Full water glass

7 Surprising Things about Intermittent Fasting

Diets have done an about face in the past few years. Although what you eat and how much you eat are still important, new research shows that when you eat may be as powerful. The most popular eating strategy du jour, intermittent fasting (IF), revolves around meals that are strategically timed. The intermittent fasting diet protocol has several different approaches that all juggle eating periods with non-eating, fasting, periods. Alternate-day fasting (ADF) where you eat very little — or nothing — every other day. The 5:2 approach is a version of intermittent fasting where you eat whatever you want on five days per week and “fast” by dropping to a very low-calorie diet, usually around 500 to 800 calories on two non-consecutive days per week. The fasting-mimicking diet (FMD) triggers a fasting state of ketosis by having a person eating a very low-calorie, plant-based ketogenic diet for five days at a time, one week out of a month or every few months. The rest of the month, one eats normally. Time-restricted feeding (TRF) divides each day into an eating window and a fasting window. The aim is to lengthen the fasting window; the most common protocol is the 16:8 intermittent fasting plan: a person eats all their meals for the day within an eight-hour period and then fasts, eating nothing, for 16 hours.1,2

Incorporating intervals where you eat very little — or fast completely — during a day or a week appears to improve health and body weight, and even promote longevity.3 But fasting is essentially starving yourself. And this approach to eating may lead you to wonder if it’s safe, how hungry you’ll get and what to expect. Here are seven things you should know about intermittent fasting.


Oddly, fasting is an appetite suppressant. Normally, the body relies on fat and glucose for energy. But when you stop eating, glucose supplies get depleted. After 8 to 12 hours with no food, the body undergoes what’s known as a “metabolic switch.” Fat starts to become the preferred energy source. Fat is broken down, shuttled to the liver and then converted into ketone bodies, which provide energy for many tissues and organs, especially the brain. The longer the fast lasts, the greater the elevation of ketones in the blood.3

Although some studies have found that people do feel hungry on fasting days, others have not, especially when with more time following the protocol.4 This may be because ketones have an appetite-dampening effect. “Targeting fat as the main fuel source has the added benefit of making you feel full, so you may feel less hungry,” says Dr. Stephen Anton, PhD, associate professor and chief of the clinical research division at the University of Florida’s Department of Aging & Geriatric Research.


Your first assumption might be that going for hours and hours with little to no food might make you a bit woozy in the head. Some people report the opposite. Many people report that they think more clearly once they start practicing intermittent fasting. If ketosis is achieved during a fasting period, ketone bodies can activate the gene that produces brain-derived neurotrophic factor (BDNF), a protein in the brain, as well as in other organs like the liver, heart and lungs. BDNF is responsible for the growth and maintenance of brain cells that also play a role in learning and memory. BDNF levels decrease as people age and are associated with age-related cognitive impairments. A number of factors boost BDNF levels including exercise, antidepressant medications, as well as caloric restriction. Several studies have shown improvement in memory in older adults who practice intermittent fasting.3,5,6

Intermittent fasting can help decrease body fat, including unhealthy belly fat, and can literally reverse signs of aging and improve your healthspan.


No matter which type of Intermittent fasting you do, the days or periods where you eat nothing, or very little, are counterbalanced by the days or periods where you eat as normal, or can eat as much as you want, referred to as ad libitum by researchers. This “feasting,” where you eat all that you want during the periods that you are not “fasting,” is part of the allure of this eating regimen. And while some people practice IF for its health benefits, many do it to lose weight. But that begs the question, how can you “feast” and still lose weight?

It boils down to the math. If you eat fewer calories than you burn each day, you’ll lose weight. And, even though you can eat as much as you want on feast days and you might not be consciously tracking your calories, over a period of a week or so, overall calorie intake tends to be lower.

One study published in the British Journal of Nutrition had 10 healthy, overweight or obese participants eat normally for one day, meaning they stayed in energy balance, eating as much as their body burned. On another day they ate only 75 percent of their normal caloric intake, and on a third day they fasted completely, eating nothing. After each trial, the study subjects were given a liquid meal, and their subsequent food intake was tracked to see how much more they might eat to compensate for the calorie-reduced day or for the total fast day. The researchers found that the study participants only ate 10 to 22 percent more than their norm on the feast day following a fast. In theory, they would have needed to eat 100 percent more on feast days to stay weight stable. But they remained in negative energy balance, eating 30 percent less than normal, even after two feast days following their reduced-calorie or total fast days.7

Another study published in Obesity Research had 16 lean adults practice alternate-day fasting for three weeks. They consumed only calorie-free beverages on fast days. On feast days, they could eat what they wished and were informed that they would need to eat double their usual intake in order to prevent weight loss. But they were unable to eat enough on feast days. So even though they were lean, they lost an average of 4.6 lbs. in three weeks.8 “Feast days tend to be self-limiting,” says Dr. Anton. “Most studies show that people have a hard time eating enough calories to make up for the calorie deficit created on the fast day.”


Starving yourself for periods of time isn’t all peaches and cream. When you feel hungry, you can get irritable or angry, hence, “hangry.” Several pivotal research studies that looked at calorie restriction found that study participants grew more irritable the longer the fasting continued. In the early ’90s, a well-publicized experiment on calorie-restriction was conducted at the Biosphere 2, an enclosed environment where participants were meant to sustain themselves for two years with no outside help. Although they grew their own food, it was scarce, and they ended up reducing calorie intake by 30 percent below their energy needs. Some subjects developed food fantasies and became extremely irritable. Another study into caloric deprivation, the CALERIE trials where participants ate around 25 percent less than normal, also discovered some downsides. Participants reported having trouble sleeping and a low libido.3,8,9,10,11

On the other hand, some people have reported experiencing a mild euphoria during fasts, and also while following a low carbohydrate diet. The science hasn’t caught up with this still only anecdotal phenomenon. But one researcher suggests that it’s an effect of ketosis, where higher levels of beta-hydroxybutyrate (BHB) may play a role in this enhanced sense of well-being. Since ketosis signals that the body has entered a sort of starvation state, author Dr. Andrew Brown, PhD, hypothesizes, “From an evolutionary perspective, mild euphoria associated with short-term fasting may ease anxiety and aid the search for food.”11

Couple Drinking Coffee


The various approaches to intermittent fasting divide a week or a day into periods where you can eat or “feast” and periods of “fast,” where you drop your calorie intake to very low levels or omit food altogether. Inevitably, you might wonder what foods or drinks — if any — are kosher during the fasting window. The answer: nothing that contains calories. So, black coffee, plain tea and water can be consumed freely. The flavored beverages may help, psychologically at least, to obtain a sensation of food intake during non-eating periods. “Coffee is a natural appetite suppressant, so it might help you during longer fasting periods,” says Dr. Anton. “And there is some evidence that coffee may help your body to produce ketones.”

But the question inevitably arises…what about a little bit of creamer in the coffee or sugar in the tea? What about an almost calorie-free snack such as lettuce or other greens, or a few antioxidant-rich blueberries?

Although more research is needed to understand the mechanisms, most approaches recommend that during the fasting period, not a sip, not a bite, nor a drop of a food or drink that contains calories is allowed. That’s because once the body senses energy intake, then digestion processes kick in and different aspects of being in the physiological fasting state that confer many of the health benefits of fasting, like ketosis or autophagy, can be suspended.8,13

There’s one caveat: the fasting-mimicking diet, developed by researcher Dr. Valter Longo, PhD, and other ketogenic diets that induce a fasting state and put the body into ketosis may be able to bypass that trigger because you are able to maintain a state of ketosis, even though you are consuming some (mostly high-fat) foods.6


Intermittent fasting appears to confer a multitude of health benefits. Some health conditions may increase the health risks of fasting: diabetes or other conditions that involve the regulation of blood sugar, pregnancy or a history of eating disorders. Some studies have shown that oral glucose tolerance, or the ability to handle a sweetened drink normally, is impaired, especially in women, after fasting. The normal insulin response to shuttle the glucose to be used or stored is blunted, leading to higher than expected blood sugar levels. This is thought to be from the high levels of fat circulating in the blood that can hinder the ability of insulin receptors to function. This may only be an effect of short-term fasts, but means that anyone with diabetes or problems regulating blood sugar should consult their physician before doing intermittent fasting.7 “Many people report having more energy during the day, and during their workouts, when they do some type of intermittent fasting,” says Dr. Anton. “But if you have any health conditions that may put you at risk, follow these strategies under your doctor’s supervision.”


Most people who lose weight assume that it’s all fat loss. But that’s not the case. When a person loses weight, those lost pounds on the scale are comprised of fat, water and what is known as lean body mass (LBM) or fat-free mass (FFM).

It is common for people to refer to fat-free mass or lean body mass as muscle mass, when they refer to studies that show a gain or loss of it. But lean body mass or fat-free mass are defined in different ways, depending upon how they are measured. Generally, fat-free mass includes muscle. But it is also a measure of other soft tissues in the body and sometimes includes bone (minerals).14

What is not commonly realized is that when both lean and obese people lose weight, up to 20 to 30 percent of those pounds can come from fat-free mass, meaning that muscle can be lost when you diet. Preserving muscle is important for a variety of reasons. So, protocols to prevent or minimize fat-free mass losses are commonly included in well-designed weight-loss programs. Eating a greater percentage of foods that are higher in protein can compensate for the lower calorie intake. Exercising while losing weight, especially doing resistance training, can also minimize muscle loss.15

So, what happens to muscle mass when you fast?

You might not be doing much exercise, especially if your energy levels are low. In fact, one type of the FMD is a plant-based keto diet that is high in fat, but provides a very low number of calories for five days, advises against physical activity.6 Other types of intermittent fasting may not be as extreme. But because you’re fasting, either completely, or eating too few calories to obtain even the minimal recommended dietary allowance (RDA) for protein, losing more fat-free mass seems to be a given.

In fact, muscle tissue may be protected. “During a fasting period, the body’s metabolic switch gets activated; the lipids, or fats, in fat cells are metabolized and ketone levels increase,” explains Dr. Anton, who also blogs about fasting regimens. “Growth hormone levels increase and energy sources shift from glucose to fat, both helping to preserve muscle during the fasting period.”

One study had overweight and obese adults lose weight by either cutting calories with a standard diet plan or by intermittent fasting. After 12 weeks, both diets produced similar losses in total pounds and fat mass, but muscle was better preserved in the group who had fasted. Lean body mass increased after three cycles (over three months) of the fasting mimicking diet.6,16 It’s unclear how muscle might be affected from long-term intermittent fasting, especially among those severely reducing calories, such as when eating only one meal a day (OMAD) where it may be challenging to consume enough to meet essential nutrient needs, including protein. When practicing intermittent fasting, even though the focus is not generally on what one eats, as with any diet, it’s a good idea to choose the most nutrient-dense foods possible while eating a reduced calorie diet.15 Intermittent fasting can help decrease body fat, including unhealthy belly fat, and can literally reverse signs of aging and improve your healthspan.4 “Intermittent fasting has beneficial effects on all organs and tissues in the body,” says Dr. Anton. “It’s worth trying because it can help to create a healthy, strong and powerful mind and body.”