Studies show that this eating approach can yield a number of impressive results: weight loss1, increased fat burn, improved cognitive functioning, reduced inflammation, and potentially even increased longevity. We asked some leading intermittent fasting experts to weigh in on the topic so you can better tailor your intermittent fasting plan according to your specific goals. “There are very few definites here,” says Vincent Pedre, M.D., an integrative physician and gut health expert who frequently recommends intermittent fasting diets to his patients. “Where you start seeing those benefits depends on what you eat, how healthy your gut health is, what kind of physical activity you’re doing, etc.” For example, letting calories slip into your fasting window, overeating calorie-dense foods like pizza during your eating window, not prioritizing sleep or physical activity, or making one of these intermittent-fasting mistakes could all delay or counteract the benefits of intermittent fasting. Even if you’re formulating the rest of your diet and lifestyle perfectly, it’s important to understand that the different health benefits associated with intermittent fasting will likely kick in at different durations of fasting. For example, weight loss may be triggered by a relatively short fasting window, while entering ketosis or triggering autophagy—that “self-cleaning” cellular process that boosts brain functioning and maybe even longevity—may take significantly longer. Unfortunately, not enough research has been conducted on all the different forms of intermittent fasting to provide any hard-and-fast rules. Based on personal and patient experiences, however, the doctors and researchers we spoke with were able to offer a general idea of how long you may need to fast to reap certain benefits, which we’ll dive into below. “Twelve hours [per day] is the minimum in my experience,” says integrative physician Amy Shah, M.D. “There’s research showing2 a 34% reduction in breast cancer recurrence with approximately 13 hours of fasting per day.” This may be thanks in part to the improved blood sugar regulation3 that occurs with this level of fasting. Shorter fasts like this will also reduce your exposure to gut-associated toxins, like endotoxin, says Pedre, which is a harmful byproduct of bacterial death that contributes to metabolic syndrome, obesity, and inflammatory diseases. A daily 12-hour fast can also be beneficial for weight loss, adds Shah, but it would likely take a longer fasting period to enter ketosis or trigger autophagy. “For most people, this seems to be the sweet spot; others will need slightly longer to get those weight loss and other benefits,” he says. Shifting into the fat-burning state known as ketosis can also occur during this range. While the specific point at which you enter ketosis4 will depend on a number of factors, including what you ate during your last meal, this typically happens within 12 to 22 hours of a meal—once you’ve burned through your glycogen stores (i.e., the carbs stored in your muscles and liver). Pedre agrees that around 16 hours is a good daily fasting window for many (i.e., confining your eating to an eight-hour time frame and known as 16:8 fasting), but, he says, “You might not start seeing bigger benefits like autophagy until you reach 24 hours.” Autophagy is the natural process by which cells disassemble and clean out or recycle unnecessary or dysfunctional components so you can create new healthy cells. Autophagy reduces oxidative stress and inflammation, and taking steps to enhance autophagy may help reduce the risk of getting a number of chronic illnesses. So if you do want to experiment with these longer autophagy-inducing fasts, you’ll have to strategically space them out. “My opinion is that a longer [24-hour] episode without any caloric intake done a few times per month over decades is likely the best approach for preventing chronic diseases because it is feasible to do consistently over a lifetime—the period of time that chronic diseases like coronary artery disease and diabetes develop,” says Benjamin Horne, Ph.D., a genetic epidemiologist who conducted a trial evaluating the effectiveness of a once-per-week 24-hour water-only fast. Fasts up to 36 hours have also shown promise for people with type 2 diabetes, says Pedre, but these would absolutely need to be done under the supervision of a medical professional. “What matters more, I think, is the duration of fasting that works for you,” says Pedre. “If you can’t stick with it or it isn’t working for whatever reason—say, you can’t concentrate—that’s a sign benefits are starting to drop off and you may need to scale back.” “Increase your fasting time to a few days a week or do a longer fast one day a week,” suggests Pedre. “In the end, what works best is mixing it up for your body and keeping it guessing when food is coming.” Also, be sure you know exactly what breaks a fast to ensure you’re getting the maximum possible benefits.
