Restricting Eating to a 12-Hour Window of Time Each Day May Be Key to Healthy Weight

Story at-a-glance -

  • Intermittent fasting, which focuses on restricting your eating to a small time window, can have a beneficial effect on a wide array of biological functions and systems
  • Time-restricted eating can both prevent and reverse obesity and related metabolic dysfunction
  • Mice restricted to eating within an eight-, nine-, or 12-hour window remained thin and healthy, even when cheating on weekends, and/or eating a high-sugar diet

By Dr. Mercola

Intermittent fasting can provide many important health benefits, from improving your insulin/leptin sensitivity to helping your body more effectively burn fat for fuel. It's one of the most effective ways I've found to shed excess weight for these two reasons.

That notion is held up by recent research published in the journal Cell Metabolism,1 in which researchers concluded that time-restricted eating not only prevented but also reversed obesity and related metabolic dysfunction.

Even more remarkably, this was true even when the diet was less than ideal. Time-restricted eating is my personal favorite of the many intermittent fasting schedules available.

I believe it can be very helpful for those in need of shedding a few extra pounds and/or who have insulin and leptin resistance—which is the majority of the American population.

The reason so many struggle with their weight (aside from eating processed foods that have been grossly altered from their natural state) is because they're in continuous feast mode and rarely ever go without a meal.

As a result, their bodies have adapted to burning sugar as its primary fuel, which down-regulates the enzymes that utilize and burn stored fat. Fasting is an excellent way to "reboot" your metabolism so your body can start burning fat as its primary fuel, which will help you shed your unwanted fat stores.

Once your insulin resistance improves and you are normal weight you can start eating more frequently, as by then you will have reestablished your body's ability to burn fat for fuel—that's the key to sustained weight management.

Intermittent Fasting as a Therapeutic Intervention

To assess the effectiveness of different time-restricted feeding schedules on weight and metabolic function, the researchers2 divided adult male mice into four groups, each of which was fed a specific diet:

  1. High-fat diet
  2. High-fructose diet
  3. High-fat and high-fructose diet
  4. Regular mouse kibble

In each of these four diet groups, half of the mice were allowed to eat whenever they wanted during waking hours (ad libitum), while the other half only had access to food during a restricted period of time of eight, nine, 12, or 15 hours per day.

The experiment lasted for 38 weeks. At the end, the mice that ate at all hours had become obese and suffered with metabolic dysfunctions, while the mice restricted to eating within an eight-, nine-, or 12-hour window remained thin and healthy.

This was true even for select groups of mice that were permitted to cheat on weekends, and eat any time they wanted. Mice that were switched from an ad libitum schedule to a restricted eating schedule midway through the experiment also shed some of the excess weight they'd accumulated. As reported by The New York Times:3

 "Time-restricted eating didn't just prevent but also reversed obesity,' says Satchidananda Panda, an associate professor at the Salk Institute who oversaw the studies. 'That was exciting to see'... Precisely how a time-based eating pattern staved off weight gain and illness is not fully understood, but Dr. Panda and his colleagues believe that the time at which food is eaten influences a body's internal clock. 'Meal times have more effect on circadian rhythm than dark and light cycles,' Dr. Panda says. And circadian rhythm in turn affects the function of many genes in the body that are known to involve metabolism."

Mini-Fasting—The 5:2 Plan

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Another intermittent fasting schedule is the 5:2 Plan as popularized by Dr. Michael Mosley and his FAST diet. Here, you're allowed to eat normally five days a week and only fast on two. On fasting days, you restrict your eating to 600 calories or less for men, and 500 calories or less for women. This kind of restricted eating plan has been shown to have similar effects and benefits as continuous modest calorie restriction, which includes weight loss, improved insulin sensitivity, and improvements in other biomarkers for health. In some cases it may even be more effective.

As reported by NPR:4

"A study5 by researchers at the University of Manchester found that when overweight women followed a 5:2 approach, they lost more weight and body fat and improved their insulin resistance compared with women who followed a more traditional diet of limiting calories seven days per week One explanation for the success of the 5-2 dieters could be that a day of mini-fasting can lead to a diminished appetite...There may be an evolutionary explanation for this because humans (and other animals) have fasted intermittently for much of our time on Earth, after all. As a recent paper6 in the Proceedings of the National Academy of Sciences notes, "The most common eating pattern in modern societies, three meals plus snacks every day, is abnormal from an evolutionary perspective."

How and Why Intermittent Fasting Works

Again and again, researchers have concluded that fasting—which includes not only eating less food overall, but also eating less frequently—can have a beneficial effect on a wide array of biological functions and systems. One 2013 review7 found a broad range of therapeutic potential of intermittent fasting, even when total calorie intake per day did not change, or was only slightly reduced. Studies included in the review produced evidence that intermittent fasting may:

Limit inflammation Improve circulating glucose and lipid levelsReduce blood pressure
Improve metabolic efficiency and body composition, including significant reductions in body weight in obese individualsReduce LDL and total cholesterol levelsHelp prevent type 2 diabetes, as well as slow its progression
Reverse type 2 diabetesImprove pancreatic functionImprove insulin levels and insulin sensitivity
Reproduce some of the cardiovascular benefits associated with physical exerciseProtect against cardiovascular diseaseModulate levels of dangerous visceral fat

As noted above, one of the reasons for these health benefits relate to the fact that the human body appears to be designed to thrive in a cycle of "feast and famine." By imitating the ancestral conditions of cyclical nourishment, your body enters into a state of optimal functioning. Three major mechanisms by which fasting benefit your health include:

  1. Increased insulin sensitivity and mitochondrial energy efficiency – Fasting increases insulin sensitivity along with mitochondrial energy efficiency, and thereby retards aging and disease, which are typically associated with loss of insulin sensitivity and declined mitochondrial energy.
  2. Reduced oxidative stress – Fasting decreases the accumulation of oxidative radicals in the cell, and thereby prevents oxidative damage to cellular proteins, lipids, and nucleic acids associated with aging and disease.
  3. Increased capacity to resist stress, disease and aging – Fasting induces a cellular stress response (similar to that induced by exercise) in which cells up-regulate the expression of genes that increase the capacity to cope with stress and resist disease and aging.

Who Might Benefit from Exercising on an Empty Stomach, and Why?

The cyclical availability of food in ancient times also meant that our ancestors had to engage in physical activities, sometimes strenuous, without having anything to eat first. Modern research now confirms that this, too, has biological benefits, and that our obsession with keeping our bellies full at all times—especially if we're about to do physical work—may not be the ideal approach for all people all the time. As noted in a previous Huffington Post8 article:

"It's an ongoing debate within the world of fitness: should you eat breakfast before exercising in the morning, and are there any benefits to working out on an empty stomach? 'It all depends on what type of training you are going to do and for what purpose,' says John Rowley, Wellness Director for the International Sports Science Association (ISSA)... The main advantage to exercising on an empty stomach: the potential to burn more fat...

[E]xercisers with weight loss goals might find an advantage in waking up and exercising first thing in the morning before eating breakfast or fasting for a few hours before a mid-day or evening workout. 'The less glucose you have in your system the more fat you will burn,' says Rowley. However, if your goals are performance related (e.g. to improve strength or speed), working out without fueling up probably isn't your best bet because a lack of available energy might prevent you from putting forth your best effort."

Intermittent Fasting and High-Intensity Exercise Is a Potent Combo for Weight Loss

When it comes to shedding unwanted pounds and reworking your fat-to-muscle ratio, high-intensity interval training (HIIT) combined with intermittent fasting is a winning combination that's hard to beat. When you combine these two strategies, especially if you exercise in a fasted state, it effectively forces your body to shed fat because your body's fat burning processes are activated by exercise and lack of food. The good news is that you don't have to keep on intermittently fasting forever if it doesn't appeal to you long-term. Also if you are already your ideal weight, then there really isn't much need to exercise fasting as it will limit your performance.

If you need to lose 50 pounds, you're looking at about six months or so of intermittent fasting, after which you can revert back to eating more regularly. My only caveat is that you also need to pay attention to the quality of the food you eat. Since both HIIT and intermittent fasting help shift your body from burning sugar to burning fat as its primary fuel, it's important to feed your body the right nutrients. Even on non-fasting days, I believe it's important to eat a diet that is:

  • High in healthy fats and very low in fructose and other sugars. Those with insulin and leptin resistance will likely benefit from getting 50-85 percent of their daily calories in the form of healthy fat from avocados, organic grass-fed butter, pastured egg yolks, coconut oil, and raw nuts such as macadamia, pecans, and pine nuts.  
  • Avoid processed foods as most are very high in fructose and other health-harming ingredients, including harmful trans fats. If you're insulin/leptin resistant (and/or have high blood pressure, high cholesterol, diabetes, heart disease, or are overweight), you'd be wise to limit your total fructose consumption to 15 grams per day. For all others, the recommended daily allowance of fructose is 25 grams per day for optimal health.

  • Moderate amounts of high-quality protein from organically raised, grass-fed or pastured animals. Most will likely not need more than one gram of protein per pound of lean body weight, which for most people would equate to somewhere in the neighborhood of 40-80 grams of protein per day. If you're pregnant or exercising intensely, you would need about 25 percent more.
  • Unrestricted amounts of fresh vegetables, ideally organic.

Making these dietary shifts, and adding two or three weekly sessions of high intensity exercises is, I believe, a solid strategy for reaching your weight loss and fitness goals. Also, if you're hesitant to try fasting for fear you'll be ravenously hungry all the time, you'll be pleased to know that intermittent fasting will virtually eliminate hunger and sugar cravings.

Sure, it may take a few days or even weeks, but once your body shifts into burning fat for fuel rather than sugar, the sugar cravings will be a thing of the past. I'm a fellow of the American College of Nutrition and have studied nutrition for over 30 years, and I'd never personally encountered or experienced hunger cravings just disappearing like they did when I implemented intermittent fasting!

A tool that can be helpful to keep you on track with your intermittent fasting schedule is the Emotional Freedom Techniques (EFT). In the following video, Julie Schiffman demonstrates how to tap for emotional components that can crop up whenever you're trying to make a dietary change.