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What is 16:8 Intermittent Fasting?
16:8 is a form of time-restricted feeding* (TRF) that involves fasting for 16 hours and eating all of your calories within an 8-hour window. It is one of the most popular forms of time-restricted feeding (TRF), likely because it’s manageable enough to make it a daily practice. Just like all other forms of fasting, 16:8 is not a diet. Instead, it is a pattern of eating – it’s the “when” independent of the “what.”
*TRF is defined as an eating pattern that restricts all caloric intake to certain hours of the day and allows a daily fast of at least 12 hours.
What are the benefits of 16:8?
Circadian rhythms are 24-hour cycles of hormones and other signals within the body largely governed by the brain’s central clock in response to the light/dark cycles of the sun. Even under constant light or dark conditions, however, the same rhythms will fluctuate on a 24-hour (or near-24 hour) schedule. So what are the other inputs that keep our clock functioning on this schedule? One major factor is when we eat. When we eat out of alignment with our circadian rhythm, for example, in the middle of the night when our body is not expecting food, digestion, absorption of nutrients, and metabolism can be compromised, interrupting the natural ebb and flow of signals that tell your body to rest and repair. A consistent 16:8 fasting/feeding pattern that overlaps with our sleep/wake cycles aligns the nutrient signals with the light signals to keep your biological clock finely tuned. Ultimately, this enhances the robustness of our circadian biology, which offers several health benefits ranging from better sleep and digestion, to glucose control, brain health, and so much more.
Eating within an 8-hour window can be a great weight loss strategy because it can support a natural caloric deficit without forcing you to think too much about limiting calories. We often don’t even register the calories we consume from late-night snacks or beverages, but they can add up over time! By sticking to an 8-hour eating window that prevents these tendencies, daily caloric intake is often reduced, resulting in the potential for fat loss and improved body composition.
Anti-Aging and Disease Prevention
Over the course of the 4 -16 hours that follow your last bite of food, blood glucose levels fall, insulin slowly drops, and liver glycogen depletes as your liver churns out glucose to keep your blood sugar levels stable. At the same time, low insulin levels permit the release of stored body fat, so the body starts prioritizing fat for fuel. Depending on what you ate before your fast, your activity level, your metabolic health, and body composition, you may even start generating small amounts of ketones toward the end of this 16-hour window as a result of revving up fatty acid metabolism. This transition to burning fat reminds your cells each day what it’s like to burn something other than glucose, and over time, our cells adapt to burning fat in ways that promote greater metabolic flexibility and healthier mitochondria. Since emerging research suggests that healthy mitochondria and metabolic flexibility are protective against a series of chronic diseases – in part due to their association with reduced inflammation, reduced oxidative stress, and improved glucose levels – daily 16-hour fasts may be a practical way to promote better metabolic health, and in turn slow aging.
The interior of our GI tract – often called the “gut lining” – is a single layer of cells, making it very vulnerable to damage. When this lining is impaired, pathogens can find a way through to our bloodstream, triggering an immune response and ultimately generating inflammation. Fasting for 16 hours a day gives the gut time to rest and repair itself from a hard day of work, likely improving the integrity of the gut lining and reducing inflammation in the gut. Eating on a consistent TRF schedule also supports the circadian rhythm of the gut (yes, the gut microbiome has a clock too!), collectively, promoting better gut health.
Special Considerations for 16:8
Intermittent fasting of any type is not recommended for those with higher nutritional needs, such as children, people who are underweight, and/or women who are pregnant or breastfeeding. Beyond these exceptions, a 16:8 eating schedule is a very safe style of intermittent fasting – 8 hours should be plenty of time to consume the nutrition your body needs without any adverse side-effects. Below are a couple of special considerations to avoid any potential complications with 16:8.
Be mindful of Calorie intake
Those with higher caloric needs and/or those who are not looking to lose weight may need to make a conscious effort to meet their nutritional requirements. Because a 16:8 TRF can support a spontaneous caloric deficit, there’s a chance you could find yourself in a situation where you are undereating daily. It is essential that you still consume adequate nutrition each day, just within your 8-hour feeding window, and not drop calories to the point that you are experiencing symptoms of under-fueling (e.g., fatigue, impaired athletic performance, etc.).
Athletes can derive plenty of benefits from 16:8, especially when body composition contributes to their performance. This might seem obvious, but performance athletes generally have a higher caloric need than the rest of us mere mortals, so they need to be especially cognizant of caloric intake. Those with athletic goals may also need to adjust their eating window to optimize performance. Ideally, training takes place just before breaking your fast or within your eating window to support recovery needs, especially if building muscle and strength is your goal.
When should you eat on 16:8?
Far and away, the most popular 16:8 schedule involves skipping breakfast and starting an eating window around lunchtime, for example, 11AM – 7PM or 12PM – 8PM. This leaves room for dinner, which is typically the most social meal of the day and not the easiest to skip. That said, if you enjoy breakfast, and prefer an early dinner, opt for that. There may be even greater benefits to an earlier eating window often called “early TRF” or “eTRF”. In general, it’s best to eat somewhere between sunrise and at least 2-3 hours before your head hits your pillow.
What should you eat on 16:8?
There are no explicit guidelines for what to eat on a 16:8 TRF. It all depends on your goals! We always recommend pairing fasting with a high-quality whole-food diet and avoiding empty calories from highly processed foods. If you want to support a deeper state of ketosis and smoother transitions into the fasted state, you can pair 16:8 with a low-carb ketogenic diet. Metabolically speaking, fasting and ketogenic diets are cut from the same cloth with many overlapping benefits. But if a ketogenic diet doesn’t work for you and you find carbohydrates do not interfere with your ability to fast, dig in! Just make sure they aren’t full of sugar and processed junk. Your primary focus should be ensuring you are consuming enough protein, calories, healthy fats, and essential nutrients within your 8-hour eating window.
Using 16:8 as a tool
Generally, 16:8 hits the sweet spot between an overnight fast and a longer daily fast. It’s popular for a reason! Most people find that with a little willpower, this can be a great tool to make fasting a daily routine so you can reduce reliance on calorie counting. It can also be a tool for keeping you on track during the week so you can enjoy a more liberal eating window on weekends if that suits you, for weeks or months at a time, or maybe for the sake of simplicity when times get busy. The beauty of TRF, specifically a 16:8, is it allows for flexibility in your diet so you don’t need to obsess over your food quite so much as you otherwise might.
How to start 16:8
Most people won’t find jumping into 16:8 too dramatic, but if you are new to intermittent fasting and want to start slow, practice shorter fasts and work your way up. For example, start with a week of 13:11, followed by a week of 14:10, eventually graduating to 16:8. The Coach function in Zero should help you work your way up to 16:8 if it’s right for your goals. Working your way up will train your metabolism to burn fat and ketones more efficiently, making 16:8 feel like a breeze.
Many people find 16:8 a very reasonable style of intermittent fasting, used to derive the benefits of fasting without significantly interfering with regular daily routines or social life. Once you get the hang of it, 16:8 often gets swept up as part of your lifestyle. It can be used as a potent weight loss tool that eliminates the pressure of restricting calories or foods, but can also be a simple strategy to improve your health by aligning when you eat with your natural circadian rhythm. Ultimately, 16:8 can be helpful for better digestion, sleep, rest, and recovery. Our pro tip is to be mindful of how much you are eating within your eating window. For those pursuing weight loss, avoid restricting calories too much and for too long, and for those who are not looking to lose weight, avoid inappropriate long-term caloric restriction. It is best to position your eating window between sunrise and at least 2-3 hours before bed to give your body enough time to digest your food properly and lower your risk of disrupting sleep by eating too late. Lastly, we always recommend pairing intermittent fasting with a high-quality whole food diet. The combination of 16:8 with a healthy diet is a fantastic way to better your health.
About Kristi Storoschuk
Kristi Storoschuk, BSc (Hons.) is a science communicator with a research focus on ketogenic diets, metabolic therapies, and fasting for health optimization. She currently works alongside the world’s leading ketogenic researchers providing scientific education for the mainstream audience. Outside of her research, you will find her traveling the world, doing CrossFit, and optimizing her health through diet and lifestyle.