Not so long ago, nutrition experts advised eating small, frequent meals during 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 which involves periodic short fasts — anywhere from 14 to Two days — that can spark substantial health advantages.
“Back in 2021, 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 simply by extending the window between dinner and also the first meal of the following day: If, for instance, you finish eating at 7 p.m., don’t snack before going to bed, skip breakfast, and have an earlier lunch at 11 a.m., you'd go 16 hours without food. This is enough time between meals to trigger specific healthy changes in your body — and you’re asleep for a lot of it.
“The research on the advantages of intermittent fasting and time-restricted eating is constantly on the accrue,” says functional-medicine physician Terry Wahls, MD, author of The Wahls Protocol. “Studies show that these strategies improve insulin sensitivity, reduce obesity, and lower inflammatory cytokines. They also improve the efficiency and effectiveness of our mitochondria and increase nerve-growth factors in the brain. This a very exciting section of research.”
Still, that research is new and continuing. “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 instance, 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 advantages of IF are often chalked as much as eating less food, which could 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 results.
Notably, there are no restrictions on the quantity 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 appear to occur during the digestive process in the hours between meals.
Digestion is difficult work. Five to 15 percent of the energy you expend in almost any given day is dedicated to it. Fasting gives the GI tract an extended break from this task, and lots of people notice improvements within their digestive health.
Other benefits include autophagy, or the body’s process of cleaning out damaged cells; this seems to begin around hour 22 of a fast. Other positive effects start working earlier, around the 13th or 14th hour — and shift into high gear round the 15th or 16th.
First, IF seems 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 2 states, he explains. The very 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 levels of insulin and better insulin signaling may produce weight loss and improved body composition.
On the other hand, chronically high insulin is assigned to 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 all 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 appears to help feed them by increasing mitochondrial respiration. This improves their efficiency — as well as their ability to reproduce — which increases energy and metabolism. Lackluster mitochondria may bring about conditions like metabolic syndrome, cardiovascular disease, cancer, and neurodegenerative disorders.
IF may also slow some of the effects of aging. “If you're able to enter a fasting state, you are able to protect the length of telomeres, which means your cells can live longer and become healthier,” says Blum.
Telomeres are those parts of DNA that sit at each side of our chromosomes and help protect them. They protect chromosomes during replication. They get shorter when DNA is broken, which occurs naturally as we grow older — 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 change on the SIRT1 gene and turn off the mTOR gene. When hyperactive, mTOR is assigned to Alzheimer’s disease, cancer, and early mortality,” says functional-medicine practitioner Sara Gottfried, MD. “Sirtuin, called SIRT1, is really 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 will vary 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 every week and normal on the other five.
Modified alternate-day fasting: On fasting days, food intake is limited to 20 to A quarter of your usual consumption. You eat normally on the alternate days. Modified alternate-day fasting may be 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 results of fasting.
Time-restricted eating: You eat only within a specific window of time — say, 10 a.m. to six 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 2021 study published in Cell Metabolism, the weight-loss benefits of time-restricted eating were maintained even if interrupted by days of unrestricted eating — a regimen, they note, that is “particularly highly relevant to human lifestyle.”
“I prefer the time-restricted feeding type of intermittent fasting,” says Gottfried. “The term ‘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 wish to binge on ice cream, of course, but you can eat healthy, balanced meals.”
Research shows it’s vital that you match your eating window for 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 throughout the night put on weight, became metabolically dysfunctional, and developed diabetes type 2. “We know that eating during the night causes spikes in blood sugar,” says Boham.
The primary focus of IF may be the lengthier window of time without food, and protocols don’t dictate your food intake 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 method for a diet of doughnuts and soda. Any benefits you get from your fasting window is going to 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. However when practiced consistently, time-restricted eating even 2 or 3 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; minimizing levels of inflammation, glucose, and insulin, as well as improvements in how the body handles insulin.
In particular, they are some of the more specific issues IF may help — and how:
Digestive Health
Many people report an improvement in digestion when they begin intermittent fasting, which makes sense. Food’s total transit amount of 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 conclusion.
Weight Loss
Fasting facilitates weight loss in 3 ways. First, as insulin levels decrease, fat-burning improves.
“Insulin may be the main storage hormone for fat,” explains Fung. “Whenever we eat, insulin goes up and that 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 your body to start burning its body fat.
Second, fasting prompts cells to create new mitochondria — and the more mitochondria you have, the more fuel you burn. “There's 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 throughout an intermittent fast will sabotage any weight-loss goals as well as your overall health. This is why it’s important to eat quality food during your food window and not restrict calories.
“Whenever 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 concept that metabolic rate is stable is false,” explains Fung. “We’ve long known that it may go up or down. Therefore the question is how can you control your metabolic rate, not how many calories you put in.”
IF is an excellent method to control your metabolic rate inside a positive direction.
Type 2 Diabetes
Type 2 diabetes along with other metabolic conditions, like prediabetes and insulin resistance, are characterized by blood-sugar imbalance and chronically high insulin.
In a 2021 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 levels. All three were able to stop taking insulin to manage their diabetes.
A 2021 study published in Cell Metabolism found that time-restricted IF with an eating window in the morning improved insulin sensitivity, blood pressure, and oxidative stress.
Cardiovascular Health
Multiple studies have linked IF with 'abnormal' amounts of LDL (so-called bad cholesterol) and weight reduction, two factors that lead to optimal cardiovascular health.
Intermittent fasting’s most profound contribution to heart health might be its ability to lower inflammation. Atherosclerosis is really a chronic inflammatory condition in which plaque accumulates in the arteries; it’s the leading reason for 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 all know that high insulin and high blood sugar are associated with that inflammatory process, and that intermittent fasting will lower insulin.”
Cognitive Health
Intermittent fasting’s capability to help regulate blood sugar minimizing 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 really a lifestyle change that can have long-term benefits on our brain health.”
Evidence shows that IF promotes a process called autophagy, where 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 start 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 boosts 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 to our ancestors. “Fasting revs things up,” says Fung. “People have more energy and they concentrate better when they fast. When there is no food to eat, you don’t become weak and tired, if you do, you won’t have the ability 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'll net additional benefits, including positive metabolic and mitochondrial changes.
Cancer
Cancer involves negative cell changes and uncontrolled cell growth. Autophagy, which IF promotes, helps cull cellular waste. Dysregulated autophagy, meanwhile, is assigned to a host of degenerative diseases, including cancer.
“There’s plenty of research on not eating for 13 hours overnight for cancer prevention,” says Boham. Animal studies, for instance, suggest IF may delay the advancement of a range of cancers, as well as protecting healthy cells from chemotherapy. Periods without food improve the immune system’s stress resistance, which can combat the immunosuppression that often follows chemotherapy.
The blood-sugar stabilizing effect of fasting also protects against cancer. A 2021 study of more than 2,000 breast-cancer patients found that those who fasted for 13 hours or more overnight had significantly 'abnormal' amounts 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 could 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 perhaps a labyrinthine, eat-this-not-that regimen, it’s an approachable health insurance 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 when you focus on a whole-food, plant-heavy diet with lots of protein and healthy fats on your eating hours, you’re going to be wonderfully well fed.
Who Shouldn't Fast
Intermittent fasting (IF) may sound like a panacea, but it's not right for everyone. Experts suggest that the next groups avoid the practice:
- Pregnant or breastfeeding women. Pregnancy is really a period of growth, and fasting doesn't support what the body needs during this time. Likewise, a woman who's breastfeeding has higher nutrient and calorie requirements.
- Children. Fasting is not a good match for growing bodies. “Don't limit nutrients during this period,” says Jason Fung, MD.
- Anyone with a history of disordered eating. “Restricted eating windows can be very triggering” for people with a history of eating disorders, says Sara Gottfried, MD.
- Underweight people. Many people lose weight during IF, so if you are underweight to begin with, avoid it. “If you've got a BMI under 20, do not do it,” says Fung.
- People suffering from heart problems or high blood pressure. IF might be cardioprotective, but it is critical for anyone with heart problems or high blood pressure to consult their doctor before embarking on a fasting regimen. “Intermittent fasting might lead to electrolyte imbalances or other medical conditions in people struggling with heart problems or high blood pressure,” says Gottfried.
- Those with adrenal issues or blood-sugar imbalances. IF may cause spikes in cortisol, which may further aggravate adrenal issues making blood-sugar problems worse. “Because cortisol and insulin regulate blood sugar, if you have impairments in either of these systems, you might struggle with IF initially,” says Susan Blum, MD. She asks patients with adrenal fatigue or blood-sugar issues like diabetes or insulin resistance to start with a 12-hour fast and build from there. Gottfried advises patients with reactive hypoglycemia (dangerously low blood-sugar levels following a meal) “to avoid IF or carry it out only under careful physician observation.”
A note of caution for people with thyroid issues: While IF can ultimately be useful for people with thyroid conditions, the first stages can be tricky because of the impact on insulin and cortisol. Listen to your body, says Gottfried. “Start slow and increase step by step,” she says. “If it does not feel right, you may need to address the main cause first instead of simultaneously.”
*For details on the dos and don'ts of IF, see “The Case for Intermittent Fasting”.