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Is it true that alternate-day calorie restriction prolongs life? Doctors have anecdotally attributed improvements in a variety of disease states to alternate-day fasting including asthma, seasonal allergies, autoimmune diseases like rheumatoid arthritis, osteoarthritis, infectious diseases like toenail fungus, periodontal disease, and viral upper respiratory tract infections, neurological conditions like Tourette’s syndrome and Meniere’s disease, atrial fibrillation, and menopause-related hot flashes. The actual effect on chronic disease, however, remains unclear.

Alternate-day fasting has been put to the test for asthma in overweight adults. Asthma-related symptoms and control significantly improved, as did their quality of life, including objective measurements of lung function and inflammation, significant improvements in peak airflow, significant improvements in mood and energy. But, their weight improved too—about a 19-pound drop in 8 weeks—so, it’s hard to tease out effects specific to the fasting beyond the benefits we might expect from weight loss by any means. For the most remarkable study on alternate-day fasting, you have to go back more than a half century.

The 2017 cholesterol findings were the most concerning data I could find on alternate-day fasting. The most enticing was published in Spain 61 years earlier, in 1956. The title of the study translates as “The hunger diet on alternate days in the nutrition of the aged.” Inspired by the data being published on life extension with calorie restriction on lab rats, researchers split 120 residents of an old-age home in Madrid into two groups. Sixty residents continued to eat their regular diet, and the other sixty were put on an alternate-day modified fast. On the odd days of the month, they ate a 2,300-calorie regular diet, and on the even days were given only a pound of fresh fruit and a liter of milk, estimated to add up to about 900 calories. This continued for three years. So, what happened?

Over the duration of the study, 13 died in the control group, compared to only 6 in the intermittent fasting group. But those numbers were too small to be statistically significant. What was highly significant was the number of days spent hospitalized. Residents in the control group spent a total of 219 days in the infirmary, whereas the alternate-day fasting group only chalked up 123 days. This is held up as solid evidence that alternate-day fasting may improve one’s health span and potentially even one’s lifespan, but a few caveats must be considered. It’s not clear how the residents were allocated to their respective groups. If instead of being randomized, healthier individuals were inadvertently placed in the intermittent fasting group, that could skew the results in their favor. Also, it appears the director of the study was also in charge of medical decisions at the home. In that role he could have unconsciously been biased towards hospitalizing more folks in the control group. Given the progress that has been made regulating human experimentation, it’s hard to imagine such a trial being run today; and so, we may never know if such impressive findings can be replicated.

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