What Matters: Fasting and cancer



We are a product of our environment. But we shan’t forget that we are a product of the critical interaction between our environment and our evolutionary biology.

Back when we were roaming the plains searching for food, we likely experienced “forced fasts” (that is, we had no food). Our ancestors who were the best at surviving these periods of scarcity lived to bear us into our current period of staggering abundance. Now, we are the unhealthiest humans in history.

Dr. Jon O. Ebbert

Dr. Jon O. Ebbert

Is part of the answer to our current health problems to return to our roots and ... fast?

In a recent article by Catherine Marinac and her colleagues, patients aged 27-70 years with breast cancer in the Women’s Healthy Eating and Living study were analyzed to uncover the relationship between nightly fasting duration and new primary breast tumors and death (JAMA Oncol. 2016 Mar 31. doi: 10.1001/jamaoncol.2016.0164). Fasting was assessed through use of 24-hour dietary recall.

Fasting less than 13 hours per night was associated with an increased risk of breast cancer, compared with fasting at least 13 hours (hazard ratio, 1.36; 95% confidence interval, 1.05-1.76). Different fasting durations were not associated with breast cancer mortality.

Additional analyses demonstrated that each 2-hour increase in fasting duration was associated with significantly lower hemoglobin A1c levels and a longer duration of nighttime sleep.

The positive health benefits of fasting have become increasingly “discussed,” albeit commonly on websites advertising for fasting cookbooks. Benefits of fasting include weight loss, improved insulin sensitivity, reductions in inflammation, improved cardiovascular risk factors, enhanced brain function, reductions in Alzheimer’s disease symptoms, and extended life span.

Many of these data are preliminary, and some are based upon animal models, such as the prolonged lifespan. In one study, rats undergoing alternate-day fasting lived 83% longer than rats who were not fasted. Interestingly, human data suggest that food consumption on the nonfasting days does not result in caloric consumption to cover the caloric deficit on the fasted day.

I have to admit that I am intrigued. I am not hearing much discussion about fasting among my colleagues – although a lot them skip meals, I know. But nobody is discussing it as a recommendation to appropriately selected patients (for example, not on insulin) to combat obesity and other diseases. I tried to suggest it to a patient the other day, who had a staggering amount of central adiposity, and he laughed at me. Is the thought of skipping eating for a day so anathema to our modern consumptive culture that we can’t even consider it?

Depending on the type of fasting that one is doing, one does not have to count calories on the fasting days, because there aren’t any. That makes it easy.

In a world of abundance and limitless food options, it may seem strange (self-indulgent?) to fast. But perhaps it will be a key to help us continue the species for a couple more generations.

Dr. Ebbert is professor of medicine, a general internist at the Mayo Clinic in Rochester, Minn., and a diplomate of the American Board of Addiction Medicine. The opinions expressed are those of the author and do not necessarily represent the views and opinions of the Mayo Clinic. The opinions expressed in this article should not be used to diagnose or treat any medical condition nor should they be used as a substitute for medical advice from a qualified, board-certified practicing clinician. Dr. Ebbert has no relevant financial disclosures about this article.

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