Google ‘intermittent fasting’ and you’ll see a slew of advice, tips, and secrets on the best ways to integrate this supposedly foolproof health trend into your daily life. But the more you dig into the research on this nutritional plan, the more absurdly confusing it all becomes. Is eating every other day best? Eating only before 3 p.m.? Packing all your calorie consumption into a 2-hour window? Or should you eat normally six days of the week and only starve yourself on the seventh?
While people have been practicing intermittent fasting for thousands of years, it’s only in the past two decades that scientists began to understand what, if any, potential benefits the practice can have. Preliminary studies on mice and observational ones in humans suggest that skipping out on eating might translate to weight loss and, at least in some cases, improved metabolic health.
Here’s the good news: According to recent studies, nearly all types of intermittent fasting are physically and mentally harmless. And nearly all of them can result in some weight loss.
The bad news: There’s no evidence to show that intermittent fasting can result in more weight loss or superior health metrics compared to plain old continuous caloric restriction, which is simply eating fewer calories for an extended period of time. Plus, nearly all the best studies done to date have used mice, not humans. That can be a big problem; humans are physiologically similar to mice, but they are also different in many important ways.
The evidence is even murkier when you move away from weight loss goals and into broader health ambitions. When it comes to things like diabetes management and the prevention of cardiovascular disease, most trials have observed middle-aged or older people, or those with a chronic metabolic illness, like diabetes, obesity, or some type of heart disease. In general, researchers study healthy people far less often than they do people with a particular disease. One big problem with studying healthy people is that it is much harder to see improvements through a particular treatment regimen (intermittent fasting, for example) since positive changes will be relatively minimal. But the result is that there have not been enough studies done with enough healthy people or for a sufficiently long period of time to show how this pattern of eating could prevent disease.
What is intermittent fasting?
Intermittent fasting is, in its simplest terms, abstaining from eating or drinking for a period of time. By that definition, pretty much all of us practice it; there’s a reason we call the first meal of the day breakfast. In fact, looking at the physiological changes that occur from just that overnight fast can tell us why fasting might have a positive benefit on our health. Even fasting overnight, some studies have shown, can reduce concentrations of certain metabolic biomarkers like glucose, insulin, and other hormones. That’s precisely why doctors often force your to fast for eight to 12 hours before a blood test. That abstaining period gives your body time to reach a state where it’s not influenced by food. The obvious question, then, is whether doing that more often can positively benefit our health.
What are the different kinds?
There are a variety of takes on intermittent fasting. But there are two popular plans that involve significant eating restriction, and scientists have found similar effects from both.
Alternate Day Fasting:
People following this diet eat normally on one day and fast (consuming zero calories, or close to it) the next. That can’t be fun, but is it worth it?
A few studies have looked into the physiological effects of this diet, both in mice and in humans. A 2007 review of all the mouse studies out at the time found that alternate day fasting did cause mice to lose weight, and made their insulin and glucose concentrations—which are important factors for controlling or preventing diabetes and obesity—go down. However, the weight loss was about the same as a more traditional calorie restricting diet would provide.
Most of the studies done in humans have found similar effects: modest weight loss and a decrease in one factor involved in regulating our blood sugar levels. However, all these studies were done in groups of 30 or less. That small sample size makes it difficult to translate the results to the general population. And a final caveat: in most of the studies, researchers found that the participants reported feeling “extreme hunger” while fasting.
Modified Fasting Regimens:
This type of intermittent fasting is similar to a regular calorie restrictive diet, but much more extreme and only on certain days. On scheduled “fasting days,” practitioners consume calories to meet just 20 to 25 percent of their energy needs. There’s no set number of fasting days, but people on this diet often restrict for two days in a row followed by five normal eating days.
Does science say there’s a benefit? In one study in mice, researchers restricted rodents’ diets by 85 percent every other day. By the end of the study, their fat cells had shrunk, they lost weight, and many of the hormones that regulate body weight had leveled out.
In humans, one 2017 review analyzed nine trials looking at modified fasting in humans. Some good news: In seven of the nine trials, participants had, on average, shed pounds by the end—3 percent to 8 percent of their body weight. More good news: Overall, people reported improvements in mood and decreases in tension, anger, and fatigue. But these studies all had small sample sizes, and every participant was either overweight or obese at the outset—making it difficult to use the results to draw general conclusions—and they were short, too; the longest study lasted six months. Plus, the weight loss was about the same as you’d see in a similar group doing simple calorie restriction.
Bottom line: For weight loss, based on the data currently available, intermittent fasting is not any better than the standard “continuous calorie restrictive diet.” For healthy, normal weight people, there aren’t enough studies to determine whether it provides an added health benefit.
Is there a better method?
There’s another type of fasting (sort of) that’s worth mentioning: time restrictive eating. A person fasts for anywhere between 12 and 21 hours per day, and only consumes food in the time between.
This is based on the idea that all organisms, humans included, have circadian clocks that regulate how their bodies function, down to every organ and organ system. As such, there are times at which various bodily functions perform best, including the organs and systems involved in diet, like the liver, the endocrine system, and the digestive tract.
In terms of diet, consider this: Your pancreas produces the majority of its insulin in the morning, and that amount steadily decreases throughout the day. Insulin helps turn the glucose, or sugar, in your food into energy you can use. Without the proper amount of insulin, glucose levels build up in the blood and wreak havoc on the body.
Naturally, it would make sense to eat the biggest meal when we have the most insulin to deal with the sugar intake. And scientists have found that to be true. If you give a person the same meal, once in the morning and once 12 hours later, the meal eaten later in the day will produce a bigger blood sugar spike. Their bodies don’t have the right amount of insulin to deal with such a large surge of glucose in the evening.
The bottom line here is that there’s strong evidence to suggest eating the majority of your calories earlier in the day could be better for your health. So extending that natural fasting period at night by eating dinner earlier and avoiding evening snacks could be an easy and worthwhile diet hack.
What about religious fasting?
Humans have been fasting for religious reasons for thousands of years. Adult Muslims who are healthy enough fast from sunrise to sunset for the entire month of Ramadan, which lands at different times each year. Members of the Church of Jesus Christ of Latter-Day Saints routinely fast for periods of time as part of a healthy life. The majority of studies done on both these groups show similar results to other intermittent fasting research: A decrease in body weight and certain metabolic factors, like glucose and insulin. However, the results are mostly all transient, meaning that as soon as the fasting ends, they return to their normal baseline. Nevertheless, the fact that researchers saw those changes during the fasting process helps to confirm the results of other dietary studies.
What about skipping breakfast?
The idea of skipping breakfast—once thought of as the most important meal of the day—has actually been studied a fair bit. In one trial, researchers had obese individuals skip their morning meal for six weeks. They then measured how much the individuals ate during lunch, as well as how high their glucose and insulin levels were right after they ate. They also monitored subjects’ weight. But there was no apparent benefit to skipping breakfast on any of these metrics. So, it’s likely far better to eat a light dinner than it is to skip breakfast.
What is missing from the research now?
While we have a few decent studies on the effects of intermittent fasting on weight, there’s far less data connecting it to specific diseases like cancer, diabetes, cardiovascular disease, or Alzheimer’s, which many proponents of the diet say it can help prevent. To change that, researchers wrote in a 2017 review on intermittent fasting published last year, scientists would need to conduct trials with large numbers of people for far longer, typically over a year. Further, they say, these future studies should include other factors that influence weight and health—like sleep, exercise, and stress—to verify that the dietary scheduling is what’s doing the trick. Plus, the large populations they study should be a good mix of healthy folks and people with specific diseases, as well as covering various ages and demographics.
So yes, some small studies show that intermittent fasting can lead to relatively small amounts of weight loss. And thankfully, none of these studies have found it to be immediately harmful or detrimental to human health. But that doesn’t mean scientists have given you the green light to fast as you see fit. Before doctors can make any recommendations, far more research must be done to better understand how this affects the human body in the long term. Fortunately, the topic is just as exciting for researchers as it is for us lay people—so we might not have to wait too long for the answers to some of these burning questions.