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What Causes a Person to Lose Weight?

By Jessica Martinez ; Updated July 18, 2017

Most people welcome weight loss as a victory over unhealthy habits. However, losing weight without trying or losing a large amount of weight over a short period of time can be an indication of illness. If you experience a sudden or unexpected weight change, consult your physician to rule out any serious health problems.

Dietary and Lifestyle Changes

Whether you've recently decided to eat healthier, you've suffered a recent illness and a consequent lack of appetite or your recent hectic schedule has you regularly skipping meals, a change in your diet can cause unintentional weight loss. In addition, increasing your activity—such as going from a sedentary job to an active job, or beginning a new exercise regimen—without increasing your calorie intake can cause a drop in weight. The fact is, if you take in fewer calories than you use, your weight will go down.


Diarrhea can lead to weight loss. Most commonly, diarrhea is caused by mild viral or bacterial infections, such as gastroenteritis or food poisoning. These infections usually only last a few days, after which you can expect to regain any weight you may have lost. Chronic diarrhea, which is defined by the National Institute of Health's online resource MedlinePlus as loose, watery stools that last for four weeks or more, can cause longer-term weight loss. Chronic diarrhea can be attributed to medications, such as phenytion or tetrecyline; malabsorption problems, such as lactose intolerance; or diseases, such as chronic pancreatitis or cystic fibrosis.

Gastrointestinal Disease

Intestinal illnesses, such as celiac disease, Chrohn's disease, ulcerative colitis and peptic ulcer disease, can interfere with your body's ability to digest food and properly break down and absorb nutrients. This can cause you to lose weight, even if you eat as much or more so than usual.

Serious Illness

Serious, chronic infections, such as tuberculosis and HIV or AIDS, can cause weight loss. Weight loss is also a common symptom of cancer. Merck's online health resource notes that people afflicted with cancer may become very thin, in spite of a healthy appetite. Or, they may experience a loss of appetite, nausea or difficulty swallowing, which may make it difficult to maintain their weight. In addition, common cancer treatments, such as chemotherapy, often cause weight loss. Weight loss can be a secondary symptom of specific neurological conditions, including strokes, multiple sclerosis and dementia. It can also be a complication of advanced heart or lung disease.

Hormonal Imbalance

Your endocrine system produces and regulates your body's hormones. Endocrine disorders, such as an overactive or underactive thyroid, diabetes or adrenal insufficiency can cause hormonal imbalances that can lead to significant weight changes.

Emotional/Psychological Problems

Emotional fluctuations can cause changes in your appetite and eating habits. Chronic depression and prolonged anxiety or stress can lead to significant weight changes. In addition, eating disorders like anorexia and bulimia can cause a dangerous drop in weight and serious related health problems.

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