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.

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 explains, including new scientific findings and the rising popularity of the ketogenic or "keto" diet and one of its controversial components: intermittent fasting.

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The Science Behind 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 within a specific timeframe (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 published 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 protocol 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.

"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 explains. "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."

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Intermittent Fasting for Weight Loss

A growing body of research is linking IF with weight loss, although most studies so far have been limited to a small number of participants and short time frame.

In one study, published December 2019 in ​Cell Metabolism​, researchers had 19 people with metabolic syndrome fast for 10 hours each day for 12 weeks. They found that this amount of fasting promoted weight loss and also reduced waist circumference, body fat percentage and belly fat. (Beyond weight-loss benefits, the participants also reduced their blood pressure and total cholesterol levels, got better sleep and saw improved blood sugar and insulin levels.)

Dr. Crupain points to a larger study published October 2013 in the ​International Journal of Obesity​, which monitored the eating habits of 420 individuals over a 20-week weight-loss program. Participants consumed the same food but were grouped into two categories: early-eaters, who consumed their lunch before 3:00 p.m., and late-eaters, who dined after that time. Researchers found that those who ate earlier lost significantly more weight — 25 percent more to be exact — than those who ate later in the day.

"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."

And an October 2019 meta-analysis of 12 IF studies, published in the ​Journal of Clinical Medicine​, found that, compared to a control diet, intermittent fasting helped to significantly decrease body mass index and fat mass, although the review was limited to people without diabetes.

Some studies have suggested that IF's weight-loss benefits are due to increased calorie burn. But others, including a small July 2019 paper published in Obesity, contend that, somewhat counter-intuitively, going longer without eating actually ​decreases​ appetite.

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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Types of Intermittent Fasting Diets

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

1. Dr. Crupain's What to Eat When

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

First, consume 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 explains.

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.

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2. Alternate-Day Fasting

This eating method has been popular for a few decades, and involves alternating days of fasting (where you only consume non-caloric beverages 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 or weight maintenance, or improve heart health.

3. The 5:2 Diet (aka The Fast Diet)

In the book The Fast Diet, by writer 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 this diet, you will eat normally five days a week and then consume just 500 to 600 calories on the other two days. Those calories can be consumed 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.

In one study of 150 people, published November 2018 in ​The American Journal of Clinical Nutrition​, researchers found that this type of fasting was just as effective as a low-calorie diet for weight loss and the prevention of metabolic diseases.

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4. 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 — with low-carbohydrate and protein-rich whole and plant-based foods.

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 easy. Purported benefits include enhanced performance, weight loss, enhanced cellular renewal and improved metabolic health.

5. 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.

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 declared in a recent 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 the "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 consuming it. "Even the 'best' studies on fasting find weight loss results of just a couple pounds," she tells

Morena 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 suggests.

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 consuming food during daylight hours is a better option than 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.

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