Updated 13th July 2023

The Big IF Study: What did we find?

Written byZOE Editorial Staff
Reviewed byKate Bermingham, PhD

In October 2022, ZOE launched the largest-ever community experiment into intermittent fasting (IF). It’s called The Big IF Study

More than 140,000 people signed up to see whether IF might work for them.

We shared some very early findings with you at the start of February 2023, but we know you’re eager for more. So, let's dive in.

First, for anyone who needs it, here’s a quick recap.

Why IF?

Previous studies have shown that there are health benefits to IF. These could include improved gut health, better blood sugar control, lower cholesterol, and weight loss. 

Although these early findings are promising, up until The Big IF Study, most of the research had only recruited fairly small groups of people. 

Also, these studies tended to focus on people with chronic conditions rather than the general population.

So, in true ZOE fashion, we wanted to dig deeper. And The Big IF Study was born.

There are many forms of IF, but we focused on time-restricted eating (TRE).

TRE is a flexible method that focuses on the timing of eating rather than the quantity of food or caloric intake. So, rather than changing what you eat, you change when you eat. 

If they wanted to, participants could eat the same foods in the same amounts as usual. They just needed to eat it within a given time frame each day.

What did we do?

As part of The Big IF Study, we asked contributors to eat during a 10-hour window. 

It didn’t matter what time they started eating, but they needed to have their final morsel of the day within 10 hours of the first. 

For example, you might eat breakfast at 7 a.m. and finish your final meal by 5 p.m. Or you might only eat between midday and 10 p.m.

Outside their eating windows, participants only consumed water, black tea, and black coffee.

For the first week, our participants ate as they normally would. Then, for the final 2 weeks, they switched to the 10-hour eating window.

They also had the option to continue IF for longer, if they wanted.

OK, that’s enough of the preamble. So, what have we found?

Results from The Big IF Study

In total, an incredible 37,545 people completed the 3-week trial. Of these, around three-quarters were female. And the average age was 60.

On average, participants reduced their eating windows by 2 hours.

Sixteen weeks later, more than 5,000 of these contributors were still eating in their 10-hour windows because they loved it!

Psychological changes

On average, people experienced an 11% increase in mood and a 22% increase in energy, compared with their baseline before starting IF. 

Other mood-related symptoms, like feeling nervous, also improved during IF.

And there was a 6% reduction in feelings of hunger.

People who extended their TRE beyond the 2-week mark continued to feel these benefits.

Gut health

Around 64% of the people in the study reported that symptoms of bloating improved, with a reduction by an average of 11%.

We did measure a slight increase in headaches, but it wasn't statistically significant. In other words, it could have been down to chance. Theories as to why include low blood sugar and dehydration.


IF also influenced how much people snacked.

Overall, 58% of participants consumed fewer snacks during the IF intervention, with participants consuming on average 1 less snack per day compared with their baseline.

However, there was variation depending on when people closed their eating windows. In our analysis, people who closed their windows after 6 p.m. were considered "late TREs."

Individuals in this group tended to eat more snacks than those in the mid-TRE group, who closed their eating windows before 6 p.m.

We also found that people who are night owls tend to be more unpredictable snackers.

What’s next?

We plan to publish the results in a scientific journal in the next few months. 

So, watch this space — we’ll make sure to keep you informed as we go.

And, most importantly, a huge thank you to everyone who took part. We quite literally couldn’t have done it without you.


Dehydration and headache. Current Pain and Headache Reports. (2021). https://link.springer.com/article/10.1007/s11916-021-00966-z 

Fasting headache: A review of the literature and new hypotheses. Headache. (2009). https://headachejournal.onlinelibrary.wiley.com/doi/abs/10.1111/j.1526-4610.2009.01390.x 

The International Classification of Headache Disorders, 3rd edition (beta version). Cephalgia. (2013). https://journals.sagepub.com/doi/10.1177/0333102413485658