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Why Does Cancer Cause Weight Loss?

Certain cancers can cause significant unexplained weight loss

Why is weight loss a symptom in certain types of cancers?

Certain cancers can cause significant unexplained weight loss

Nearly everyone loses or gains weight at some point in their lifetime. Diet, exercise, pregnancy, hormonal changes and aging all can affect weight, and fluctuations are generally normal.


Unexplained weight loss, however, is not considered normal and may be a sign of a serious illness, such as cancer. In fact, weight loss can be both a symptom of cancer and a side effect of treatment or the illness itself.


Losing weight for no apparent reason does not mean you have cancer. The amount of weight lost matters. Losing a few pounds is not usually a concern, but a loss of 10 pounds or more without knowing why, may be a red flag. According to the American Cancer Society, significant weight loss happens most often with cancers that affect the stomach, pancreas, esophagus and lung.


“For many people with cancer, this unexplained weight loss is one of the first indications of the disease,” says Munveer Bhangoo, MD, a hematologist and oncologist at Scripps MD Anderson Cancer Center. “Someone who loses this amount of weight without a clear reason should see their physician to determine what is causing it.”

Why does cancer cause weight loss?

Several factors can contribute to cancer-related weight loss. Cancer cells demand more energy than healthy cells, so your body may burn more calories at rest than it normally would. The cells also release substances that affect how your body uses calories from food, which also can contribute to weight loss.


As your body tries to defend itself against cancer, the immune system produces substances called cytokines that increase inflammation. Cytokines can alter metabolism and interfere with the hormones that control appetite, causing further loss of weight.


Some types of cancer are more likely to lead to weight loss than others. Cancers that affect the mouth or throat may make chewing or swallowing difficult. Patients with nausea, a common side effect of cancer, may have little or no appetite and may be unable to keep food down. Tumors that affect organs near the abdomen, such as ovarian cancer, may press on the stomach as they grow, so patients feel full even though they’re eating less.


Weight loss also may be related to cancer treatments. Chemotherapy and radiation therapy can cause nausea and vomiting, as well as a loss of appetite. Some patients develop sores in their mouths, which make it challenging or painful to eat normally.


Cancer can take not only a physical toll, but an emotional one as well. Along with fatigue and weakness, patients may have stress, depression or anxiety — all of which can have a negative effect on appetite and energy levels.

Tips to help control weight loss

If you or a loved one have cancer and are experiencing weight loss, muscle loss or low energy, talk with your physician and care team. They can help you determine which factors are contributing to the issue and provide guidance to address them.


“Medical care is just one aspect of cancer treatment,” says Dr. Bhangoo. “At Scripps, our nurse navigators guide patients living with cancer to support services to help them feel as well as possible throughout their journey.”


Registered dietitians who specialize in cancer care work closely with patients to develop nutritious meal plans customized to their individual needs, such as consuming extra calories or adjusting meals to accommodate difficulties swallowing or chewing. Anti-nausea medications can help relieve nausea and stimulate appetite.


Exercise specialists can develop personalized exercise plans to help people with cancer prevent muscle loss and increase energy levels. Behavioral health specialists, such as psychologists and social workers, treat depression, anxiety and other emotional challenges of cancer, which can improve energy and appetite.


Finally, for many patients, simply knowing that they’re not alone can be powerful “medicine.” Members of cancer support groups often share valuable information and tips about diet, exercise and self-care.