Intermittent fasting (IF) is an uber-trendy concept in wellness circles these days. It has been shown to be a helpful tool for weight loss–a study published in Molecular and Cellular Endocrinology found that intermittent fasting reduces body fat on par with traditional calorie-restrictive diets. But it’s also tough to pull off correctly and thus not a perfect solution for everyone. (Case in point: one 2020 study found that people tend to eat more food in anticipation of an extended fasting period.)
If you’ve questioned the practicality of intermittent fasting, you’re not alone. But according to an article in the New England Journal of Medicine, eating your meals on an IF schedule can help with more than just weight loss; it also has the potential to slow aging and disease progression…and more.
What happens in the body when you fast?
When your body has finished digesting a meal or snack, it begins to draw on its own stores of energy to keep your organs functioning properly. “Energy typically comes in the form of glucose,” explains Fatima Cody Stanford, MD, obesity medicine physician at Massachusetts General Hospital and Harvard Medical School in Boston, MA. Glucose is a simple sugar, derived from food, that travels through the bloodstream to supply the body with energy.
“When we’re fasting, we’re having to take either what we already have stored or produce [energy] stores through other mechanisms,” Dr. Stanford says. If you go for an extended period without eating (at least 16 hours in a 24-hour period), your body uses up its glucose stores and starts burning fat instead of carbohydrates, inducing a state called ketosis.
This is the idea of an intermittent fasting plan: you deprive your body of energy from food for a certain number of hours each day, then eat within the other allotted hours. IF gives your body a break from digestion and teaches it to use its own energy stores more effectively–a process known as metabolic switching.
Mark Mattson, Ph.D., professor of neuroscience at Johns Hopkin’s University in Baltimore, MD, is a principal author of the NEJM paper on IF’s benefits. He explains that fasting mimics the eating patterns of our ancestors, who had to hunt or forage for food before they could sit down for a meal. “Predators have to expend a lot of energy in a food-deprived state to catch their prey, and their brains have to be functioning really well in the fasted state, or they’re not going to be successful in getting food and passing on their genes,” he says. The humans who could function well on a calorie deficit were the ones who ultimately survived and had an advantage.
How do you fast?
Intermittent fasting takes many forms, depending on what works best for each individual. Here are the most popular ways to do it:
- 16:8 – You fast for 16 hours and eat your meals within an 8-hour window.
- 18:6 – You fast for 18 hours and eat your meals within a 6-hour window.
- 20:4 – You fast for 20 hours and eat your meals within a 4-hour window. (Not for the faint of heart!)
- 5:2 – You eat normally for 5 days per week, then severely calorie-restrict on the other two days.
- Alternate day fasting (ADF) – You eat normally every other day, with severe calorie restriction on your fasting days.
An important thing to note: IF isn’t technically a diet because it doesn’t involve limiting your overall calorie intake. Instead, Mattson calls it an eating pattern, a way of structuring your day to reap the benefits of your body’s metabolic processes.
What are all the benefits?
IF continues to be a hot research topic for nutrition scientists. Here’s what they know so far about its beneficial effects, summed up in the NEJM paper by Mattson and his co-author, Rafael de Cabo, Ph.D.
Intermittent fasting can help with weight loss. This is the main reason most people start IF in the for first place. Forcing your body to use its own energy stores each day results in a more effective metabolism. “Having been subjected to the fasting period, cells are better able to take up nutrients” such as amino acids and glucose, Mattson explains. Fasting can help increase insulin sensitivity, which keeps your blood sugar levels stable. It can also provide a natural framework for calorie restriction if you do want to limit your overall food intake to help you lose weight.
It reduces inflammation in the body. Dr. Stanford explains that when we are eating every few hours throughout the day, our bodies don’t get time to rest and recover. “Sometimes when we’re constantly feeding the body, especially highly processed foods, this can cause inflammation,” she says. Inflammation is a form of physical stress that can contribute to several chronic conditions and autoimmune diseases, such as IBD or MS.
It may help slow disease progression. Early research on mice with multiple sclerosis has revealed that fasting could help delay onset of symptoms. The NEJM paper also notes that IF is likely beneficial for patients with arthritis due to its anti-inflammatory effect. Plus, it’s been shown to reverse insulin resistance in people with prediabetes or type 2 diabetes.
It supports brain function. When the body is in fasting state, Mattson explains, it produces more of a protein called BDNF that supports learning and memory. “It’s thought that decreases in BDNF during aging contribute to the vulnerability of neurons in Alzheimer’s and Parkinson’s,” he says. The NEJM paper noted that an alternate-day fasting program has potential for slowing progression of neurogenerative disorders.
It could help you live longer. Fasting improves blood pressure, resting heart rate, and good cholesterol – all of which are seriously beneficial for your heart health. It is also thought to inhibit the ability of cancer cells to grow and multiply. Combine that with the cognitive and anti-inflammatory benefits, and your body is much more prepared to fight off physical signs of aging. “We know that overeating and being sedentary shortens lifespan by increasing risks for many chronic diseases,” Mattson says, especially if you are maintaining an unhealthy body weight.
What’s the catch?
Intermittent fasting is not a holy grail lifestyle change that works for everyone. The biggest issue? It’s hard to pull off, especially in the long-term. “This is often difficult to sustain,” Stanford says, adding that your body can’t truly reap the benefits of intermittent fasting until you’ve been doing it consistently for several months or years.
She notes that for some people, fasting fits easily within their lifestyle, but for others (those with less control over their mealtimes), it’s tough to keep it going. Fasting can be a challenge for your social life if your friends want to schedule plans outside your normal eating window, or if your family likes to have dinner together but aren’t all practicing this eating pattern.
Is there anyone specific who shouldn’t be trying IF?
The most important advice to remember here is to consult a professional before trying this. “For anyone that has chronic health conditions, I would work with a physician to make sure you’re doing this safely,” Dr. Stanford advises.
Are you diabetic? The changes in insulin sensitivity mean you’ll have to be extra careful to monitor your body as you’re easing into a fasting lifestyle. Improved insulin sensitivity can lead to a severe drop in blood pressure, Dr, Stanford explains, which could be dangerous. Still, IF could ultimately help folks with Type 2 who approach it with caution. “People who are overweight and have Type 2 diabetes can benefit from intermittent fasting,” Mattson says. “They should be careful and consult with their physician because of potential for hypoglycemia. Same goes for people with Type 1 diabetes whose bodies naturally don’t produce insulin. Don’t try fasting unless you have approval and support from your doctor.
Do you have chronic kidney and heart diseases? You also need close supervision on a diet like this, Dr. Stanford urges. “For people that have heart disease, which is very common in the U.S., it may affect their fluid status,” she says. Again, not that it’s not worth trying. “I just think it should be monitored, with the doctor being closely affiliated or tied into that person's care, to make sure that nuances of anything that could go wrong are easily and quickly addressed,” she explains.
Trying to conceive? Now is probably not the time to start a new fasting regimen. “It can impact fertility,” Dr. Stanford notes. This is a controversial subject, and much more research is needed on the specifics, but small studies have shown that caloric restriction may impact a woman’s hormonal balance, which could disrupt her menstrual cycle and thus make it harder to get pregnant.
Do you have osteoporosis? IF may also decrease bone density, so it’s not a great idea for people with osteoporosis. “Intermittent fasting can lead sometimes to weakening of the bone or even increased fracture risk,” Dr. Stanford says.
Too much on your “plate” already? Anyone who’s going to be too stressed out or anxious about maintaining a diet like this should consider their mental health first. Living a generally healthy lifestyle confers many of the same benefits on your body over time. “Healthy people who have a normal body mass index, are already exercising, and have a normal weight–we think there’s some benefits [to IF], but they are relatively minimal,” Mattson explains. “They’re already doing a lot to reduce their risk for these chronic diseases.”
Ultimately, what works for you may not work for everyone, which is why there are so many versions of IF–not to mention other lifestyle changes that can benefit you, such as daily exercise and a whole-foods diet. “Learn what works for you and continue to sustain it indefinitely,” Dr. Stanford suggests. “If it doesn’t work, then it’s fine. That doesn’t mean you’re a failure; it just means that it’s not right for you.”
If you are curious about intermittent fasting and think it could help you, reach out to your doctor to get their advice. If implemented properly, this eating pattern could be a great addition to your chronic treatment regimen, not to mention your overall health.