With all the reports about the effectiveness of an intermittent fasting schedule for weight loss along with other benefits, you may be surprised to learn that researchers have not yet identified hard and fast rules that can be universally applied. But if you’re interested in trying this way of eating, there are guidelines you can use as a jumping off point.
The terms intermittent fasting and time-restricted eating (or time-restricted feeding) are often used interchangeably in the media and, based on the research done so far, we know that the potential health benefits of the two approaches are the same, but there are differences between the two:
Taking a 30,000-foot view, the primary benefits are improvements to both the metabolic and the immune systems. Overeating, especially when the foods are processed, can induce inflammation and put a strain on metabolism. Studies indicate that both IF and TRE may lead to better blood sugar and cholesterol control and, because of calorie restriction, weight loss. Even more impactful may be the reduction in systemic inflammation, thought to be at the root of so many chronic diseases. Much current research is focused in this area.
One theory about IF and TRE is that they give the body a rest from the work of digesting food and allow it to recalibrate, so to speak, and stay in fit form. It’s somewhat like training to run a marathon. You wouldn’t train by running a marathon every day: That would leave you exhausted and likely to crash and burn. You need regular breaks.
For most of human evolution, we did not have constant access to food, and the body was not designed to eat late into the night. The current overabundance, or overnutrition, that we experience certainly in this part of the world is relatively new, and our bodies may not be particularly well suited to it—it goes against our internal body clock. In some ways fasting represents a return to the more natural state that humans experienced for most of recorded time.
When it comes to time-restricted eating, arguably the best-known work has been and is being done by Satchin Panda, PhD, professor and the director of the Regulatory Biology Laboratories at the Salk Institute for Biological Studies. His lab found that eating within an eight- to 12-hour period, which people did as recently as 100 years ago, might stave off chronic conditions such as obesity, high cholesterol, and diabetes. Some research done at other institutions suggests that the more restrictive eight hours should be the goal.
Mark P. Mattson, PhD, of Johns Hopkins published a study in 2019 that found considerable health benefits from eating within just a six-hour period each day. Others are looking at whether time-restricted eating can help conditions such as type 2 diabetes and fatigue from cancer.
It’s important to know that, while different regimens have been tested in various (and often small) studies, there has not yet been a comprehensive side-by-side comparison of them to answer questions such as: What is the ideal number of hours per day for eating? At what time of day—such as morning or afternoon—should eating start? Also, most studies have been done on mice, not people.
With so many unknowns, choosing a timetable is going to be highly personalized (and so will be the results). You want to find the right balance for you when choosing a new eating pattern. Anecdotally, I will share that, to combat the metabolic slowdown and resulting weight gain that typically occur after age 50, I recently chose to stop eating lunch on most days—a completely different option from typical IF and TRE patterns. I’m a breakfast eater, so I didn’t want to delay my first meal of the day. I use the noon hour to be productive and don’t have to succumb to the lunch cart at the office. And then I really enjoy dinner. After five months, I’ve experienced GLP-1-type benefits for weight loss without taking a GLP-1 drug.
It’s always a good idea to talk with your primary care doctor as well as a registered dietitian or nutritionist before embarking on a major dietary change. Most physicians are aware of intermittent fasting, but each one may have his or her own view of the best approach. A nutritionist can help you develop a balanced, step-by-step plan to reduce the number of eating hours or calories and help you achieve a lifestyle change that is sustainable and that makes you happy and not “hangry”.
He or she can offer advice to stop the common knee jerk reaction of, “I’m going to eat twice as much during my eight hours than I would have normally” as well as make sure the food you do eat is as healthful as it can be. Here are more tips that may help:
A 12/12 schedule might be a better strategy for you than the more restrictive 8/16 schedule, especially when starting out. If you’re a grazer who likes to eat small portions throughout the day, rather than set meals, your focus might instead be on general caloric restriction. Decide what you can cut out and/or what lower calorie food trades you can make to still feel satisfied.
Change takes time, so give yourself some leeway and don’t berate yourself if you find that you need to eat beyond your goal time restrictions. Full disclosure: I do have lunch on occasion.
It’s common to want to make drastic changes, especially after hearing that your HbA1C, cholesterol, and/or blood pressure is too high. But you may have better success with a more gradual approach. For instance, reduce the number of eating hours by one hour at a time over a period of weeks or months as needed. It takes time to rewire the region in the brain responsible for pleasure, especially if you’ve been feeding it with a nightly 10 p.m. bowl of ice cream.
Wearable health tracking devices, like the Apple watch, offer valuable health information, such as improvements in sleep and heart rate variability, a measure of your heartbeat. I have colleagues who are motivated by positive feedback from such devices. They work hard to see improvements, which helps them stay on track.
Seeing your weight drop on the scale is a nice indicator, but weight can fluctuate back and forth, and not everyone loses weight easily. A more concrete motivator is seeing improvement in blood markers, like HbA1C and cholesterol. IMPORTANT: It won’t happen in two or three weeks, but it will over three to six months so, again, give yourself time.
These options are all “intermittent.” Prolonged fasting, on the other hand, can cause more damage than good because the body will eventually go into starvation mode and different systems will start to shut off.
My colleague Miriam Merad, MD, PhD, published research three years ago showing that monocytes, immune cells created in bone marrow that circulate in the blood at the ready for healing after an injury and infection, seem to retreat or hibernate during prolonged fasting. In other words, if you over fast, you risk dysregulating your immune system to the extent that it may not respond properly when you need it to kick in. The take-home message: balance.
