One of the popular strategies for weight loss over the past decade has been intermittent fasting, or as some researchers call it, time restricted eating.
In case you don’t know what this is, it’s simply setting a restricted schedule around when you eat and when you don’t. Studies have shown that the typical American spends more than 15 out of every 24 hours in a “feeding” state, meaning that they eat shortly after waking in the morning, and then consume multiple meals and snacks throughout the day right up until shortly before bedtime. As a result, the longest stretch they ever go without eating in a typical day is no more than 9 hours, most of which consists of the time they are asleep overnight.
By contrast, somebody who is intermittent fasting might follow one of the following schedules:
Fasting after waking until 12 noon (water and non-caloric drinks such as black coffee, tea, and sparkling water are ok), and then cutting themselves off of all calories after 8pm, so that they are eating 8 hours out of the day and fasting for 16 hours out of the day (this is called “16:8” intermittent fasting).
Eating only once per day, for example fasting after waking until 4pm, then having a large meal around 4:00 pm, and then fasting again until 4:00pm the next day (this is called “OMAD,” an acronym for “One Meal A Day”)
Not eating one day out of the week, for example finishing dinner at a normal time on Sunday night, eating nothing again that evening, eating nothing all of Monday, and not eating again until breakfast Tuesday morning. (This is referred to as a “36 hour fast”)
There are many other fasting protocols that have been tried. I simply mention the above because these are some of the more popular and common ones.
As with any lifestyle or medical intervention, the two obvious questions about intermittent fasting are: 1) is it safe, and 2) is it effective?
The first issue is relatively straightforward: yes, fasting is generally safe. Pregnant women, children, and people who take medication for chronic medical conditions such as diabetes should consult with their doctor before embarking on a fasting regimen, but for the average reasonably healthy adult there is no known harm to fasting.
And if we think about it logically, why would there be? The idea of eating three meals per day only came into vogue in the 1800s, and snacking between meals is an even newer phenomenon that became popular in the past 50 years. For almost all of human history, it was standard practice to eat one or two meals a day with no snacks in between (in ancient Rome, for example, it was considered a sign of gluttony to eat more than once per day). And our hunter-gather ancestors would have frequently had the experience of going several days between meals, as food could often be scarce.
In modern times, experiments have been performed in which people fasted for multiple days without any serious ill effects, and the US military teaches the “rule of 3s” to soldiers as a way to help them prioritize in a survival situation, namely: “A human can survive three minutes without oxygen, three hours without shelter, three days without water, and three weeks without food.” So in sum, if you are interested in fasting and aren’t sure it’s for you – especially if you take medications for a chronic condition – check with your doctor. But otherwise you aren’t going to cause yourself any serious harm by skipping a few meals. (Do make sure to drink water while fasting though – staying hydrated is important.)
The second question, about whether fasting is beneficial, is more nuanced. Studies in animals have shown some unique benefits of fasting. For example, in mice who fast, a process called autophagy (a type of cellular “clean up” mechanism inside the body that heals injury and reduces cancer risk) is increased during fasting. The problem with this sort of data however is that humans are obviously different from animals, and moreover we live on very different timelines – since a mouse may only live a year or two whereas humans live on average for many decades, a sixteen hour fast in a mouse might be the equivalent of a several week fast in a human. Therefore a lot of the unique benefits of fasting that are seen in animals may not apply in humans.
Where fasting does really seem to shine for some people is as a calorie control mechanism. Research shows that, when controlling for calories, there is no real difference between people who fast versus people who eat frequently throughout the day. However, many people have a much easier time eating fewer calories by skipping meals than they do by eating frequent small portions.
As a personal example: I’m rarely hungry in the morning and it’s no challenge for me to just have a black coffee and some water, and not eat until lunch time. Moreover, if I do eat breakfast, it doesn’t seem to make me any less hungry at lunch or dinner. So skipping breakfast is an easy way for me to cut a few hundred calories out of my day. I’ve met many patients over the years who have the same experience. I’ve even met people who have no real struggle going 24 or 48 hours once a week without eating. I would find it very difficult to adhere to this schedule, but we are each unique. And speaking of being unique, I have also known patients who really don’t do well with any type of fasting, and are much more successful on a regimen of frequent small meals.
The key point about fasting is that it is useful mainly in so much as it helps somebody to manage their calories. For this reason, it often works best when paired with another dietary strategy, such as being low carb, or counting calories, during the eating window. By contrast, if you fast for 18 hours out of day and then spend the next six hours shoving thousands of calories of hamburgers and ice cream down your throat, you are probably going to gain, not lose, weight.
In summary, fasting is like a lot of lifestyle interventions. It’s a useful tool that works really well for many people, but it is not for everyone. The key to maintaining a healthy body weight is to find the collection of tools that work best for you. If you’ve never tried it, it’s probably worth a go to see if fasting is helpful for you.
