You Are When You Eat? How Intermittent Fasting Affects Weight Loss

Should you be just as focused on when you're eating as what you're eating?

"You are what you eat" is an old adage that gets hammered into our heads at a young age — and for good reason, according to pretty much every health expert on the planet.


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In recent years, though, that mantra has begun to shift a bit: Sure, quality and quantity are clearly very important dietary factors. But many health experts are starting to think that timing may be just as crucial. "We have always focused on what we eat, but almost never on when we eat," Michael Crupain, MD, MPH, Medical Unit Chief of Staff at ​The Dr. Oz Show​ and author of What to ​​Eat When, tells


There are a number of reasons why this thinking is changing, Dr. Crupain says, including new scientific findings and the rising popularity of the ketogenic diet and one of its controversial components: intermittent fasting for weight loss.

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What Is Intermittent Fasting?

Fasting is nothing new — in fact, the concept dates back to ancient times. But the structured method of intermittent fasting (IF), in which you restrict eating to a specific timeframe (like certain hours of the day or days of the week), has gained popularity recently with the rise of the keto diet. The idea is that abstaining from food for longer-than-normal periods of time helps put your body into a state of ketosis, prompting it to burn fat for energy rather than carbohydrates.


But not everyone who practices IF follows the keto diet. Other purported benefits of IF on its own include weight loss, lower blood pressure, improved mental state, reduced inflammation and improved cellular repair, according to a December 2019 review in ​The New England Journal of Medicine​.

The review notes that human studies on IF are limited, and much more research needs to be done, but the evidence so far shows that the eating style may also improve insulin sensitivity (which might help reduce the risk of type 2 diabetes) and reduce belly fat.

Dr. Crupain attributes the IF movement to the evolving science supporting the idea that our bodies' circadian rhythm — aka our internal clock — influences our health, according to January 2013 research in ​Physiological Reviews​.

"Our circadian rhythm isn't just about sleep. It actually changes our metabolism throughout the day, so that when we eat can make a big difference," he says. "By eating in line with when our body is best primed to use food, we basically hack our metabolism and optimize health. By contrast, eating out of sync with our circadian rhythm can lead to weight gain and other health issues."

Types of Fasting Diets

In addition to keto and intermittent fasting, there are a few other diets that put a serious emphasis on meal timing, fasting and weight loss, including:

1. Dr. Crupain's What to Eat When

The concept of fasting can be a little daunting, but it shouldn't be, according to Dr. Crupain, whose book outlines his method of food timing and offers readers a few principles for how to eat in alignment with this clock.

First, eat your meals during daylight hours. "The sun sets our circadian rhythm and our body is primed to eat when the sun is out and fast when it is dark," he says.

Second, he stresses the importance of eating more earlier in the day, and less later. Breakfast and lunch should be your biggest meals of the day, and dinner the smallest. "Ideally, you should eat your traditional dinner foods — protein, veggies, etc. — earlier in the day and avoid simple carbs like sugar as well as saturated fat," he says.

Following these two principles, Dr. Crupain points out that you should get a window of at least 12 hours or more of fasting each day.

2. The 16:8 Diet

Similarly, the 16:8 diet involves eating during an eight-hour window, followed by 16 hours of fasting (which includes the time you're asleep), according to Johns Hopkins Medicine.

This type of intermittent fasting is popular because of its simplicity: Outside of the eating and fasting windows, there are no other rules to follow.

3. Alternate-Day Fasting

This eating method has been popular for a few decades, and involves alternating days of fasting (where you only drink non-caloric liquids like coffee and water) with days of eating normally.

However, despite its popularity, there has been little scientific evidence to support claims that it can help with weight loss, weight maintenance or heart health.

4. The 5:2 Diet (aka the Fast Diet)

In the book The Fast Diet by Mimi Spencer and Michael Mosley, MD, the duo explain that fasting two days each week induces your body to burn fat and promotes weight loss.

On the 5:2 diet, you will eat normally five days a week and then eat just 500 to 600 calories on the other two days. Those calories can be eaten during one sitting or spread out over the day. You can also choose when to do the fast days, staggering them throughout the week or doing them consecutively.

Research has found that this type of fasting was just as effective as a low-calorie diet for weight loss and the prevention of metabolic diseases, according to a November 2018 study in ​The American Journal of Clinical Nutrition​.

5. The ProLon Diet (aka the Fasting Mimicking Diet)

Created by Valter Longo, PhD, an Italian biologist and researcher, the Fasting Mimicking Diet reportedly gives the body all the health benefits of fasting while still providing it with nutrition from low-carbohydrate and protein-rich whole and plant-based foods, according to the ProLon website.

Meals include nut bars, soups, kale crackers and olives. Longo even sells five-day, prepackaged meal kits on his website, in order to make the diet easier. Purported benefits include enhanced performance, weight loss, enhanced cellular renewal and improved metabolic health.

6. The Warrior Diet

The Warrior Diet is an extreme type of intermittent fasting that includes 20 hours of under-eating followed by four hours of overeating. The diet has three phases, each a week long and with different (but very strict) rules around what you can eat. It was created by a former member of the Israeli Special Forces who does not have a background in nutrition.


The Warrior Diet can deprive you of essential nutrients, which may be especially unsafe for athletes, people who are pregnant and people who have chronic illness or blood sugar issues. If you'd like to try a fasting diet, it's best to skip this one and try another program.

Intermittent Fasting for Weight Loss

So, how does intermittent fasting help you lose weight? A growing body of research links IF to weight loss, although most studies so far have been limited to a small number of people and a short time frame. Nonetheless, here are some of those potential benefits:

1. It May Help Burn More Fat

Intermittent fasting may work for weight loss by prompting your body to burn more fat. That's because when you go a while without eating, your body runs out of sugar to use for energy and instead starts to burn stored fat, according to Johns Hopkins Medicine.

For instance, a small study of 19 people with metabolic syndrome found that fasting for 10 hours each day for three months promoted weight loss and also reduced waist circumference, body fat percentage and belly fat, per December 2019 research in ​Cell Metabolism​.

The fasting also helped reduce people's blood pressure and total cholesterol and improved their sleep, blood sugar and insulin levels.

2. It Can Help Regulate Insulin

On a similar note, you might also try intermittent fasting to lose weight because it can help regulate your levels of the hormone insulin, according to University of Michigan Health.

Your insulin levels drop when you aren't eating, which prompts your body to start burning sugar for energy. And when insulin levels stay low for long enough, your body starts to burn fat, which can contribute to weight loss.

3. It Can Sync With Your Circadian Rhythm

Fasting may also help you lose weight because your meal times can work in harmony with your body's natural rhythms. As Dr. Crupain pointed out, it may be best to eat most earlier in the day when your body is primed for processing food.

Indeed, a study monitoring the eating habits of 420 people over a 20-week weight-loss program found that people who ate their lunch before 3 p.m. lost significantly more weight — 25 percent more to be exact — than those who ate later in the day, even though they ate the same food, per October 2013 research in the ​International Journal of Obesity.

"Perhaps rather than a rigid 'no food after 4 p.m.' rule, the old adage 'breakfast like a king, lunch like a merchant and supper like a pauper' would be something to try in your own lifestyle."

4. It May Improve Metabolic Health

Intermittent fasting may also help with weight loss by supporting your metabolic health. For instance, research has found that IF may improve blood sugar regulation, which can help decrease body mass index and fat mass, according to an October 2019 review in the ​Journal of Clinical Medicine.

However, this review was limited to people without diabetes.

5. It May Decrease Calorie Intake

An intermittent fasting diet could also support weight loss because you may eat fewer calories overall.

In fact, an April 2022 study in ​The New England Journal of Medicine​ found no difference between time-restricted eating and overall calorie restriction for weight loss in a small group of people with obesity over 12 months.

The researchers split the participants into two groups, who ate the same number of calories per day, with one group restricting eating to between 8 a.m. and 4 p.m. The participants lost about the same amount of weight, which seems to indicate that it's the calorie restriction that causes weight loss, not the fasting aspect.

But there's also some research to suggest that (somewhat counter-intuitively) going longer without eating can actually decrease your appetite and lead to eating fewer calories, per a small July 2019 paper in Obesity.

In the report, researchers found that after restricting eating to between 8 a.m. and 2 p.m., people had lower levels of the hunger-linked hormone ghrelin in the morning and higher levels of a satiety hormone at night compared to when they ate within a 12-hour window instead.

However, because only 11 people were included in the study, more research is needed to determine whether or not IF truly tamps down appetite.

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Is Intermittent Fasting for Weight Loss Safe?

Though fasting diets are gaining acceptance for their possible weight-loss benefits, there are some safety considerations to keep in mind when trying this eating style.

For instance, it may not supply enough consistent nutrients for the following people, according to the University of Michigan Health:

For others, weight-loss fasting diets can trigger disordered eating. As a result, it's best to steer clear of IF if you have or have had an eating disorder, per University of Michigan Health.


If you have any other underlying illnesses, check with your doctor before trying a fasting program to make sure it's safe for you.

Some Skeptics Don't Buy the Hype

"It doesn't matter when you eat or when you don't eat, all that matters is how much you eat," celebrity trainer Jillian Michaels said in an interview with ​Well + Good.

While Michaels doesn't believe intermittent fasting will help with weight loss, she does try to keep her food intake to the period between 8 a.m. and 8 p.m. in order to reap supposed anti-aging benefits.

Monica Auslander Moreno, RD, LDN, dietitian and founder of Essence Nutrition, agrees that if you are trying to lose weight, you should focus more on the quality and quantity of the food you are eating than when you are eating it. "Even the 'best' studies on fasting find weight loss results of just a couple pounds," she tells

Moreno mentions, though, that restricting your meals to earlier in the day might help you feel more comfortable digestion-wise and improve your sleep.

"Perhaps rather than a rigid 'no food after 4:00 p.m.' rule, the old adage 'breakfast like a king, lunch like a merchant and supper like a pauper' would be something to try in your own lifestyle," she says.

Is Intermittent Fasting Right for You?

When it comes to your diet, you should always take a variety of factors into consideration, with timing being just one of them. While science has yet to definitively conclude whether fasting diets are beneficial, most experts will agree that eating food during daylight hours is a better option than eating at night.

If you decide you want to tap into fasting, though, be sure to speak with your health practitioner or a medical expert first, especially if you have any preexisting health conditions.