When you think about malnutrition, images of exceedingly thin children in third-world countries may come to mind. However, malnutrition affects cancer patients, as well, and it can have serious consequences. And it’s common: The National Cancer Institute estimates that 30 to 85 percent of cancer patients may be malnourished.

Malnutrition isn’t just about getting enough food or being underweight. It’s about nutritional deficiencies, says Dr. Pankaj Vashi, medical director of gastroenterology/nutrition and metabolic support at Cancer Treatment Centers of America. “Even people who are obese can have malnutrition.”

In some ways, obesity is a paradox when it comes to cancer, says Dr. Darrell Gray, a gastroenterologist at Ohio State University Comprehensive Cancer Center — Arthur G. James Cancer Hospital and Richard J. Solove Research Institute. “Obesity is a risk factor for colorectal cancer. However, being overweight can be protective once you’re diagnosed with colorectal cancer.”

[See: 8 Questions to Ask Your Doctor About Colon Cancer.]

What Causes Malnutrition in Colorectal Cancer Patients?

Cancer itself can contribute to malnutrition, Vashi says. Cancers produce chemicals that change the way your body uses — or absorbs — some nutrients (malabsorption). This can cause a condition called cachexia, which is characterized by loss of appetite, weight loss, muscle loss and overall weakness. Additionally, the metabolic activity of cancer requires significant energy, which can cause patients to lose weight. Finally, a tumor can cause a blockage in the bowel and disrupt absorption. Gray says weight loss often triggers alarm in people who don’t know they have cancer but become concerned they’re losing weight for no apparent reason.

Cancer treatment is another big cause of malnutrition, Gray says. Chemotherapy, a common treatment in colorectal cancer, can cause nausea, vomiting, diarrhea and abdominal pain. Treatment can change the way food smells or tastes or your ability to eat enough to get sufficient nutrients. “If you don’t feel well, you don’t want to eat,” Gray says. “This predisposes you to malnutrition if it goes on over time.” Recovering from surgery can also increase your body’s need for nutrients and energy, according to the NCI. You may not be eating enough to keep up with your body’s demands.

Anorexia, or loss of appetite, is the most common cause of malnutrition in cancer patients. It’s often present at diagnosis and may arise during treatment. In fact, the NCI says, anorexia is widespread in people with metastatic cancer.

Vashi says treatment also makes patients susceptible to infection. “Chemotherapy suppresses the immune system,” he says. “If you get an infection, you’re in a high metabolic state, burning more calories than you are taking in.”

Finally, Vashi says, patients who have underlying depression or psychological stress (which, he says, is often undiagnosed) are also more vulnerable to malnutrition. “If a patient’s state of mind is better, they do better with treatment.”

Vashi says good nutrition is about getting the right amounts of two categories of nutrients. Macronutrients are the basic components of food: protein, fat and carbohydrates. Micronutrients — important chemical elements in food — include things such as electrolytes (sodium, potassium and calcium, for example) as well as vitamins and trace elements in the blood, such as selenium, zinc and beta carotene. Micronutrients may be lost in people who have persistent diarrhea or vomiting from cancer treatments.

[See: 5 Rare Diseases You’ve Never Heard Of (Until Now).]

Preventing Malnutrition

According to the NCI, treating cachexia early is important, because it’s hard to correct after it persists. So, the first step is to diagnose malnutrition. Vashi says the Subjective Global Assessment, a nutritional assessment tool, is the gold standard for diagnosing malnutrition. The questionnaire asks patients about weight loss, symptoms, fluid retention and performance status (how much can you do?). Based on the answers, the SGA puts patients into one of three categories: well nourished, moderately malnourished or severely malnourished. Early detection is important, Vashi says. You don’t want to wait until a patient loses weight. “Nutritional intervention is key,” he says. Being properly nourished can also help patients withstand the rigors of treatment and facilitate recovery.

Gray says the first step is to address the underlying cause of malnutrition and then to tailor treatment appropriately. For example, he says, a cancer can grow to the point where it creates a blockage and the pain prevents a patient from eating. Or, some patients can’t absorb nutrients, even if they can eat.

In severe cases, patients may require assistance in obtaining appropriate nutrition. For some patients, that means a PEG tube, which delivers food directly to the stomach via a tube placed in the nose or an opening in the abdomen. If the patient has extensive disease, or requires extended nutritional support, he or she may require parenteral nutrition delivered directly into the blood stream.

[See: 10 Lessons From Empowered Patients.]

A nutritionist or dietitian should be part of every patient’s cancer care team. These experts can create tailored interventions to ensure patients get sufficient calories — and the right types of calories. If you’ve been recently diagnosed or are undergoing treatment for colorectal cancer, talk to your doctor about your nutritional needs before, during and after treatment.

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What You Need to Know About Malnutrition in Colorectal Cancer originally appeared on usnews.com