Peak Fasting — How Long Should You Intermittently Fast?
- Intermittent fasting helps reset your body to burn fat for fuel, and helps optimize insulin sensitivity and mitochondrial health and energy production
- Intermittent fasting involves cutting calories in whole or in part, either a couple of days a week, every other day, or even daily. It’s one of the most effective interventions for normalizing your weight
- Fasting has a number of health benefits, including improved cardiovascular health, reduced cancer risk, gene repair, increased longevity, and dementia prevention
By Dr. Mercola
Do you struggle with excess weight? Are you showing signs of insulin and leptin resistance? Is your fasting blood sugar above 100? If you answer yes to one or more of these questions, you may want to reconsider not only what you eat but when you eat as well.
One lifestyle factor that appears to be driving not only obesity but also many chronic disease processes is the fact that we eat too frequently.
Research reveals that a vast majority of Americans eat all day long.1 Most also consume a majority of their daily calories late in the evening, and this type of eating pattern is a recipe for weight gain and metabolic dysfunction.
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 rarely, if ever, skip a meal.
As a result, their bodies have adapted to burning sugar as its primary fuel, which down-regulates enzymes that utilize and burn stored fat. In addition, our ancestors didn’t have access to food 24/7, and biologically your body simply isn’t designed to run optimally when continuously fed.
Biological Repair and Rejuvenation Occurs During Fasting
Research has confirmed that many biological repair and rejuvenation processes take place when there’s an absence of food, and this is another reason why all-day grazing triggers disease. Your body never has the time to clean out the garbage and regenerate.
When you go without food for a period of time, the resulting metabolic changes stimulate a natural cleansing process known as autophagy, or mitophagy in the case of mitochondrial autophagy, in which your body detoxifies and rids itself of damaged cells.
When you’re in constant “feast mode,” your body forgoes many of these benefits. That does not mean you need to (or should) starve yourself for extended periods of time though.
Simply cycling between periods of eating and fasting on a daily or weekly schedule has been shown to provide many of the same benefits as complete fasting, where you don’t eat for several days.
What’s so Great About Intermittent Fasting?
Intermittent fasting is a term that covers an array of different meal timing schedules. As a general rule, it involves cutting calories in whole or in part, either a couple of days a week, every other day, or even daily, as in the case of the scheduled eating regimen I prefer to use myself.
As noted by Time magazine, intermittent fasting is becoming increasingly popular, and for good reason — it works. And it works whether you’re trying to lose weight or simply improve biomarkers for optimal health.
But what exactly makes it so effective for weight loss when other calorie-cutting diets have such a high failure rate? As noted in the featured article:2
“The body converts food into glycogen — a form of energy that it can store for later use. Your body then squirrels away that glycogen in both fat cells and in your liver.
‘If you’re eating all day, the stores of glycogen in your liver are never depleted,’ [neuroscientist Mark Mattson, Ph.D.] says.
On the other hand, after about 12 hours without food your liver runs out of glycogen, at which point your body starts drawing energy from the glycogen stored in your fat cells.”
In a nutshell, your body was designed to: a) run on fat as its primary fuel, and b) cycle through periods of feast and famine. Today, most people do the complete opposite. They eat sugar and net carbs (total carbs minus fiber), which is virtually identical to sugar metabolically, all day long.
So, by mimicking the eating habits of our ancestors, who did not have access to food around the clock, you restore your body to a more natural state that allows a whole host of biochemical benefits to occur.
Fasting May Hold Key to Cancer and Dementia Prevention
Besides normalizing your weight, intermittent fasting is also one of the ways by which you can significantly boost mitochondrial health and energy efficiency, which is important for chronic disease prevention, thereby cutting your risk for health problems like heart disease and cancer.
Intermittent fasting can also have a very beneficial impact on your brain function, and may even hold the key to preventing Alzheimer’s disease.
Mark Mattson, Ph.D. has conducted animal studies showing that when mice, genetically engineered to develop Alzheimer’s, are put on an alternate day fasting diet, they develop Alzheimer’s around the age of 2 years, which in human terms is equivalent to being 90.
Normally, they develop dementia in half that time — around 1 year, equivalent to the age of 40 or 50 in humans. When he put them on a junk food diet, they developed Alzheimer’s around 9 months! Mattson’s research suggests that alternate day fasting can boost a protein known as brain-derived neurotrophic factor (BDNF) by anywhere from 50 to 400 percent, depending on the brain region.
BDNF activates brain stem cells to convert into new neurons. It also triggers other chemicals that promote neural health, and has been shown to protect brain cells from adverse changes associated with Alzheimer’s and Parkinson’s disease.
Which Intermittent Fasting Schedule Is Right for You?
There are several intermittent fasting schedules to choose from, and the “right” one for you is the one you will actually comply with. Here are a few of the most popular eating schedules. For even more fasting protocols, including ones that are specifically designed to be combined with exercise, please see this previous intermittent fasting article.
|5-day fast||In one recent experiment,3 people who fasted five consecutive days once a month for three months in a row saw improvements in biomarkers for cell regeneration. Risk factors for diabetes, cancer, cardiovascular disease, and aging also declined.
You do not abstain from food entirely during these days. On the first day, you eat about 1,000 to 1,100 calories, followed by 725 calories on the remaining four days. Your diet during these days should be primarily plant-based, low in carbohydrates and protein, and high in healthy fat.
Beware that it can be quite challenging to go a full five days with very little food, especially if you’ve never fasted before, so you may want to work your way up to this kind of schedule.
|5:2 fast||On the 5:2 plan, you cut your food down to one-fourth of your normal daily calories on the two fasting days of your choice (about 600 calories for men and about 500 for women), along with plenty of water and tea. On the other five days of the week, you can eat normally.|
|Alternate day fasting||This program is exactly as it sounds: one day off, one day on. On fasting days, you restrict your eating to one meal of about 500 calories. On non-fasting days, you can eat normally.
When you include sleeping time, your fast can end up being as long as 32 to 36 hours. According to Krista Varady, Ph.D., author of “The Every-Other-Day Diet,” alternate day fasting can help you lose up to 2 lbs. per week.
Another benefit to alternate day fasting is that your body tends to adapt to the regularity of the program, whereas the randomness of the 5:2 plan can be more difficult to adjust to.4 In trials, about 90 percent of participants are able to stick to alternate day fasting, whereas the other 10 percent drop out within the first two weeks.
One caveat: more recent research5 shows that if you want to lose weight, you cannot binge on non-fasting days when you’re on an alternate day fasting plan, which is something you can typically do on the 5:2 plan. Exactly why is still unclear, but it may have to do with the fact that there’s less regularity in the pattern on the 5:2 plan.
Restrict daily eating to a six- to eight-hour window (the key here is to eat breakfast or dinner, but not both)
|Here you avoid eating for 13 to 18 hours. This strategy is more aggressive and, as a result, people tend to see results sooner. The specific time is based on your blood sugar readings. This is my personal preference as it’s really easy to comply with once your body has shifted over from burning sugar to burning fat as its primary fuel.
At that point, you cease to experience frequent hunger pangs, and can go for hours without a dip in energy. Fat, being a slow-burning fuel, is what allows you to keep going without suffering from the dramatic energy crashes associated with sugar.
In order to make this schedule work, you need to skip either breakfast or dinner. Which one to omit is up to you. However, if you chose to eat dinner, it’s important to avoid eating for at least three hours before going to bed.
The rationale for this recommendation has to do with the way your body produces energy. When you’re sleeping, your body needs the least amount of energy, and if you feed it at a time when energy is not needed you end up creating a situation in which your mitochondria creates excessive amounts of damaging free radicals.
I have recently appreciated that this is another important factor that can help optimize your mitochondrial function and prevent cellular damage from occurring.
What You Eat Still Matters
While some intermittent fasting programs claim you can binge on whatever you want on non-fasting days, I strongly recommend paying attention to the quality of your food regardless of the program you choose.
Since you’re eating less, you’ll want to make sure you’re getting high-quality nutrients from your food. Healthy fats are especially important, as intermittent fasting pushes your body to switch over into fat burning mode. If you feel tired and sluggish, it may be a sign you need to increase the amount of healthy fat in your diet.
Cutting net carbs (total carbs minus fiber) is equally important. Fructose is particularly troublesome as it activates a key enzyme, fructokinase, which in turn activates another enzyme that causes your cells to accumulate fat and resist letting any of it go. If you’re overweight, insulin-resistant, or diabetic, reducing sugar consumption is really key. So, as a general rule — whether you’re fasting or not, and regardless of the fasting schedule you’re on — I believe it’s important to eat a diet that is:
- High in healthy fats. Many will benefit from 50 to 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.
- Moderate amounts of high-quality protein from organically raised, grass-fed or pastured animals. Most will likely not need more than 40 to 80 grams of protein per day. (I recommend limiting protein to one-half gram of protein per pound of lean body mass.)
- Unrestricted amounts of fresh low net carb vegetables, ideally organic.
Peak Fasting — How Long Should You Fast?
Besides when and what to eat, another common question relates to duration. Just how long must you continue intermittently fasting? While some embrace it as a lifestyle (and this tends to be particularly true of those who restrict their daily eating to a specific window of time), it’s not something you have to do for the rest of your life. I don’t recommend any of the other types of fasting as they have major shortcomings from a metabolic perspective that I will discuss in my new book on this topic.
As a general rule, I recommend a new type of intermittent fasting that I call Peak Fasting, which is done every day rather than a few days a week. However you can certainly cycle in off days due to schedule or social commitments. The key is flexibility. But if circumstances allow, seek to do it every day. The process is simple.
Stop eating three hours before bed and don’t have your first meal for at least 13 hours. Measure your blood sugar at that time. You can do this every half hour, and when it starts to dramatically rise, this is an indication that you need to break your fast and eat food.
Why? Because suddenly rising blood sugar when you haven’t eaten is a sign that glucogenesis is setting in. By definition, glucogenesis refers to the production of glucose from a nonglucose precursor, such as protein. Once your body starts converting protein to glucose, you’re breaking down your lean muscle mass, and this is NOT healthy by any means.
This is also why I strongly recommend avoiding longer complete fasts. Research shows you can lose about ¼ pound of muscle mass per day if you fast for two days or longer! If you reach 16 to 18 hours and your blood sugar still hasn’t spiked, feel free to eat if you want to.
If you’re overweight and/or have symptoms of insulin and leptin resistance, such as high blood pressure, high cholesterol, or full-blown type 2 diabetes, continue intermittent fasting until your insulin/leptin resistance improves, and your weight, blood pressure, cholesterol ratios, or diabetes normalizes. As an example, if you need to lose 50 pounds, you’re looking at about six months or so of intermittent fasting, after which you can revert to eating more regularly.
After that, all you need is a “maintenance program.” Keep track of your markers, and if they start sliding, go back on the fasting program of your choice again for a number of weeks or months. Alternatively, you could intermittently fast for say one month, twice a year, as a form of maintenance.
Remember over half of the U.S. population is either diabetic or prediabetic, so measuring blood glucose is a powerful and cheap test. You can purchase the Bayer meter6 on Amazon for $7 and the strips are less than 25 cents apiece. I have tested many meters and this is the clear value winner. For less than $1 or $2, you can accurately and simply identify the ideal time of your intermittent fast.
If you are new to fasting, it may take some time to work up to 13 hours, but once you start activating your fat burning system you will easily achieve this. The most effective way is to limit your net carbs (total carbs-fiber) to under 40 grams per day and do not exceed more than 1 gram of protein per kilogram of lean body mass.
Tips for Making It Through the Transition Period
The toughest part of any intermittent fasting plan is getting through the initial transition, which can take anywhere from 7 to 10 days. Maybe even longer for some people, depending on how insulin-resistant you are, and other factors, like your weight, blood pressure and cholesterol levels, and if you are not consistent with the fasting and wind up cheating.
About 10 percent of people will report headaches as a side effect when they first start fasting, but the biggest complaint is hunger. It may be helpful to remember that part of why you’re craving food is because your body has not yet made the switch from burning sugar to burning fat as its primary fuel. As long as you’re running on sugar, which is a fast-burning fuel, frequent hunger pangs will be the norm. Fat is far more satisfying, as it’s a much slower-burning fuel.
Remember, a diet high in carbohydrates severely inhibits your body’s ability to produce lipase and use fat as an energy source. Lipase is inhibited because of high insulin levels, and your insulin rises in response to eating foods high in carbohydrates, so it’s important to replace carbs with healthy fat in order to successfully make that metabolic switch-over and become an efficient fat burner.
Another factor that can trip you up during the transition period is purely psychological. If you’re used to grazing throughout the day, it may take some time to break the habit. One trick is to drink more water. Oftentimes people mistake thirst for hunger.
Peak Fasting May Be the U-Turn You’ve Been Looking For
The vast majority of Americans are overweight and most would therefore benefit from intermittent fasting for a period of time. (Adrenal-fatigued individuals are perhaps an exception to this rule). When done correctly, you will inevitably lose weight and your insulin and leptin receptor sensitivity will be optimized, which is really important for optimal health. Fasting also:
- Decreases the accumulation of oxidative radicals in your cells, thereby preventing oxidative damage to cellular proteins, lipids, and nucleic acids associated with aging and disease.
- Raises human growth hormone. Research7 has shown fasting can raise HGH by as much as 1,300 percent in women, and 2,000 percent in men, which plays an important part in health, fitness and slowing the aging process. HGH is also a fat-burning hormone, which is another reason why fasting is so effective for weight loss.
- Inhibits the mTOR pathway, which plays an important part in driving the aging process by increasing mitophagy and mitochondrial biogenesis.
If you decide to attempt intermittent fasting, be sure to pay careful attention to hypoglycemic signs and symptoms, and if you suspect that you’re crashing, make sure to eat something, like coconut oil. I do not recommend fasting if you’re living with chronic stress (adrenal fatigue), or have cortisol dysregulation. Pregnant or nursing mothers should also avoid fasting, as your baby needs plenty of nutrients during and after birth, and there’s no research supporting fasting during this important time.