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

"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. But what about the intermittent fasting diet, which targets when you eat?

Indeed, 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 LIVESTRONG.com.

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: the intermittent fasting program.

Fasting is nothing new — in fact, the concept dates back to ancient times. But the structured method of intermittent fasting diet (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.

So, how does intermittent fasting help you lose weight? 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, per Johns Hopkins Medicine.

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."

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. Here are the different types of fasting for weight loss:

The concept of fasting can be a little daunting, but it shouldn't be, says Dr. Crupain, whose book outlines his method of fasting intervals 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, rather than fasting in the morning. "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.

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.

So what are the pros and cons of intermittent fasting with the 16:8 plan? This program is popular because of its simplicity: Outside of the eating and fasting windows, there are no other rules to follow, like how many calories you have to eat during 16:8 intermittent fasting.

The Leangains intermittent fasting program is similar to the 16:8 diet — you eat during an eight-hour window and then spend the next 16 hours fasting, per the diet's website.

The main difference is that this lean fasting plan is meant to support strength training, as the nutrition guidelines center around the macronutrients you should include in pre- and post-workout meals.

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.

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.

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.

The Warrior Diet is an extreme type of fasting plan for weight loss 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 Monk program is another extreme type of IF that involves a 36-hour fast every week, according to the program's website. The main purported 36-hour fasting benefit is that it encourages your body to start burning fat for energy.

But while the plan asserts this benefit of a 36-hour fast, in reality, 36 hours of fasting can be unsafe and deprive your body of necessary nutrients, according to Harvard Health Publishing. So if you'd like to try intermittent fasting, opt for a safer plan.

So, what happens when you don't eat for 12 hours (or whatever fasting window you choose)? 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:

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.

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.

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."

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.

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.

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

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

• People younger than 18
• People who are pregnant or breastfeeding
• People with diabetes or blood sugar issues

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, according to the University of Michigan Health.

IF can also lead to some temporary health issues, says Bonnie Taub-Dix, RDN and author of ​‌Read It Before You Eat It - Taking You from Label to Table‌.

"Besides the discomfort of a rumbling belly, fasting could bring uncomfortable side effects," she tells LIVESTRONG.com. "Skipping meals could cause blood sugar levels to plummet making some people feel lightheaded, dizzy and weak. It can also lead to irritability and a lack of focus."

Proponents of fasting claim that hunger pangs subside over time and actually list mental clarity as a benefit of this way of eating, but these are definitely concerns worth considering.

Intermittent fasting can do a number on your hormones. A March 2019 study in ​‌Nutrients‌​ validates that excessive calorie restriction can lead a disturbance in the menstrual cycle for people assigned female at birth (AFAB) and a decrease in testosterone for people assigned male at birth (AMAB).

Additionally, animal studies (including this January 2013 research in ​‌PLOS ONE)‌​ have found that intermittent fasting disrupts the reproductive cycle in females and lowers testosterone levels in males.

"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 LIVESTRONG.com.

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.

There are pros and cons to any intermittent fasting program. But regardless of the plan you follow, here are some mistakes to avoid:

While the variety of options is great because one size doesn't fit all when it comes to intermittent fasting regimens, jumping right into an extreme approach like the Warrior Diet isn't the best idea. It can be shocking for your body, and you're a lot less likely to stick with it, according to Johns Hopkins Medicine.

‌The fix:‌ Think about approaching IF like you would a fitness goal. If you're new to running and you start to train for a half marathon, the best and safest approach is not going for a 10-mile training run right out of the gate. By doing this, you would be putting yourself at risk for injury, let alone the very real potential that you might not finish.

The same goes for intermittent fasting. Consider where you're starting and make adjustments to your eating from there. If you normally eat throughout the day, for example, you might want to start with shaving off an hour or two at night.

Be realistic about your current lifestyle and what is feasible for you, otherwise your IF program may not be sustainable.

Case in point: A July 2017 study in the ​‌Journal of American Medical Association of Internal Medicine‌​ looked at both intermittent fasters and those who just restricted calories and found that while both groups lost similar amounts of weight, the fasting group had a higher dropout rate.

‌The fix:‌ Pick a plan that most naturally suits your usual routine so you don't have to adjust your lifestyle in order to fast.

Not losing weight while intermittent fasting with the 16:8 or other plan? It may be because you're eating high-calorie, low-nutrition foods like baked goods or fried food when you're not fasting, according to Johns Hopkins University.

Even though you're fasting, it's still important to prioritize nutritious, balanced meals — without them, you're setting the stage for digestive trouble.

"[When intermittent fasting], people are usually prone to eat very large meals during eating times, which can cause gastrointestinal stress leading to indigestion and bloating," says dietitian Patricia Bannan, RDN. "This is particularly concerning for people with irritable bowel syndrome, who already have a more sensitive gut."

‌The fix:‌ The best meals for intermittent fasting include vegetables, fruits, whole grains, lean proteins and beneficial fats. You can also plan your meals ahead of time so they're ready to eat once your fasting window ends.

Because the amount of time you're "allowed" to eat is limited, it can be hard to eat as much as you should — especially enough of all the nutrients your body needs. This puts you at risk for nutrition deficiencies and potentially stalled weight loss (even if you're initially losing weight while intermittent fasting), according to Harvard Health Publishing.

"Depending on how long you fast and what you eat when you are eating, fasting could lead to vitamin and mineral deficiencies in addition to an inadequate amount of macronutrients like protein, carbohydrates and fats," says Taub-Dix. ‌"‌​It's also likely that you won't meet your needs for fiber."

‌The fix:‌ Planning your meals can help ensure you're eating enough of the right foods. If you find you're eating too few calories, consider adding more nutrient- and calorie-dense foods like avocados and nuts. You may also need to extend your eating window if you find you're not able to eat enough.

Make sure that you follow an IF plan that allows you to refuel after workouts. Without a post-exercise snack or meal, your muscles may not be able to recover properly, according to the Mayo Clinic.

For instance, if you're following the 5:2 method, you probably don't want to plan a strenuous workout on the day you're only consuming 500 calories. The same goes for other methods like 16:8 — you don't want to work out later in the evening when you're not able to refuel properly afterwards.

‌The fix:‌ Make sure your eating windows and workout times are compatible. Recovery nutrition, or eating post-workout, is crucial because it allows your body to refuel and rehydrate as well as help your muscles repair and grow, per the Mayo Clinic.

Skipping recovery nutrition can make you more tired overall, increase muscle soreness and put you at an increased risk for injury as well as limit your potential gains from the training session.

Whether you're following the 5:2 method or 16:8, skipping meals earlier in the day and eating more of your calories later in the afternoon or evening may not be serving you. While it's important to pick a plan that works with your schedule, eating late can get in the way of weight loss, according to Penn Medicine.

‌The fix:‌ Eat more in the a.m. than p.m. Indeed, a July 2019 study in ‌Obesity‌ found that eating early in the day and stopping in the afternoon reduced hunger hormones and decreased appetite compared to eating early and continuing on later into the evening.

A March 2020 study in ‌The Journal of Clinical Endocrinology & Metabolism‌ furthers these results. It found that when more calories are eaten in the morning versus later in the day, the diet-induced thermogenesis — or the increase in metabolism post-meal — was 2.5 times higher. And appetite, especially for sweets, was increased in those who ate more of their calories later in the day.

Intermittent fasting needs to work with your life, and if you strive for absolute compliance, you'll likely crash and burn, according to the Mayo Clinic.

‌The fix:‌ Change comes with consistency, not perfection. If you really want to give IF a go, give yourself some flexibility.

If you have a celebration or a social outing with friends, don't skip out on enjoying the cake or dinner on account of it being outside of your eating window. Give yourself some wiggle room to enjoy these important moments.

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 first speak with your doctor about which is the most effective intermittent fasting plan for you, especially if you have any preexisting health conditions.