Cancer can affect every aspect of your health, including your appetite and diet. Selvi Rajagopal, M.D., a specialist in internal medicine at Johns Hopkins Medicine, explains why your diet is so important during cancer treatment, and provides tips on foods to add and avoid.

How does cancer treatment affect your diet?

Treatments such as chemotherapy and some forms of radiation therapy can cause a variety of side effects, including:

• Constipation, which can cause discomfort and further reduce your desire to eat
• Diarrhea, which can drain your body of nutrients
• Fatigue, which means you’re less active, so you burn fewer calories and don’t feel as hungry throughout the day
• Loss of taste, which can make food unappealing
• Nausea and vomiting, which might reduce your appetite and cause weight loss

“Sometimes it also depends on the specific type of cancer you have,” explains Rajagopal. “Treatment for breast cancer and blood cancers often involve steroids. Steroids can actually increase your appetite and increase your blood sugar levels, which might lead to insulin resistance and weight gain. So instead of losing weight, it’s possible to gain quite a bit of weight from the combination of medications and a more sedentary lifestyle during cancer treatment.”

Some people also have hormone therapy after chemotherapy for breast cancer or endometrial cancer. The drugs suppress production of estrogen, a hormone that plays an important role in metabolism. If your metabolism slows down, you may put on weight.

People with pancreatic cancer often find it difficult to maintain their weight. Since the pancreas isn’t functioning the way it should, they may not be able to digest food normally. This can lead to weight loss or malnourishment.

Why is your diet important during cancer treatment?

Since cancer treatment can lead to fluctuations in appetite and body weight, it’s important to pay close attention to your diet. In addition to helping you maintain a healthy weight, eating a balanced diet during chemotherapy or radiation therapy can:

• Help manage treatment side effects
• Increase energy
• Increase muscle tone
• Preserve immune function
• Reduce inflammation

What foods should you add to your diet during cancer treatment?

“Anyone with a chronic illness, even if it’s not cancer, should eat foods high in protein, healthy fats, whole grains, and vitamins and minerals,” Rajagopal emphasizes. “If possible, make these dietary adjustments before cancer treatment begins so you’ll be healthier going into treatment.

Plant-based Proteins

Some of the best foods to eat during chemotherapy or other cancer treatments are plant-based proteins. They offer the highest levels of vitamins and minerals, Rajagopal says. This means eating lots of vegetables as well as beans, legumes, nuts and seeds. If you do eat animal proteins, choose lean options like chicken or fish.

Healthy Fats

Monounsaturated and polyunsaturated fats also have health benefits. Avocados, olive oil, grapeseed oil and walnuts are all high in omega-3 fatty acids, which help combat inflammation and improve cardiovascular health.

Healthy Carbs

When choosing carbohydrates, opt for foods that are minimally processed, like whole wheat, bran and oats. These have soluble fiber, which helps maintain good gut bacteria. Soluble fiber also promotes the production of short-chain fatty acids (SCFAs), which lend a hand to everything from metabolism to cellular repair.

Vitamins and Minerals

“Vitamins and minerals help our bodies’ enzymatic processes, which play a big role in boosting immune function and reducing inflammation,” Rajagopal says. When possible, select foods fortified with vitamin D. These may include milk, orange juice, yogurt and some cereals.

Should you take supplements during cancer treatment?

If you’re not eating as much as usual during treatment, or if you have side effects like vomiting and diarrhea that cause you to lose vitamins and nutrients, you might consider taking a multivitamin.

“Vitamin D tends to be the most common vitamin deficiency,” says Rajagopal. “Vitamin D helps keep your immune system strong, reduces fatigue and supports bone health. Especially if you’re on steroids, you’ll be at risk for bone density loss.”

Talk to a registered dietitian and your oncologist before adding any vitamins or supplements to your diet.

How can your diet help manage cancer treatment side effects?

Some dietary changes can help you manage side effects after your treatment begins. These side effects include:

• Appetite loss. Eat small meals or nutritious snacks throughout the day, rather than three large meals.
• Constipation. Drink plenty of water, consider a fiber supplement, and add veggies and beans to your meals.
• Diarrhea. Choose foods or drinks with sodium (sports drinks or broth) and potassium (bananas and all natural fruit juices).
• Loss of taste. Knowing what to eat when you can’t taste can be difficult. Consider trying new foods with different spices or marinades. You can also add strong flavors, such as lemon or lime juice.
• Nausea. Anti-nausea foods include citrus, ginger and peppermint oil. You can suck on a slice of lemon, drink ginger tea or eat ginger chews.

What foods should you avoid during cancer treatment?

Be aware of what’s going into your body during cancer treatment. Read nutrition labels and prepare as much of your own food as you can. It’s best to stay away from highly refined, processed food. You should also avoid fried foods that contain a lot of hydrogenated oils, which can increase inflammation.

Since people with cancer often have compromised immune systems, consider skipping foods that carry the risk of foodborne illnesses, including:

• Lightly cooked or raw fish, such as sushi
• Soft-cooked eggs or foods that contain raw eggs, such as homemade mayonnaise
• Unpasteurized cheeses and dairy products
• Unwashed fruits or vegetables

Planning your cancer treatment diet

Registered dietitians have specialized training in the nutritional needs of people with specific diseases. Your dietitian can help you plan meals that give you the right number of calories and nutrients.

“It’s also important to build an eating plan that’s practical for you,” says Rajagopal. If you’re busy in the evenings and don’t have the time or energy to cook, try to select healthy takeout options. If you’re on a budget, adding inexpensive, nutritious foods like beans or frozen fruit or vegetables to simple meals can go a long way.