless. When there is weight loss due to serious medical disease, there will be one or more other symptoms even before the weight loss is significant.

Failure to gain weight or weight loss in children

In children not gaining weight adequately may be serious. It results from decreased intake of calories (due to illness or lack of food), increased calorie requirement, mal-absorption or loss of nutrients and acute fluid loss. The nutritional and psychosocial causes of failure to gain weight are more common in smaller children. However the doctor will also evaluate for possible bodily diseases leading to weight loss or failure to gain weight.

Causes of involuntary weight loss:

Several conditions cause an undesirable loss of weight. These include:

• Hyperthyroidism: In the disease of thyroid gland that produces abnormally excess amount of thyroid hormone, there is weight loss despite good appetite and food intake, as energy is burnt in excess.
• Diabetes mellitus: In this disease where blood sugar is excessively high due to decreased insulin production, especially in new onset type I diabetes, there is weight loss despite increased appetite and intake, as there is excess fluid loss and loss of lean muscle/body mass.
• Adrenal insufficiency: In the chronic primary disease of adrenal gland, there is weight loss due to decreased appetite and loss of fluid.
• Malignancy/cancer: There is a significant weight loss in all types of cancer due to different processes including decreased appetite and intake and decreased muscle mass.
• Gastrointestinal diseases: In several diseases of the digestive system, involving different parts of gastrointestinal tract, there is weight loss from decreased intake, decreased digestion or absorption.
• HIV and other chronic infections: Such patients have decreased intake or loss from vomiting or diarrhea.
• Advanced diseases: Advanced diseases of heart, lungs, kidney or brain have weight loss as constitutional symptom.
• Psychiatric diseases: Psychiatric illnesses can cause weight loss in many people when they stop eating voluntarily in depression, feeding disorders, memory loss, etc.
• Drugs: Some medicines taken from over-the-counter or herbal products cause a loss of weight. This may also be a side effect of some prescription drugs. Substance abuse like alcohol dependence, amphetamines, opiates and withdrawal from marijuana also causes weight loss.

Overenthusiastic weight loss in some overweight and even some normal persons through excessive exercise, self-induced vomiting, avoidance of food etc, may be of concern at times, but is mostly harmless.

When to seek help

Unintentional significant weight loss (particularly more than 10% of usual weight in a non-obese person) increases the chances of dying, due to specific serious diseases. So you need to consult if the weight loss is associated with other bodily symptoms and the rate of loss is significantly high despite good appetite and fluid intake.

Evaluation and treatment

As there are distinctly different causes of weight loss, evaluation is individualized. This means the findings in the history and physical examination will help to choose which investigations to do. This will enable your doctor to find out the cause of weight loss in most of the cases. 

Some of the investigations your doctor may order or perform include:

• Blood tests – Counts, glucose, electrolytes, renal and liver function tests, thyroid function test.
• Imaging – Chest X-ray, Ultrasound.
• Others – Urine and stool tests.

Treatment is based on the cause of weight loss. Broadly it includes:

• Medicine: antibiotics for infection, anticancer drugs, drugs to treat hormonal problems.
• Surgery or radiation therapy for cancer.
• Behavioral therapy for psychiatric illnesses.
• Supportive care and nutritional therapy.