Not so long ago, nutrition experts advised eating small, frequent meals throughout the day and never, ever skipping them. Today, some of those experts have changed their view. They now recommend a practice called intermittent fasting, or IF, an approach to food timing that involves periodic short fasts — anywhere from 14 to 48 hours — that can spark substantial health benefits.
“Back in 2013, I wrote about the importance of eating breakfast,” says functional-medicine practitioner Susan Blum, MD. “Today I say, ‘Most people can skip it.’”
For many, IF is as simple as extending the window between dinner and the first meal of the next day: If, for instance, you finish eating at 7 p.m., don’t snack before bedtime, skip breakfast, and have an early lunch at 11 a.m., you would go 16 hours without food. This is enough time between meals to trigger specific healthy changes in your body — and you’re asleep for much of it.
“The research on the benefits of intermittent fasting and time-restricted eating continues to accrue,” says functional-medicine physician Terry Wahls, MD, author of The Wahls Protocol. “Studies show that these strategies improve insulin sensitivity, reduce obesity, and reduce inflammatory cytokines. They also improve the efficiency and effectiveness of our mitochondria and increase nerve-growth factors in the brain. This a very exciting area of research.”
Still, that research is new and ongoing. “We don’t know enough yet about intermittent fasting to say ‘this is the best,’” explains Elizabeth Boham, MD, medical director of the UltraWellness Center in Lenox, Mass. “It depends on the person.”
For people who are pregnant or underweight or who have a history of eating disorders, for example, it’s best to steer clear. For others, however, it may be a useful tool.
So, what’s happening in the body during a longer period without food? Who can safely fast — and who should avoid the practice? We dug into the research to get a better understanding of this practice.
What Happens When We Fast
The benefits of IF are often chalked up to eating less food, which can happen naturally during a shorter eating window. Yet reducing calories isn’t the primary objective, and experts believe that eating less overall accounts for only some of intermittent fasting’s positive effects.
Notably, there are no restrictions on the amount of food you consume during the eating window, though choosing nutrient-dense whole foods and maintaining good hydration are essential. The majority of IF’s benefits seem to occur during the digestive process in the hours between meals.
Digestion is hard work. Five to 15 percent of the energy you expend in any given day is dedicated to it. Fasting gives the GI tract a longer break from this task, and many people notice improvements in their digestive health.
Other benefits include autophagy, or the body’s process of cleaning out damaged cells; this appears to begin around hour 22 of a fast. Other positive effects kick in earlier, around the 13th or 14th hour — and shift into high gear around the 15th or 16th.
First, IF appears to regulate hormones. “When you don’t eat, certain hormones, like insulin, go down and certain hormones, like cortisol, go up,” says Jason Fung, MD, author of The Obesity Code.
The body exists in two states, he explains. The first is the “fed state,” where insulin is high, helping to facilitate the storage of caloric energy. The second is the “fasted state,” where insulin is off duty and levels are low. Lower insulin levels and better insulin signaling may produce weight loss and improved body composition.
On the flip side, chronically high insulin is associated with type 2 diabetes, cancer, cardiovascular disease, and cognitive decline. (Fasting is a form of healthy stress that can increase cortisol production, which may be problematic for those with adrenal issues: see “Who Shouldn’t Fast” on page 57.)
Fasting also boosts mitochondria, the furnaces in our cells that burn fuel for energy. Fasting seems to help feed them by increasing mitochondrial respiration. This improves their efficiency — and their ability to reproduce — which increases energy and metabolism. Lackluster mitochondria may contribute to conditions like metabolic syndrome, cardiovascular disease, cancer, and neurodegenerative disorders.
IF may also slow some of the effects of aging. “If you can enter a fasting state, you can protect the length of telomeres, which means your cells can live longer and be healthier,” says Blum.
Telomeres are those sections of DNA that sit at both ends of our chromosomes and help protect them. They protect chromosomes during replication. They get shorter when DNA is damaged, which occurs naturally as we age — though it’s increased by exposure to toxins, smoking, stress, and poor diet. Shorter telomeres are associated with accelerated aging and cancer.
Fasting may also protect our genes through epigenetics. “Intermittent fasting can turn on the SIRT1 gene and turn off the mTOR gene. When hyperactive, mTOR is associated with Alzheimer’s disease, cancer, and early mortality,” says functional-medicine practitioner Sara Gottfried, MD. “Sirtuin, called SIRT1, is a gene that protects you from diseases of aging by revving up the mitochondria, which tend to conk out as you get older.”
How To Fast
There are different approaches to intermittent fasting:
Alternate-day fasting: Days without food are alternated with days of normal eating.
The 5:2 diet: Food intake is limited on two nonconsecutive days each week and normal on the other five.
Modified alternate-day fasting: On fasting days, food intake is limited to 20 to 25 percent of your usual consumption. You eat normally on the alternate days. Modified alternate-day fasting is the template for the 5:2 diet.
Fast-mimicking diet: This is a short-term, plant-based, ketogenic-style diet, in which calories are reduced by about 60 percent for five days. It was developed by cell biologist Valter Longo, PhD, to mimic the physiological effects of fasting.
Time-restricted eating: You eat only within a specific window of time — say, 10 a.m. to 6 p.m. Some people do this every day, others several days a week. This is the most popular method of intermittent fasting, perhaps because you sleep through most of the fast.
Dee Harris, RDN, CDE, a functional-medicine certified nutritionist and diabetes educator, recommends practicing intermittent fasting or time-restricted eating two to three days weekly. (She noted that her clients who did daily 18-hour fasts tended to overload on carbohydrates late in the day and sabotage their weight-loss efforts. They avoided this issue during daily 12–14 hour fasts.)
In a 2014 study published in Cell Metabolism, the weight-loss benefits of time-restricted eating were maintained even when interrupted by days of unrestricted eating — a regimen, the researchers note, that is “particularly relevant to human lifestyle.”
“I prefer the time-restricted feeding model of intermittent fasting,” says Gottfried. “The word ‘fasting’ for many people implies restriction or suffering. In reality, you are sleeping during most of the fasting window, and during the eating window, you don’t have to ‘restrict’ your calories. You don’t want to binge on ice cream, of course, but you can eat healthy, balanced meals.”
Research shows it’s important to match your eating window to your body’s 24-hour circadian rhythm — which means not eating too close to bedtime. In animal studies, mice given an eight-hour eating window during the night put on weight, became metabolically dysfunctional, and developed type 2 diabetes. “We know that eating at night causes spikes in blood sugar,” says Boham.
The primary focus of IF is the lengthier window of time without food, and protocols don’t dictate what you eat during your eating windows. Still, experts encourage common sense: Stick to whole foods low in sugar and high in phytonutrients, make sure you’re getting plenty of healthy fats and protein, and sidestep processed fare. Intermittent fasting doesn’t make way for a diet of doughnuts and soda. Any benefits you get from your fasting window will be erased by the oxidative stress of a poor diet.
Because of the relative ease of time-restricted eating, it can feel like cheating — or like it might not do the body enough good. But when practiced consistently, time-restricted eating even two or three days each week appears to produce powerful results. Studies have tied the approach to weight loss and maintenance; better cholesterol markers and overall cardiac health; and lower levels of inflammation, glucose, and insulin, as well as improvements in how the body handles insulin.
In particular, these are some of the more specific issues IF may help — and how:
Many people report an improvement in digestion when they begin intermittent fasting, which makes sense. Food’s total transit time in the body averages 53 hours. To eliminate waste, the digestive system deploys the migrating motor complex (MMC), which propels it toward the exits. The MMC is active between meals but shuts down when we eat. Longer breaks from food offer the MMC more time to do its work, and good elimination is key to digestive health.
The fasting-mimicking diet also offers gut-healing potential. In animal studies, it helped reverse signs of inflammatory bowel disease, encouraged gut-tissue regeneration, and fostered the growth of beneficial gut bacteria. When researchers observed the fasting-mimicking diet in humans, they found that participants with higher-than-normal levels of inflammation at the start of the fasting cycle had reduced inflammatory markers at the end.
Fasting facilitates weight loss in three ways. First, as insulin levels decrease, fat-burning improves.
“Insulin is the main storage hormone for fat,” explains Fung. “When we eat, insulin goes up and we store fat, or glycogen, in the liver. When insulin goes down, we start to break down glycogen and burn fat.” Low levels of insulin tell the body to start burning its stored fat.
Second, fasting prompts cells to make new mitochondria — and the more mitochondria you have, the more fuel you burn. “There is a direct line between mitochondria and metabolism,” says Blum. “Your mitochondria take carbs and other nutrients and burn them for energy. That’s how you live.”
Third, when eating windows narrow, people tend to eat fewer calories. But focusing solely on calorie restriction during an intermittent fast will sabotage any weight-loss goals and your overall health. This is why it’s critical to eat quality food during your food window and not restrict calories.
“When you restrict calories on a chronic basis, your metabolism goes down,” says Harris. Your body thinks it’s starving, so it slows your metabolism to help you store fat for the famine.
“The idea that metabolic rate is stable is false,” explains Fung. “We’ve long known that it can go up or down. So the question is how can you control your metabolic rate, not how many calories you put in.”
IF is one way to control your metabolic rate in a positive direction.
Type 2 Diabetes
Type 2 diabetes and other metabolic conditions, like prediabetes and insulin resistance, are characterized by blood-sugar imbalance and chronically high insulin.
In a 2018 article Fung published in BMJ Case Reports, three adult patients with type 2 diabetes used therapeutic intermittent fasting to reverse insulin resistance and control their blood sugar. All three were able to stop taking insulin to manage their diabetes.
A 2018 study published in Cell Metabolism found that time-restricted IF with an eating window early in the day improved insulin sensitivity, blood pressure, and oxidative stress.
Multiple studies have linked IF with lower levels of LDL (so-called bad cholesterol) and weight loss, two factors that play a role in optimal cardiovascular health.
Intermittent fasting’s most profound contribution to heart health might be its ability to lower inflammation. Atherosclerosis is a chronic inflammatory condition in which plaque accumulates in the arteries; it’s the leading cause of vascular disease worldwide.
“Vascular disease occurs when there is inflammation and oxidative stress that causes changes in cholesterol and in the arterial lining,” says Boham. “We know that high insulin and high blood sugar are associated with that inflammatory process, and that intermittent fasting will lower insulin.”
Intermittent fasting’s ability to help regulate blood sugar and lower insulin is one key way it helps protect against neurodegenerative diseases.
“Abnormal glucose signaling is responsible for 60 percent of cognitive decline,” says Gottfried. “By helping to regulate and keep blood sugar under control as we age, intermittent fasting is a lifestyle change that can have long-term benefits on our brain health.”
Evidence suggests that IF promotes a process called autophagy, in which the body breaks down and discards dysfunctional cell components. Autophagy, which becomes less efficient as we age, is linked to inflammatory conditions, including neurodegenerative diseases. “When toxins build up inside our cells, disease can begin and progress,” says Harris.
Autophagy clears out that toxic buildup, Harris says, “providing a reset button that signals the cell to rejuvenate.” It’s worth noting that other forms of healthy stress, like exercise, also promote autophagy.
IF also increases the production of brain-derived neurotrophic factor (BDNF), which helps boost resistance to cognitive degeneration.
For some, IF can also improve mental focus and energy. This harks back to our ancestors. “Fasting revs things up,” says Fung. “People have more energy and they concentrate better when they fast. If there is no food to eat, you don’t become weak and tired, because if you do, you won’t be able to go out and find food and save yourself.”
Likewise, if you plan to exercise on a non-eating day, plan your workout in the morning before you break your fast. Your body will be primed for activity and you may net additional benefits, including positive metabolic and mitochondrial changes.
Cancer involves negative cell changes and uncontrolled cell growth. Autophagy, which IF promotes, helps cull cellular waste. Dysregulated autophagy, meanwhile, is associated with a host of degenerative diseases, including cancer.
“There’s lots of research on not eating for 13 hours overnight for cancer prevention,” says Boham. Animal studies, for instance, suggest IF may delay the progression of a range of cancers, as well as protecting healthy cells from chemotherapy. Periods without food help build the immune system’s stress resistance, and this can combat the immunosuppression that often follows chemotherapy.
The blood-sugar stabilizing effect of fasting also protects against cancer. A 2016 study of more than 2,000 breast-cancer patients found that those who fasted for 13 hours or more overnight had significantly lower levels of hemoglobin A1c (a measure of blood sugar), a reduced risk of breast-cancer recurrence and mortality, and reduced risk of all-cause mortality.
Still, the relationship between autophagy and cancer is complex and may depend on tumor type, stage of progression, and genetics. Consult with a nutritional oncologist or trusted healthcare provider if you have active cancer and want to try IF.
A Strategy for Health
Because IF emphasizes food timing rather than denial of certain foods or a labyrinthine, eat-this-not-that regimen, it’s an approachable health and weight-loss strategy. In many ways, IF can enhance your eating pleasure: With only the fasting window to consider, you may find yourself worrying less about what you eat overall.
“When you eat during only eight hours of the day, you are probably not overeating,” says Blum. And if you focus on a whole-food, plant-heavy diet with plenty of protein and healthy fats during your eating hours, you’re going to be wonderfully well fed.