Intermittent Fasting: Is it worth the hype?

​One of the most intriguing and popular diets to hit the market in the last 10 years is intermittent fasting. While the diet presents in various forms and regimens, by definition it is any designated period of fasting, followed by a period of  free (or ad libitum) feeding. Intermittent fasting has been credited for health benefits such as weight loss, lower blood sugar,  improved aging and vitality as well as anti-cancer benefits. With all of it’s accolades, it might surprise you to know that the bulk of that research on intermittent fasting  has NOT been done in humans. The majority (about 80%) has been done in pre-clinical models such as cells and rodents. In fact, there is very limited research examining the effects of intermittent fasting in humans, and only a fraction of that examines any outcome other than weight loss. Let’s take a look at what we know so far.

Intermittent fasting first gained popularity in the early 2000s when studies conducted in mice suggested fasting may provide benefits for aging and recovery from brain injury. Since then many metabolic studies have examined the effects of various fasting regimens on other health factors such as cardiovascular health, diabetes, and the ever-elusive weight-management. While rodent studies have been promising, few human trials have been able to replicate these results. 
Before we go any further, let’s get clear on some definitions. As I mentioned above, intermittent fasting is technically any period of fasting followed by a period of free feeding. However this has been implemented via various regimens. Most studies to date have examined what is called “alternate day fasting” or ADF. ADF typically involves a 36 hour fast followed by 12 hours of free feeding, followed by another 36 hour fast, and so on. Some studies have adapted this slightly to a more consistent 24 hour fast and 24 hour feed, however it is implicit that at least 7-10 of the 24 “feeding” hours are spent sleeping, ultimately making the fast closer to 30 hours. A newer regimen that has been examined in recent years is the 5:2 regimen wherein subjects eat freely for 5 days per week and “fast” the remaining two. There are two caveats to this regimen. First, the two fasting days are never consecutive. Second, they are not true fasts. Typically subjects eat less than 25% of their daily calorie goal (<500 calories) on the “fasting” days. The final regimen is the one that is both most popularized and least studied is time-restricted eating or TRE. Time-restricted eating refers to eating and fasting windows within a 24 hour period, such as the popularized 16:8 regimen. Now that we have our definitions, let’s look at what the research actually says.

Perhaps the most consistent result in human trials is that intermittent fasting, regardless of specific regimen followed, seems to produce weight loss and better blood sugar control in people with diabetes. That said, trials comparing intermittent fasting with standard calorie restriction (200-500 calorie deficit daily) show no differences in weight loss or blood sugar control. So while intermittent fasting may be a method of creating a calorie deficit for weight loss, it does not appear to have any inherent advantage.
Recent studies have also directly compared ADF, 5:2 and TRE. In 2018 Harris et al. conducted a study in overweight or obese individuals who followed the ADF, 5:2 or TRE regimen for 12 weeks. At the end of the 12 weeks, all participants had lost weight, but there were no differences between the various regimens. Similarly, most individuals saw a decrease in blood cholesterol, however there was no difference amongst the various experimental groups. They did find that the 5:2 regimen only lowered fasting insulin and it also produced the biggest change in weight circumference. In 2019, Stekovic et al expanded on these findings by placing a small cohort of normal-weight men on either an ADF diet or control diet (no modifications) for 6 months. At the end of the trial, individuals in the experimental (ADF) group had decreased LDL cholesterol compared to those in the control group. They also had higher levels of ketones in their blood. And finally, a summary in the New England Journal of Medicine stated that while ADF and 5:2 regimens have been shown to increase ketone, and improve memory impairments in older adults, the link between these two phenomena is still poorly understood. They go on to state that most trials have only been conducted over a period of 3-6 months, and the long-term effects of these diets are still unknown.
If you read the last two paragraphs and now have more questions than answers- you’re not alone. The simple fact is we currently don’t have a good understanding of what impact intermittent fasting has on the body and various diseases. While there are lots of “wellness” sites out there singing the praises of intermittent fasting, the science simply doesn’t support those claims at this time. It’s also worth noting that while some of the data we do have at this time is promising in the management of diseases such as diabetes and heart disease, these studies are very preliminary, have been done only in small cohorts of mostly healthy subjects and have only extended as far as 3-6 months. At present, what we DON’T know about intermittent fasting far exceeds what we do. And my advice is to be skeptical of sources that tell you otherwise.

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References: 
https://pubmed.ncbi.nlm.nih.gov/29419624/
https://academic.oup.com/cardiovascres/article/116/3/e36/5740708
https://www.nejm.org/doi/full/10.1056/nejmra190513