Intermittent Fasting
In Americans’ never-ending quest to lose weight, various diets — all with their own benefits and drawbacks – are pushed on a rolling basis. Keto, Atkins, and South Beach have all had their moments in the spotlight. One that has reemerged is intermittent fasting.
What is Intermittent Fasting?
Intermittent fasting is often called a “diet,” but it’s not what most people think of when they think about diets. Intermittent fasting focuses on “when” to eat rather than “what” to eat, so instead of a long list of foods that are “off-limits,” this plan divides each day (or week) into two periods: eating and fasting. Depending on the schedule you’ve chosen, you’ll eat during a certain period of the day, or on specific days of the week. Once the eating time frame is over, you fast until the next eating period.
The theory behind intermittent fasting is that, by allowing insulin levels to drop low enough, fat cells can then release their stored fat to be used as energy. This process is what burns fat, which may or may not result in weight loss — that depends on how much the person eats during the eating phase and how long the fasting phase is. But assuming that the person is fasting long enough and insulin levels get low enough, intermittent fasting can result in fat loss.
A published in the New England Journal of Medicine (NEJM) sifted through multiple intermittent fasting studies and found some interesting correlations as to why time-based eating may be so helpful for the body. Fasting triggers cells to perform essential duties — far more important than losing weight or burning fat. The NEJM researchers ultimately found that fasting can support metabolism and joints, blood sugar regulation, and can even help flush out toxins and dead or damaged cells.
However, if weight loss is the reason behind a person choosing to try intermittent fasting, it’s not a sure bet. Intermittent fasting doesn’t require calorie-counting, nor does it restrict the types of foods you can eat during the eating period.
“The first thing I tell clients who are interested in trying intermittent fasting is that it does not change the rules of energy balance,” says , NASM Certified Personal Trainer and ACE Certified Health Coach. “Meaning if they are doing intermittent fasting with a goal of weight loss, a calorie deficit is still necessary. A benefit of intermittent fasting while dieting is that it can make a calorie deficit easier.”
Foods that contain calories break the fast, but calorie-free beverages, such as water, black coffee, and unsweetened tea are allowed during fasting times, and are actually encouraged to ensure the body doesn’t become dehydrated.
Intermittent Fasting Benefits
Generally speaking, fasting develops metabolic flexibility — the ability of the body to go back and forth between burning carbohydrates and fats for fuel. This is an evolutionary adaptation which is important for overall health. However, there are some key benefits that research has been able to uphold on a consistent basis.
Time Shifting vs. Calorie Counting
One of the largest benefits to intermittent fasting is that it doesn’t require individuals to change the types of foods they’re eating — only that they restrict the times of day in which they’re consuming calories. If it works, the potential benefits include , fat loss, , a lower risk of , improved , and more efficient .
Cell Rejuvenation
“Autophagy is the self-eating process where a cell breaks down its own damaged components and remakes them – this is essentially the cell rejuvenating itself – and is triggered under conditions of nutrient deprivation (AKA fasting),” says Dr. Ralph E. Holsworth, D.O. and Director of Clinical and Scientific Research at . “That cell rejuvenation is the expected health benefit as the dysfunctional cells (like cancer cells) or dysfunctional cell components tend to be ‘eaten’ first.”
Cardiovascular Benefits
Another potential benefit lies within the cardiovascular system. A examined the findings of many intermittent fasting studies. Researchers concluded that “by affecting the biochemical transformations of lipids, [intermittent fasting] decreases body mass and has a positive influence on lipid profile parameters — it reduces the concentration of total cholesterol, triglycerides, and LDL cholesterol.”
The researchers were careful to point out that there are still unknowns regarding intermittent fasting and the benefits they found. In their conclusion, they wrote: “It remains questionable if these benefits are solely due to weight loss or non-weight loss mechanisms. The success of every type of diet depends on rule compliance—following a prescribed diet according to the circadian rhythm.”
In other words: were the cardiovascular benefits because of the fasting mechanism, or because of the decreased body mass?
Cancer Treatment Synergies
Along the same lines as Holsworth alluded to above, some research suggests that intermittent fasting can be helpful for cancer patients. found that fasting increased the potential efficacy of chemotherapy and radiation treatments, suggesting that a fast prior to treatment could have a positive impact.
While the researchers advocated for clinical trials, they concluded that “[caloric restriction] by fasting is likely an effective method to potentiate the cytotoxicity of chemotherapy and [radiation therapy] because of the overlapping induction of molecular profiles, and it may also provide a beneficial means of improving the overall health and metabolic profiles of patients.”
In his book The Longevity Diet, Professor Valter Longo, PhD discusses this very topic, including the potential for fast-mimicking diets to that have been linked to some forms of cancer. The clinical trials are still ongoing, but Longo believes that the early results are promising.
He points out that fast-mimicking diets are only intended to be part of the overall treatment and prevention of cancer, saying that “prophylactic mastectomies and other surgical procedures can reduce the incidence of genetically induced cancers, but nutrition and FMD may also help.
Of course, not everyone experiences all of these benefits. Each body is different.
Intermittent Fasting Schedules
The 16:8 Method
Perhaps the most popular intermittent fasting schedule is the 16/8 method. On this schedule, a person eats during an eight-hour period and fasts for 16 hours. This window could be whatever time of day is preferable for each individual person — for example, 9:00 am to 5:00 pm, or noon to 8:00 pm.
“The 16:8 is by far the most sustainable,” says . “As you know, the key to success for any lifestyle habit is sustainability. Within a week or two people realize that they can easily skip breakfast. The first meal is lunch around 11:00 am or 12:00 noon and then dinner is around 6:00 pm or 7:00 pm. This schedule allows for family dinners and a sense of normalcy when you’re home after work with the kids and your partner.”
The 5:2 Diet
Also known as The Fast Diet, the 5:2 method of intermittent fasting involves five days of eating normally and two non-consecutive days of fasting, such as Monday and Thursday each week. While some people choose to abstain from all calories on their fasting days, others choose to eat about 25% of what they’d usually eat — perhaps about 500 calories. This method is also very popular among intermittent fasters.
Eat-Stop-Eat
The Eat-Stop-Eat intermittent fasting schedule is very similar to the 5:2 schedule in that it involves fasting for an entire 24-hour period at least one day per week. This plan is more serious about consuming zero calories during the fasting period. If you choose to fast for more than one day in the week, make sure the days aren’t consecutive.
Alternate Day Fasting
Alternate day fasting involves a continuous schedule of 24 hours eating and 24 hours fasting. For example, if Monday is an “eating” day, you’d fast on Tuesday, eat Wednesday, etc. This is another schedule in which some people choose to eat a very small amount of calories on their fasting days.
The Warrior Diet
The Warrior Diet is based on the way in which ancient warriors are believed to have eaten — they largely fasted during the day and then ate a significant meal in the evening. In terms of hours, this schedule follows a 20-hour fasting window and a 4-hour eating window.
Theoretically, you’re allowed to eat a little bit during the fasting period, such as hard-boiled eggs, raw fruits and vegetables, and dairy. Of course, if you consume too many calories and gorge at night, you’ll defeat the purpose of the fast.
Which Method is the Best?
Ultimately, the schedule that works “best” is the one that an individual can stick to. “Some people do really well on a 16:8 fast. I’ve had others find that a 24-hour fast and eating one meal a day works better for them,” says . “Timing of when they break their fast is also important. Some people just feel better if they do have breakfast, while others do fine with skipping the morning meal and waiting to eat later in the day.”
Regardless of the chosen fasting schedule, Dr. Kellyann Petrucci, MS, ND incorporating bone broth during fasting periods to help with hunger pangs, shaking, or headaches. Bone broth is nutrient-rich, low-calorie and has no carbohydrates, so it won’t give you so many calories that it breaks your fast. “It allows you to realize the immediate benefits of fasting without the drawbacks.”
Is Intermittent Fasting Safe?
Overall, intermittent fasting is relatively safe — with a few caveats.
“Anyone who suffers from binge eating disorder, anorexia, bulimia or any other type of eating disorder should not experiment with intermittent fasting,” says eating disorder coach . “Extreme diets are a major risk factor and can ultimately lead to eating disorders. According to the , 35% of teens and young adults who diet periodically may develop an obsession with dieting. Research has found that eventually, between 20 and 25% of habitual dieters will be diagnosed with an eating disorder.”
Individuals shouldn’t try intermittent fasting without speaking to their practitioner first. Some underlying health conditions (such as diabetes) are not conducive for this type of eating schedule, and it may do more harm than good.
One potential downside to intermittent fasting is that people force themselves to ignore their hunger cues (when they’re hungry, when they’re full, etc). When they’re finally allowed to eat, it’s easy to start binge eating because they’re so hungry that they can’t stop, or they don’t recognize the cue that they’re full.
For those who exercise in the morning, intermittent fasting via the 16:8 method or the Warrior Diet may be a challenge. Without any food in the body, it may not have the energy it needs to get through the workout.
don’t recommend intermittent fasting for athletes at all because the body needs to be replenished after a hard workout, and it needs carbohydrates for fuel. But as Martinez stated, some research implies that the , and can be used by nearly anyone (even athletes).
If your patients are considering intermittent fasting, be sure to warn them about signs that it’s not a good fit for them. “Look for any signs of low blood sugar (hypoglycemia). These symptoms could include feeling dizzy, tired, headache, nausea, and poor memory,” says Martinez.
Potential Health Risks
Intermittent fasting is not recommended for women who are pregnant, trying to become pregnant, or are breastfeeding. The irregular consumption of calories is potentially dangerous, and will not support the level of energy to make breastmilk.
Fasting has the potential to cause complications for people with existing heart rhythm problems and higher risk of stroke. Patients who are elderly or infirm may not be good candidates for intermittent fasting.
Additionally, a frequent complaint of those who intermittently fast is headaches or migraines. Headache specialists don’t typically recommend this type of eating schedule for people who already experience or are prone to headaches.
Does Intermittent Fasting Work for Women?
Intermittent fasting affects men and women differently. While more research is needed to determine the exact reasons, many suggest that it’s simple biology.
“I’ve noticed that some female clients don’t respond well to intermittent fasting,” says , NASM Certified Personal Trainer and ACE Certified Health Coach. “Their morning hunger signals never seem to adapt, so we take things on an individual basis.”
Other practitioners have found that women need a more customized approach to the fasting schedule. “The difference for women and intermittent fasting is related to issues with hormones and some issues with cortisol levels, especially in the longer fasting duration,” says Kasey Hageman, MS, RD, LD. “For women interested in doing intermittent fasting, sticking with a larger eating window (such as a 10 to 12-hour period) would be better.”
Another hypothesis as to the difference is the regulation of glucose. “Men and women’s livers may perform differently when regulating glucose while fasting. Women release less glucagon (a hormone released to help raise blood sugar levels when they are low) after exercise compared to men,” says .
“Female bodies are extremely sensitive to calorie restriction. When calorie intake is low — such as from fasting for too long or too frequently — the hypothalamus is affected. This can disrupt the secretion of gonadotropin-releasing hormone (GnRH), which helps release two reproductive hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). When these hormones cannot communicate with the ovaries, you run the risk of irregular periods, infertility, poor bone health to name a few adverse effects.”
Additionally, some women have reported that intermittent fasting did not give them more energy, but left them feeling tired during the fast. Again, pregnant women, women planning to become pregnant, or nursing moms are not advised to try intermittent fasting.
Are Supplements Important for Intermittent Fasting?
There are no special supplement recommendations for people who want to try intermittent fasting. Many practitioners recommend a high-potency daily multivitamin anyway, which would also be a good idea on this type of diet.
“When incorporating intermittent fasting, it’s critical to make sure that you’re eating nutrient-dense whole foods that provide all of the vitamins and minerals necessary for optimal health,” says Dr. Scott Noorda, DO and Creator and CEO of the .
“We make personalized supplement recommendations for our patients based on micronutrient testing,” Noorda continues. “For those that are deficient in specific vitamins and minerals (which includes many of our patients), we recommend supplements that will help correct the deficiency in addition to a nutrient-dense diet. We advocate for counting nutrients instead of calories and many of our patients find that it is much simpler to focus on restricting their eating window than restricting calories.”
It’s most important that people who are choosing an alternate eating schedule are continuing to eat nutrient-rich, healthy foods. The Mediterranean Diet is largely believed to be the healthiest possible way to eat, and an intermittent fasting schedule doesn’t change that.
Do Practitioners Recommend Intermittent Fasting?
As with any other dietary question, the answer is: it depends.
“Intermittent fasting is just one of the tools in my tool kit when discussing strategies with patients,” says Hageman. “As a professional working with a population looking to lose weight, it is important to find a strategy that works long-term for them. If intermittent fasting can fit that long-term strategy for a client and they are interested in trying it, I fully support them.”
The most important question for patients who express interest in intermittent fasting is why they want to do it. What are they hoping to accomplish? If they want to cut out late-night snacking, or restrict the number of hours during the day in which they’re eating, intermittent fasting might be a worth a shot.
Are you interested in learning more about fasting? Check out these articles with additional pro tips.