What you need to know about fasting and cancer

People fast—abstain from food or drink—for a variety of reasons. Some fast to prepare for a medical procedure or blood test. Others do it for religious reasons. And the hottest diet trend these days is intermittent fasting, a weight-loss strategy that focuses more on when you eat rather than what you eat.

You may also have heard that fasting reduces cancer risk or improves your ability to fight it. Could fasting-related changes to your metabolism really make a difference, either by hurting cancer cells or enhancing the effectiveness of cancer-fighting drugs?

While research on the subject is tantalizing, there’s little clinical evidence involving humans to substantiate the claims. Studies on the potential impacts on cancer treatment from various forms of fasting or calorie restriction, including the possibility that they reduce side effects, have been limited.

Cancer patients who plan to fast should first consult their oncologist or care team. Fasting may not be appropriate for malnourished individuals or those with cancer cachexia, which results in a continuing loss of skeletal muscle mass, or for people with chronic diseases. Those with diabetes need to be very careful, because of the risk of hypoglycemia, says Carolyn Lammersfeld, MBA, MS, RD, CSO, LD, Vice President of Integrative Care Services at City of Hope.

Fasting also is not recommended for people with a history of eating disorders, including anorexia nervosa and bulimia, or for those with wounds that need adequate nutrition to heal.

For those hungry for more information about fasting and cancer, here are some things to know:

Fasting vs. dieting

While fasting may be part of a diet plan, not all diets involve fasting. What’s the difference between fasting and dieting?

What is fasting?

Fasting programs tell you when you can and can’t eat, not what you can and can’t eat. With fasting, you eat no food for specific periods of time, which may be for a continuous portion of a day, week or month. Many fasting plans allow you to eat what you like when you’re allowed to eat.

Intermittent fasting

Intermittent (short-term) fasting allows you to abstain for shorter periods on a regular basis. If you get eight hours of uninterrupted sleep a night, you already have an eight-hour intermittent fasting period. If you don’t eat for the four hours before you go to sleep and for four hours after you wake up, you’re doing the 16:8 fast, meaning a 16-hour daily continuous fast with eight hours during which you can eat anything you want. Other fasting plans call for abstaining for a full day twice a week, or three days every two weeks or so.

Fasting-mimicking diet

The fasting-mimicking diet (FMD) allows you to consume just enough calories every day to get by. When you’re eating, you consume healthy, natural foods. As the name says, this isn’t fasting, but the reduction in food is supposed to fool the body into thinking you’re fasting.

What is dieting?

Dieting often involves calorie-restriction programs that limit your total intake of food. They may also suggest what types of food you should eat to get those calories.

Fasting for cancer: What does the research say?

It’s widely accepted that a healthy diet intended to reduce weight may also reduce cancer risk. Obesity is a known risk factor linked to at least 13 cancers, including breast, colorectal and pancreatic cancers. Research into how fasting as a weight-loss strategy may impact risk or treatments is inconclusive, and more studies are needed to investigate this issue.

Topics researchers have explored include those listed below.

Breast cancer recurrence: Researchers at several universities in San Diego suggest that fasting for 13 hours a day may reduce breast cancer recurrence and death from the disease. But, they say, randomized trials “are needed to adequately test whether prolonging the nightly fasting interval can reduce the risk of chronic disease.”

Prevention and improved treatments: Scientists at two other California universities concluded separately that calorie reduction and fasting may help reduce cancer risk and improve the effectiveness of the treatment in attacking cancer cells. A 2016 University of Southern California study suggests fasting may make cancer cells more sensitive to chemotherapy and may protect normal cells and promote stem cell production. A 2014 University of California San Francisco study in mice theorized that fasting may help the immune system regenerate. The researchers also suggested that a two-day fast combined with chemotherapy was better at slowing cancer progression than chemotherapy alone. “Both [calorie reduction] and fasting are the subject of ongoing research, and definitive conclusions await the results of those studies,” they wrote.

Slowing cancer growth: Fasting may reduce glucose levels in the blood, making it harder for cancers to grow. Cancer cells feed on glucose, consuming it at a much higher level than normal cells do. During fasting, one way cells try to conserve energy is by making it easier for cell membranes to metabolize insulin. This improves insulin’s ability to remove glucose from the blood. Reducing the glucose available to cancer cells may hinder cancerous growth.

Promoting cell regeneration: Can fasting help the body replace unneeded cells and other damaged parts during a process called autophagy? Studies suggest that a lack of autophagy decreases the levels of tumor-suppressing genes. One way to activate autophagy is through fasting, which puts the body’s cells under stress. Autophagy kicks in to make the cells function more efficiently.

The bottom line is that more research is required before reaching any conclusions on the impact of fasting on cancer risk and treatments.

If you are planning to fast, and your care team is supportive of the plan, keep the following additional tips in mind.

Try a sensible, healthy diet. Many nutrition recommendations for people concerned about cancer risk include what types of food they should eat. Diets that limit the amount of red meat, processed foods and alcohol may reduce the risk of cancer or cancer recurrence. These diet strategies encourage eating fruits, vegetables, nuts, herbs, beans and whole grains, along with fish and seafood, poultry, dairy and eggs.

Lean proteins like fish or a chicken breast, plain Greek yogurt, a protein shake or beans, peas and lentils are healthy ways to get to a minimum of 50 grams of protein daily, which will help you maintain muscle mass, Lammersfeld says.

Be careful of dehydration. Your body needs water, maybe even more so when placed under the stress of a fasting program. So, when you’re not consuming food, you should still drink plenty of water to stay hydrated. Good hydration helps transport nutrients to your cells, balance acid levels, regulate hormones and flush out damaged cells and toxins that may be broken down during fasting.

Cancer patients should pay particular attention to their hydration levels. Due to side effects from cancer or cancer treatment, you may already be experiencing water loss through diarrhea, nausea and vomiting or fever. Some medications you’re taking may also contribute to dehydration.