Intermittent Fasting: Diet Review | What is Intermittent Fasting?

What exactly is intermittent fasting? Is there good research behind it? Is it right for you? All the details on this intermittent fasting diet review.

Intermittent fasting has been all the rage, especially the last few months. Everywhere you turn celebs like Hugh Jackman and Tim Ferriss are giving it a whirl. Well, what does the research say about intermittent fasting? And what exactly is it?

Disclosures: None. This post was not created in partnership with any product or brand. 

What is intermittent fasting?

While there is no consensus on an exact definition of intermittent fasting, intermittent fasting is either abstinence from food and calorie-containing beverages for certain times of the day or for certain days of the week, or severely limiting calories consumed on certain days of the week.

Here are some of the most popular methods of intermittent fasting:

16-8 Intermittent Fasting

One of the most popular intermittent fasting methods is to fast for 16 hours each day and eat during the other 8 hours of the day. This is called time-restricted feeding, and this method of intermittent fasting limits eating to specific hours each day. There are other variations of time-restricted feeding, like fasting for 14 hours and eating during the other 10, or similar.

5 & 2 Fasting

This method became popular with the book The Fast Diet which came out in 2013. With the 5 and 2 intermittent fasting method, calories eaten are severely restricted two days of the week (to 25% of your body’s calorie needs) and you can eat normally the other five days of the week. Technically this is a form of “modified fasting” since a small amount of calories are allowed to be consumed on fasting days.

Alternate Day Fasting

Alternate day fasting includes complete fasting (calorie-free beverages are permitted) every other day. With this method, you can eat normally on “days off.”

What Sets Intermittent Fasting Apart from Other Diets

What’s unique about intermittent fasting is this: it’s pretty simple. Intermittent fasting primarily only limits when you eat, not what you eat. Because there is less decision making around food and less exposure to food, it could potentially set you up for greater success.

Intermittent Fasting & Weight Loss

I reviewed more studies than I’d like to count on intermittent fasting, but I thought I’d first highlight a few on intermittent fasting and weight loss.

One 6-month, randomized study evaluated a calorie-restricted diet vs intermittent fasting (n=89 of 107 starting participants completed the study) for six months in overweight or obese young, adult women (1). The calorie-restricted group followed a 25% calorie deficit daily with a Mediterranean dietary pattern. The intermittent fasting group had a 75% calorie restriction for two consecutive days followed by adequate calories the other 5 days of the week, likewise Mediterranean. The overall net calorie restriction throughout the week was the same between the two groups. Weight loss was comparable between both groups. Interestingly, 58% of the intermittent fasting group compared to 85% of the calorie-restricted group planned to continue their respective diets at the conclusion of the study, suggesting a calorie-restricted diet might be more favorable in terms of long-term adoption.

Similarly, a parallel, randomized controlled trial in adults with type 2 diabetes (n=58 completed the study) who were overweight or obese (BMI ≥27 kg/m²) found that weight loss between a calorie-restricted group versus an intermittent fasting group was again comparable (2). In this study, the intermittent fasting group had severe energy restriction two days each week (allowed to eat 400-600 kcals) and ate normally the other five days a week. The calorie-restricted group ate 1200-1550 kcals daily.

I liked this next study, although very small and short, because it let participants eat ad libitum (as much as they wanted) after fasting to observe their behavior. A small, two-day experimental, crossover, randomized study (n=18) on lean, healthy adults found that overall, compensation to a day of severe restricting gleaned a net negative energy (calorie) balance (3). This study compared appetite regulation and ad libitum food intake after a day of severe energy restriction (25% of calorie needs allowed to be eaten) versus an adequate-calorie day. Calorie intake increased the day after severe restriction to about 7% more calories than a typical day, and normalized on two days after severe restriction. A separate study likewise found participants were unable to comply with study protocol to eat 25% of calorie needs one day and 175% of calorie needs the following day. This suggests that, left to their own devices, someone who is fasting intermittently will likely end in an overall negative energy (calorie) balance.

A review of human trials evaluating multiple methods of intermittent fasting found 11 of 13 studies showed statistically significant weight loss with intermittent fasting, regardless of the fasting regimen (4).

Overall, the it appears that intermittent fasting can promote weight loss, however it does not necessarily outperform daily calorie restriction for weight loss (4,5,6). Some researchers are suggesting it might be a viable option for those who struggle to maintain consistent daily calorie restriction for weight loss, however there may be lifestyle barriers to using intermittent fasting as a long-term, sustainable weight loss method.

Intermittent Fasting, Metabolic Health & Longevity

Intermittent fasting is proposed to improve metabolic health and longevity by leveraging circadian rhythms (limiting food intake to daytime), influencing the gut microbiome (potentially limiting inflammation and influencing energy absorption, expenditure and storage), and modifying lifestyle factors in beneficial ways (4,5).

There has been significant research on the positive effects of calorie restriction (without malnutrition) on longevity. What’s difficult to peel apart, is whether the effects on longevity we are seeing from intermittent fasting stem from intermittent fasting itself or from the overall result of calorie restriction. Calorie restriction in humans appears to be protective against obesity, type 2 diabetes, oxidative stress, inflammation, cardiovascular disease and overall slows aging to maximize lifespan (7).

What made researching intermittent fasting challenging

Three factors made reviewing the research on intermittent fasting challenging:

First, a lot of research has been done in animals. Studies in humans are very small; most had less than 40 participants, with the largest having about 80 participants.

Secondly, there are a lot of different types of intermittent fasting, which makes it hard to compare studies and a full-on meta-analysis or systematic review non-existent. The studies vary by number of hours fasted, number of days fasted, whether or not the participants could eat ad libitum or still had to maintain a caloric restriction when they were not fasting, whether they fasted completely on fasting days or could maintain some specified caloric intake, and some studies evaluate data from Ramadan, a religious fast.

Lastly, are the positive results we’re seeing from intermittent fasting specific to intermittent fasting or the result of overall caloric restriction?

Final Consensus on Intermittent Fasting

In my opinion and from the research I reviewed (both outlined here and numerous studies I reviewed but did not mention here), it appears intermittent fasting is safe, both mentally and physically, and may result in some positive health benefits in terms of weight control and potentially lending to anti-aging or improved longevity. To me it does not appear to be better than caloric restriction on any front, but it might be easier for some to follow at least in the short-run. It is unlikely to be sustainable for a majority of individuals for long-term use. Further randomized, controlled trials in humans need to be conducted with more participants, and studies evaluating the effectiveness of intermittent fasting methods against one another also need to take place.

That said, I wouldn’t necessarily recommend intermittent fasting (full disclosure: I do fast once monthly for religious purposes myself) because I’m a bigger fan of sustainable changes for long-term results, but I’m not opposed to someone who feels strongly they would like to try intermittent fasting. For that individual, I would probably recommend a multivitamin with minerals to maintain micronutrient stores if they are fasting completely on some days (like a 5 & 2 intermittent fast) and suggest they first get clearance from their primary care physician and secondly check in regularly with a Registered Dietitian Nutritionist.

I hope you found this intermittent fasting diet review helpful. I wasn’t able to dive into every detail here (like diabetes, changes in blood lipids, intermittent fasting for athletes, etc.), but let me know if you have more questions you’d like to address and I’m open to a Part 2 blog post on intermittent fasting!

Have you ever tried intermittent fasting?

What other questions do you have about intermittent fasting?