Many health trends come and go, but one dietary trend that has seemed to endure the craze is intermittent fasting.
The primary reason intermittent fasting (IF) has remained so popular is because it offers flexibility to fit a busy schedule, and (while I don't typically advise mindlessly eat as long as your fasting) it doesn't ultimately demand restricted eating.
What is intermittent fasting, and how is it done?
Intermittent fasting is defined as diet regimen that cycles through a period of time in which the body is driven into a catabolic state through fasting, where no significant calorie intake occurs, met with periods of fed states where no calorie restriction occurs. The windows for fasting are typically around 16 hours, met with an 8 hour fed state.
Intermittent fasting can be used in a few different ways:
The most common approach is time restricted feeding. Now, most guidelines state that timing of these fed/fasted windows won’t matter – ie a fed state from 9 am – 5 pm will produce the same results as a fed window from 1 pm – 9 pm. We’ll talk about why that’s not always true in a moment. But first, lets look at the research that demonstrates the health benefits to intermittent fasting.
Are There Benefits to Intermittent Fasting?
Where the research falls short:
The first shortcoming to intermittent fasting is that when you compare apples to apples, research to date shows that intermittent fasting provides no further long term benefit over calorie restriction, and calorie restriction provides no long term benefits in weight loss. In fact, most long term evidence shows calorie restriction has negative impacts to long term weight loss.
Now let me be clear here – daily calorie deficits are a good thing. Eating the same/slightly less and exercising more could never be a bad thing. However, when we examine some of the major clinical studies conducted on weight loss (the TODAY study, the Women’s Health Initiative study, the Diabetes Prevention program), data shows that despite initial weight loss compared to control groups, continuous calorie restriction did not amount to long term changes in weight, or body composition. In fact, the members of these trails not only gained back all their weight, but also now have to follow a calorie restricted diet to maintain it. (4,5,6)
With this being said, the long term data for intermittent fasting benefits just simply doesn’t exist yet. Understanding that calorie restriction and intermittent fasting both have underlying catabolic actions call for the need for more long term studies with follow ups greater than 1 year would be needed before drawing conclusions to the long term safety of intermittent fasting on metabolism.
The second biggest pitfall to intermittent fasting research is that majority of it is done in men. No large scale, long term follow up trials have yet to be conducted on female subjects, which is a problem. Numerous trials to date show higher negative sequelae to dietary restrictions in women, such as carbohydrate deprivation inducing under functioning thyroid in as little as 6 wks, and ketosis caused menstrual dysfunction in 45% of women after 6 months (7). And while we don't have great human trials for intermittent fasting for women, rat trials show significant impacts to reproductive hormones in as little as two weeks following IF.8This information should be extrapolated with caution when deciding if intermittent fasting is right for you.
When should intermittent fasting be avoided?
While health benefits from IF exist for many people, the following people should not engage in restrictive dieting without the counsel of a medical professional:
My general advise remains to assess things on a case by case basis to determine if intermittent fasting is right for you, and for how long. Until more research confirms more long term benefits and conclusive benefits in women, I usually advice sticking to IF 2-3 days a week, and maintaining a whole foods, plant rich diet the rest of the time.
To find out if intermittent fasting and which methods are right for you, please feel free to contact Dr. Courtney Holmberg, ND at 647-351-7282 to schedule an appointment today!