When You Start Intermittent Fasting, This Is What Happens To You

Intermittent fasting is the concept of eating during a certain restricted time window each day and fasting (aka not eating) the rest of the time. "The idea is that fasting for long enough allows insulin levels to fall low enough that our body will use fat for fuel," physician Monique Teller explained in a Harvard Health article. In other words, instead of being fueled by food, long periods of not eating would force our bodies to eat their own stores of fat, leading to weight loss.

While there's a lot left to learn about intermittent fasting, there's no denying that it's popular. There are several ways to do it, but the most common is the 16/8 approach, which involves an eight-hour eating window per day — say, from noon to 8 p.m. — and fasting for the rest of the time. Other popular ways to do it are the eat-stop-eat method, which recommends one or two 24-hour fasts per week. Keeping in mind that there's still so much left to figure out, let's take a look at how styles of intermittent fasting can affect your body.

Despite intermittent fasting being popular among both researchers and regular people, experts don't really have a clear picture of everything it can do to the human body. As  nutrition researcher Benjamin Horne explained in an article for Self, our current interest in intermittent fasting comes from a bunch of animal studies done in the early 2000s. 

Such studies found that intermittent periods of fasting have beneficial health effects. Specifically, researchers found that intermittent fasting could induce ketosis, a process in which the body burns stored fat for fuel instead of converting food to glucose for fuel. They also found that periods of fasting caused cells to enter into a period of renewal and rejuvenation, which could reduce chronic disease risk and increase longevity. 

But, as Horne said, all of these benefits were found in animal (primarily rodent) studies. While this means that intermittent fasting may have promise, it doesn't mean that intermittent fasting necessarily will have all of the same benefit for humans.

You may have heard that intermittent fasting can slow aging and increase longevity — that is, help you stay healthier and live longer. That's not totally true, but it's not a total myth, either. While there's not enough high-quality evidence to say whether intermittent fasting has this effect on humans, there is solid evidence that it has this effect on animals.

A team of researchers from the National Institutes of Health studied the effect of intermittent fasting on mice. Of the 300 mice included in the study, half were fed a whole food, low-sugar diet while the other half were fed a processed, higher-sugar diet. Next, the researchers separated each group into three subgroups: one group had access to food all day, one group was fed a calorie-restricted diet, and the final group were fed a regular amount of food, but only once per day. What the researchers found was surprising.

While the type of food eaten didn't have a significant effect on health or lifespan, the mice that were only fed once per day were healthier and lived longer than the other groups of mice. Again, we don't know yet whether or not this is true for humans, but it is promising.

There aren't any large, long-term trials that have observed the effect of intermittent fasting on cholesterol levels. However, research from small clinical trials suggests that intermittent fasting probably won't have any benefit here.

In a 2017 randomized controlled trial of 100 obese adult men, researchers assigned participants to either alternate-day fasting, a low-calorie diet, or no diet. What they found was that while intermittent fasting increased HDL ("good") cholesterol after six months, it also raised LDL ("bad") cholesterol after one year. Bottom line: You shouldn't expect intermittent fasting to magically lower your cholesterol levels.

While this trial was small and more research certainly needs to be done, it's unlikely that intermittent fasting has the power to positively impact your cholesterol. Instead, the American Heart Association recommends limiting saturated fat, red meat, and sodium, and eating plenty of fresh fruits and vegetables, whole grains, beans, nuts, and seeds. 

Intuitively, it makes sense that if you restrict your eating to only certain hours in the day, you'll eat less overall. Chalk it up to no late-night snacking, if you will. And it's reasonable to believe that eating less will lead to weight loss. Research says this might be true in the short term, which could be good news for people trying to lose weight.

Citing research from the year prior, a 2015 review study revealed that people on intermittent fasting regimens — either eating for only a certain period of time each day or alternating between days of eating and days of not eating — typically lost between 3 and 8 percent of their body weight in the first three to 24 weeks. However, there was one major caveat. These studies all lasted six months or less. As such, there's just no way to know whether or not people were able to maintain that weight loss or continue losing weight long term.

While some studies have found that intermittent fasting leads to short-term weight loss, others have found that it doesn't. In a 2017 randomized controlled trial of 100 obese adult men published in the Journal of the American Medical Association, researchers assigned participants to either alternate-day fasting (one day of eating 125 percent of estimated calorie needs, then one day of eating just 25 percent of estimated calorie needs), a low-calorie diet, or no diet.

After a few months, those on the alternate-day fasting regimen had lost anywhere from 3 to 7 percent of their total body weight. At 12 months, this amount of weight loss had averaged out to about 6 percent of their body weight. This seems to reveal that some of the participants may have actually gained weight back in the second six months. 

Another thing to note is that the participants on alternate-day fasting regimens didn't lose any more weight than those on low-calorie diets. In other words, there probably isn't anything magic about intermittent fasting when it comes to weight loss. Any intervention that causes you to eat less will likely help you lose weight in the short term.

One big problem with weight loss is that it often leads to a loss of both fat and muscle. However, early research suggests that intermittent fasting may prevent this muscle loss. 

In a small randomized controlled trial published in Nutrition Journal, researchers followed 32 subjects for 12 weeks. Half of the subjects followed an alternate-day fasting regimen (one day of eating as much as they wanted then one day of eating just 25 percent of their estimated calorie needs) while the other half continued eating as usual.

At the end of the 12 weeks, the alternate-day fasting group had lost weight and fat mass but had not lost any lean body mass, which means they hadn't lost any muscle. This is good news, as having adequate muscle mass is essential to staying healthy and fighting disease (via The American Journal of Clinical Nutrition). However, the study only lasted three months; it's impossible to say whether or not these effects will last in the long term.

While intermittent fasting might help you lose weight without losing muscle, it probably isn't a good idea if your main goal is to gain muscle. In a small 2014 randomized controlled trial published in the European Journal of Sports Science, 18 young men who did not strength train regularly were assigned to an eight-week weight training program. Half followed a normal diet while the other half followed an intermittent fasting regimen that required them to eat all of their food in a four-hour window, four days a week. 

What the researchers found was not good news for the intermittent fasters. Those on the regular diet experienced "a gain in lean soft tissue" of some 5 pounds, unlike the intermittent fasting group. Bottom line? If you're hoping to put on muscle, you'd be better off pairing strength training with a normal diet, instead of trying intermittent fasting.

One big question that researchers are trying to answer about intermittent fasting is whether or not it helps with insulin sensitivity — that is, whether or not intermittent fasting helps with the hormonal regulation of blood sugar levels. Preliminary research suggests that this might be the case.

In a 2014 review of existing studies published in Translational Research, the authors found that, across all studies they looked at, various forms of intermittent fasting were shown to reduce blood sugar by 3 to 6 percent, and reduce fasting insulin between 20 to 31 percent (via Healthline).

Essentially, this means that intermittent fasting could help control your blood sugar levels, which is important for both diabetes management and prevention. It also suggests that intermittent fasting makes you more insulin-sensitive. That's a good thing, because it means that your body is able to keep your blood sugar steady after a meal, preventing big and unhealthy blood sugar swings.

If you've read about intermittent fasting in magazines or on celebrity blogs, you've no doubt heard the word "autophagy." For example, the Dubrow Diet, a diet that relies on intermittent fasting, claims that the eating style can "activate a process known as autophagy, your cells' self-cleaning process and an antiaging game changer." That sounds great, but is it true? The jury is still out.

No studies have successfully examined how intermittent fasting affects autophagy (a process that, by the way, happens naturally) in humans. However, there is evidence to suggest that intermittent fasting increases the rate of autophagy in rodents.

A small 2010 study published in Autophagy found that mice who fasted for 24 to 48 hours had higher rates of autophagy than those who did not fast. And, the longer the fast, the higher the rate of autophagy. While this is promising, it's very important to keep in mind that just because this is true for mice, that doesn't mean that it's necessarily true for humans.

First things first: Inflammation isn't inherently a bad thing. Inflammation is constantly happening in our bodies and it's what helps us fight disease, recover after a workout, or heal a wound. However, these are examples of acute (short and temporary) inflammation. Chronic inflammation, on the other hand, happens as a result of disease, poor diet, and other lifestyle factors — and this form of inflammation is not a good thing. 

A small 2007 clinical trial published in Free Radical Biology & Medicine put ten patients with asthma on an intermittent fasting regimen for eight weeks. At the end of the trial, they had, on average, lower levels of oxidative stress and inflammation than they did before fasting. Likewise, a 2012 study published in Nutrition Research followed 50 adults during Ramadan (an annual religious fast that's central to Islam) and found that levels of inflammation in their bodies had decreased slightly while fasting.

While this might seem like good news, far more research needs to be done before we fully understand how intermittent fasting affects inflammation, and what this means for our overall health.

Here's the thing about intermittent fasting: It's a framework for when you eat, not for what you eat. That means that it's possible to eat a very nutritious diet while intermittent fasting, but it's also possible to eat a pretty unhealthy diet. And there's absolutely no evidence to show that fasting can cancel out the effects of a bad diet. 

If you decide to give intermittent fasting a try, make sure you also focus on eating a balanced diet overall. According to the Centers for Disease Control and Prevention, a healthy diet contains plenty of fruits, vegetables, whole grains, beans, and low-fat or fat-free dairy. It also includes lean meat, protein, eggs, nuts, and seeds. At the same time, a healthy diet limits saturated fat, trans fat, salt, and added sugar. It also stays within your daily calorie needs.

The potential benefits of intermittent fasting will be negated if you don't stick to a balanced diet. As Healthline explained, "It's very important to primarily eat healthy foods during your eating window. This method won't work if you eat lots of junk food or an excessive number of calories."

Many experts worry that intermittent fasting, like other restrictive diets, could lead to binging. In an article for Psychology Today, Riccardo Dalle Grave, a medical doctor and head of the Department of Eating and Weight Disorders at the Villa Garda Hospital in Italy, wrote, "There is no robust evidence that intermittent fasting produces additional clinical benefits, compared to healthy and regular eating patterns." What's more, he explained, "Instead, there is some evidence that delaying eating increases the risk of developing episodes of overeating and binge eating." While the goal of intermittent fasting and dieting is to eat less, it often actually leads to eating more.

The takeaway? If you decide to try intermittent fasting and find that you're absolutely ravenous during periods when you're "allowed" to eat, that might be a sign that it isn't for you. For most people, it's probably better to eat regularly when you're hungry throughout the day, instead of confining eating to a short window.

Think that intermittent fasting could lead to lasting weight loss? That might be the case for some people, but it's certainly not the case for everyone. In an article for Psychology Today, Riccardo Dalle Grave, a medical doctor and head of the Department of Eating and Weight Disorders at the Villa Garda Hospital in Italy, explained that, "research has ... shown that dieting significantly predicted future weight gain in normal-weight individuals." Although intermittent fasting is not like a typical diet in every sense, it still calls for eating restrictions, which is a hallmark of a diet.

Skeptical about future weight gain? A 2013 review published in Frontiers in Psychology looked at previously conducted studies and found that people who tried to restrict their eating in some way were more likely to gain weight in the future than people who didn't actively try to restrict.

While that's not good news for intermittent fasting, it is good news in general. Instead of trying to be strict and regimented in your eating, focus on eating a balanced overall diet without getting too caught up in the details.

There's no single way to try intermittent fasting. While most people take the 16/8 approach of eating for eight hours a day and fasting for 16 hours, others do it differently. Alternate-day fasting involves eating normally one day and eating nothing or very little the next day. And, the 5:2 approach calls for eating normally five days per week, then eating 500 to 600 calories the remaining two, non-consecutive days, Healthline revealed. 

"If you want to give IF [intermittent fasting] a try, be prepared to figure out what works best for you," registered dietitian Anna Taylor told the Cleveland Clinic. "It might take some trial and error first." Cleveland Clinic explained that ultimately, any benefits of intermittent fasting are due to eating fewer calories overall. If you're hoping to lose weight or improve your health, choose whatever type of intermittent fasting helps you eat less overall without feeling too hungry or lethargic.