Updated August 12, 2026

Cancer can affect every aspect of your health, including your appetite and diet. Selvi Rajagopal, M.D., an internal medicine specialist 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 
• Weight gain or weight loss, depending on the medications and type of cancer 

“Sometimes it also depends on the specific type of cancer you have,” explains Rajagopal. “Treatment for breast cancer and blood cancer often involves 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. These 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, these patients 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 Whole Foods

Some of the best foods to eat during chemotherapy or other cancer treatments are plant-based whole foods, including vegetables as well as beans, legumes, nuts and seeds. They are often rich in vitamins, minerals and fiber.

• Healthy Fats

Monounsaturated and polyunsaturated fats also offer health benefits. Avocados and olive oil are rich in monounsaturated fats, while fatty fish and walnuts provide omega-3 fatty acids, which may help reduce inflammation and support cardiovascular health.

• Complex Carbohydrates

When choosing carbohydrates, opt for foods that are minimally processed, like whole wheat, bran and oats. Lentils and beans are also a wonderful source of complex carbohydrates. Soluble fiber helps nourish beneficial gut bacteria, which produce short-chain fatty acids that support gut health and other body functions.

Should you take supplements during cancer treatment?

If you’re not eating as much as usual during treatment, or if you have side effects such as 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.

What foods should you avoid during cancer treatment?

• Highly processed and fried foods, especially those high in unhealthy fats 
• Foods that carry the risk of foodborne illnesses, including:
• Soft-cooked eggs or foods that contain raw eggs, such as homemade mayonnaise
• Unpasteurized cheeses and dairy products
• Unwashed fruits or vegetables
• Raw sprouts
• Deli meats unless heated thoroughly
• Undercooked meat or seafood

Planning Your Cancer Treatment Diet

People with cancer often have compromised immune systems, so it’s important to be aware of what’s going into your body during cancer treatment. 

• Read nutrition labels.
• Prepare as much of your own food as you can.

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 such as beans, frozen fruit or vegetables to simple meals can go a long way.

Medically reviewed by Selvi Rajagopal, MD MPH