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? (Image: Getty Images/JubaPhoto)

"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.

However, in recent years, 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 key components: intermittent fasting.

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 your day's eating to within a specific timeframe, 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, improved mental state, reduced inflammation and improved cellular repair, according to a June 2013 paper published in the Canadian Medical Association Journal. A preliminary study in human and mouse cells found that fasting reduced the amount of monocytes — inflammatory immune cells that can cause tissue damage — in the blood, according to the August 2019 report published in the journal Cell. The researchers found that just 4 hours of fasting reduced monocyte levels but 20 hours without food — a much more challenging fast — led to even bigger drops.

Some research, such as a February 2018 study in BioMed Central's journal Trials, also link the method to increased energy levels.

Dr. Crupain also attributes the 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."

"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."

Dr. Crupain points to an October 2013 study published 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.

Another 2013 study published in the journal Obesity, which allowed participants to consume the same amount of daily calories at various times of the day, reared similar results. Researchers found that individuals who consumed 50 percent of their calories in the morning lost more than twice the amount of weight as those who consumed the same amount at dinner. The front-loaders also reaped other benefits. Not only did they report being less hungry, but they also saw improved insulin, triglyceride and cholesterol levels — factors that could lower the risk of health problems like metabolic syndrome, diabetes and heart disease.

Some studies have suggested that IF's weight-loss benefits are primarily 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 July 2019 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.

Diets That Tap Into Meal Timing

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.

2. Alternate-Day Fasting

This eating method has been popular for a few decades, and involves alternating days of extreme calorie restriction (fast days) with days of eating normally (feast days).

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 fast the other two, consuming just 500 to 600 calories. 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.

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.

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 and nutrition consultant for RSP 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 Meal Timing 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 extreme 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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