Just in time for people on the e-hunt for New Years’ lifestyle resolutions, the New England Journal of Medicine published a review on intermittent fasting. The media has naturally jumped all over this review with overarching, exuberant clickbait promises of living longer if you follow an intermittent fasting diet.  But is this really the correct conclusion to draw from this article?
The New England Journal review gives an excellent overview of the physiology by which intermittent fasting (IF) may be beneficial for our health. Basically, when we fast for 10-14 hours, our bodies deplete glycogen (carb) stores and switch to fatty acids as a fuel source. The liver generates ketones from these fatty acids, which becomes a source of energy production. The fatty acids and ketones also activate a number of genes and hormone cascades throughout the body (including the brain).  These signaling changes triggers repair of cells and promotes cell survival.  Totally fascinating.
The article then goes on to discuss 3 types of intermittent fasting: alternate day fasting, 5:2 intermittent fasting (fasting 2 days per week), and time restricted feeding.  In the first two forms, the fasting day is actually a very low calorie day (500-700 cal per day), and in time restricted feeding,  you eat for only 6-8 hours per day.  (The article also discusses caloric restriction data, but this is not actually the same thing as intermittent fasting.)
Now, let’s get to the data – what can the IF diet actually do for human health?
As for the effect of IF diets on lifespan, data in rats, mice and monkeys mostly favor increased lifespan (but not all studies have shown this).  There is no human data to show increased lifespan with intermittent fasting.
There is human data to suggest there may be improvement in insulin resistance, blood pressure, and markers of inflammation (see previous blog here). However, also as blogged previously, a clinical trial in people with type 2 diabetes showed no benefit of IF vs daily calorie restriction.  Limited human studies have suggested there may be improvement in memory.  There are cases reported suggesting benefit in some cancer types, but no actual clinical trials have been completed (there are some in progress).  Animal models suggest there may be some benefit in Alzheimer’s and Parkinson’s, but there is not yet any human data.  There are a couple of pilot studies in multiple sclerosis suggesting some benefit, but again, no clinical trials.
It is important to note that many of the studies that have shown benefit of IF are short term.  We need long term data (1-2 years or more) to actually know if there is a benefit to health over the longer term.  One of the larger, longer (1 year) studies on IF that has been done actually showed no difference in any metabolic parameter other than slightly worse cholesterol compared with daily caloric restriction, and people were less likely to be able to adhere to the IF diet (as blogged previously).
The article suggests a plan that doctors can use to guide their patients on intermittent fasting regimens.  I think it’s great that guidance is being provided to doctors on how they can support people who would like to try to follow IF as a permanent lifestyle change.  However,  it’s important to remember that while the theory and some initial data is exciting, there is not conclusive data for benefit, and that no lifestyle program works for everyone.

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