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Updated 7th November 2022

The Big IF Study: Does the time you eat make a difference?

Written byYella Hewings-Martin, PhD
Reviewed byKate Bermingham, PhD

    Download the free app now to try intermittent fasting with the 100,000 people who have already started the study.

    ZOE’s mission is to understand and improve health for everyone, whether that’s through the ZOE Health Study or our work on nutrition and the microbiome

    Using our community science approach, we’re now investigating how our lifestyles affect every aspect of health. Together with our Contributors, we’re tackling some of the biggest health challenges we all face.

    ZOE’s latest study — The Big IF Study — looks at intermittent fasting, which some people call time-restricted eating. We know that what you eat makes a difference to your health, and we’re now studying if when you eat makes a difference as well.

    We’re inviting all Contributors to take part to see if changing when they eat can benefit their health. You can sign up here.

    What’s intermittent fasting?

    Intermittent fasting is a way of eating that involves eating and fasting for set periods of time. 

    There are a few different types of intermittent fasting. Time-restricted eating is one of them, and it’s the most popular. 

    It’s important to note that time-restricted eating is different from the 5:2 type of intermittent fasting, where you reduce your calorie intake to 500–800 kcal on 2 days per week and eat your regular diet on the other days.

    Doing time-restricted eating means that you eat during certain hours of the day and fast — or don’t eat — the rest of the time. For most people, this way of eating means extending the natural fast that happens when they sleep. 

    With time-restricted eating, you can pick which eating window best suits you and your lifestyle, and it doesn’t involve changing how much or which foods you eat. Instead, you eat your regular meals during a set window of time. 

    If you fast during a 14-hour window and eat during the remaining 10 hours, you’re following a 14/10 time-restricted eating pattern. 

    This could mean that you eat your first meal at 8 a.m. and start your fast at 6 p.m., after your last meal. Or you might eat your first meal at midday and start your fast at 10 p.m. 

    There are also 16/8 and 20/4 versions of time-restricted eating. 

    What does the science say about it so far?

    Many people who eat a typical Western diet have their food during a 12-hour window each day, with dinner and post-dinner snacks making up nearly 45% of their average daily energy intake.  

    But scientists are increasingly seeing links between meal timing and health. The science of how food and meal timings interact with your body clock is called chrononutrition.

    The time you spend not eating is one aspect of chrononutrition. Scientists investigating this have shown that a number of things happen to your body when fasting:

    • Your body can swap from using sugar to using fat for energy.

    • Your cells can improve their resistance to stress and disease.

    • The beneficial bacteria in your gut may increase.

    When you sleep, your gut activity also slows down. This allows time for your gut microbes to clean up your gut lining and keep it healthy. Extending the natural fasting window that happens when you sleep may help strengthen your gut barrier

    Research also indicates that fasting may have several other long-term health benefits. 

    These health benefits include better blood sugar control, improved heart health and cholesterol levels, as well as weight loss. There’s also some evidence that fasting may help with brain function during aging and could reduce the symptoms of mental health conditions like anxiety and depression.  

    A recent study with 150 firefighters doing shift work saw the participants’ health and quality of life improve after they followed a 14/10 time-restricted eating pattern — eating between 9 a.m. and 7 p.m. — for 3 months. 

    Why are we running The Big IF Study?

    Most of the studies about intermittent fasting have included relatively small groups of people or have combined time-restricted eating with a lower calorie intake. And often, the participants had one or more chronic health conditions. 

    What’s not clear from these studies is how much the findings will apply to the broader population. 

    The ZOE Health Study is different. Together with all of you, we can do research on a much larger scale. 

    This unique intermittent fasting study aims to be the largest research study in the world yet. With it, we’re examining whether time-restricted eating has benefits for individuals, focusing on hunger, energy levels, mood, and other daily symptoms in the real world, rather than in a clinical trial setting. 

    If you take part in The Big IF Study, you’ll be following a 14/10 time-restricted eating pattern. You won’t need to restrict your calorie intake, and you can pick the 10-hour eating window that suits you best. 

    The study will last a minimum of 3 weeks, with an option to extend for longer. During the first week, you’ll log how you feel and when you eat your first and last meal each day — all while following your regular eating pattern. 

    This allows our scientists to understand your regular eating times and measures of health. 

    After that, you’ll practice time-restricted eating for 2 weeks or longer. This means eating your regular meals within a 10-hour window. 

    During the study, we’ll ask you to log your mood, energy, hunger levels, and health symptoms each day so we can see if these change when you are intermittently fasting.

    After the study finishes, we’ll play back your personal data insights, as well as those of the wider community. 

    By participating, you’ll contribute to important scientific research by being part of the largest intermittent fasting study in the world.

    You can also understand whether time-restricted eating works for you and your body, and whether it benefits your mood, energy, and hunger levels. 

    Who can join The Big IF Study?

    All Contributors to the ZOE Health Study app in the U.K., who are at least 18 years old and who are reporting for themselves — and who have opted to take part in ZOE’s Wider Health Studies — can join The Big IF Study. 

    When you sign up to the study, we’ll ask you to give your consent to take part. 

    There are some groups of Contributors that we recommend check with their doctor before taking part:

    • if you have type 1 diabetes

    • if you have a history of disordered eating

    • if you have an uncontrolled chronic medical condition

    • if you are pregnant or breastfeeding

    Also, if you're currently in the testing phase of your ZOE nutrition program, we recommend that you don't sign up for the study until 3 months after you've started. This ensures that you get the most from your ZOE program.

    How can you get involved?

    You can take part in The Big IF Study through the ZOE Health Study app from the 24th of October 2022.

    Join The Big IF Study here.

    You can start the study at any time. Pick the day that best suits you. 

    During the first week of the study, we’ll ask you to eat as usual and report daily, as before. You’ll log your usual meal times, your hunger and energy levels, and your mood. 

    In the following 2 weeks — or more if you decide to extend the study — you’ll eat as usual during a 10-hour eating window. We’ll ask you to continue logging your daily report, energy levels, and mood, as well as the time you start and finish eating each day.  

    This means if you have breakfast at 8 a.m., you’ll start your fast by 6 p.m., to make sure you’re eating during a 10-hour window and fasting during a 14-hour window. 

    Or you could choose to do a late eating window, starting your first meal at midday and starting your fast by 10 p.m.  

    You can change the time you begin your 10-hour eating window during the study. We want you to explore what works best for you. 

    If you're not in the U.K., don’t worry — you can still sign up and we’ll keep you updated if the study becomes available in your area. 

    You can also check out The Big If Study facebook community page here.

    What can you eat during The Big IF Study?

    During your fasting window, you shouldn’t eat any food. You can drink beverages that don’t have any calories, including tap, still, and sparkling water, as well as tea (including herbal tea) and coffee without milk, sugar, or sweeteners.

    During your eating window, we recommend that you eat your normal meals and snacks. You don’t need to change or reduce the amount of food that you eat during the study. 

    What can you expect to happen?

    Many people follow intermittent fasting because they want to improve their health. 

    When people first start fasting, it’s not uncommon to experience fatigue and some irritability. For most people, this goes away quickly. 

    Based on the research around intermittent fasting so far, we anticipate that, overall, you may actually experience improvements in your energy levels and your mood. 

    However, we don’t know if this will be the case for everyone, and that’s what we want to find out.

    We know from our research that people are unique and respond differently to food and meal times. By taking part in The Big IF Study, you can help us understand how time-restricted eating works in a real-life setting, what type of eating windows people prefer, how they feel, and whether their daily symptoms change. 

    We’ll share our findings with you regularly to keep you updated with our latest insights.