Over the past year, weight loss medications have skyrocketed in popularity. However, as more people turn to Ozempic and other semaglutide-based regimens to shed excess pounds, others find themselves looking for non-medical alternatives that won't cause the drug's notorious set of side effects—or cost its astronomically high price tag. Enter: intermittent fasting (IF).
The concept of IF has been around for a long time, but it was popularized in 2012 by Michael Mosley's BBC documentary Eat, Fast, and Live Longer. In it, the journalist reforms his own diet after learning that despite appearing to be in relatively good shape, his metabolic health, as measured by blood tests, told a different story. While meeting with experts, Mosley learns that intermittent fasting can not only lead to weight loss but also comes with a shockingly wide range of health benefits.
Since then, others have continued to probe the potential benefits of intermittent fasting. Two popular books—Kate Harrison's book The 5:2 Diet and Dr. Jason Fung's 2016 bestseller The Obesity Code—agree with Mosley's assertion that when it's carried out responsibly, IF can not only lower your weight but also prevent chronic illness and lower your overall risk of mortality.
Still, you may have questions: What is intermittent fasting, and can it actually help with weight management, health, and longevity? Here's everything you need to know when considering the strategy for yourself.
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What Is Intermittent Fasting?
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Intermittent fasting is any time-restricted eating schedule in which you only consume calories during certain times of day. "It involves no pills, no injections. and no hidden costs—it's all about what you eat. Or rather, what you don't eat," Mosley says in his TV documentary.
Many people use intermittent fasting techniques—following a healthy diet and utilizing strategic fasting periods—as a way to not only lose weight but also to improve their metabolic health and reduce their risk of certain chronic illnesses.
Is It Beneficial for Weight Loss?
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The research suggests that when done safely, intermittent fasting can definitely help you lose weight. For instance, one meta-analysis of 40 studies found that people lost an average of seven to 11 pounds over the course of 10 weeks of intermittent fasting.
Harvard Health Publishing explains that this is because when you eat food, the nutrients are broken down by enzymes in the gut and directed to the bloodstream.
"Carbohydrates, particularly sugars and refined grains (think white flours and rice), are quickly broken down into sugar, which our cells use for energy. If our cells don't use it all, we store it in our fat cells as, well, fat," they explain. "But sugar can only enter our cells with insulin, a hormone made in the pancreas. Insulin brings sugar into the fat cells and keeps it there."
"IF makes intuitive sense," Harvard's experts further note. "Between meals, as long as we don't snack, our insulin levels will go down and our fat cells can then release their stored sugar, to be used as energy. We lose weight if we let our insulin levels go down. The entire idea of IF is to allow the insulin levels to go down far enough and for long enough that we burn off our fat."
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What Can I Have While Fasting?
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Cory Rice, DO, chief clinical advisor at Biote, notes that during your fasting period, it's important to restrict your calorie intake completely.
"Intermittent fasting must be done on a routine basis if you want to get fatty acid metabolism," Rice tells Best Life. "The reason people fail at intermittent fasting is simply falling short of the fasting timing goals. Also, many people think they are fasting because they only 'eat a little bit in the morning.' As I tell my patients, you are either fasting or you aren't. Unfortunately, you cannot be 'nearly' fasting."
However, it's important to stay hydrated. "While doing intermittent fasting, patients can drink water, unsweetened tea, and black coffee. They should definitely avoid all diet drinks and sugar substitutes," the doctor says.
Katrina Mattingly, MD, chief medical officer at Options Medical Weight Loss, adds that bone broth can also make a good addition to your fasting day.
"It's a great choice for getting nutrients in a low-calorie manner while fasting. Bone broth contains collagen, gelatin, amino acids, vitamins, and other nutrients, so it's easy to see why this is a nutritionally sound choice for some people," she says.
What Should I Eat When Not Fasting?
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It's crucial to distinguish between intermittent fasting and crash diets that restrict calories down to dangerous levels. Regardless of when you eat, it's important to consume a well-rounded diet of whole foods with an adequate amount of total calories.
Harvard Health Publishing says you should still avoid processed foods, sugars, and refined grains during your eating window. "Instead, eat fruits, vegetables, beans, lentils, whole grains, lean proteins, and healthy fats (a sensible, plant-based, Mediterranean-style diet)," their experts say. They also recommend avoiding snacks between meals, as this can bring insulin levels back up and prevent weight loss.
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What Is the Ideal Schedule for Intermittent Fasting?
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IF can take many forms and can follow many different schedules. "A growing body of research suggests that the timing of the fast is key, and can make IF a more realistic, sustainable, and effective approach for weight loss, as well as for diabetes prevention," writes Harvard Health Publishing.
One popular approach is the 5:2 diet in which you eat normally five days a week, then follow a time- and calorie-restricted diet two days a week. On those two days, you might consume between 500 to 600 calories in total. Alternate day fasting uses this same concept, but switches off between fasting and non-fasting days.
Another approach is to follow a less extreme daily fasting schedule. "A typical intermittent fasting schedule is the 16/8 model," explains Rice. "With this model, you do not consume calories for 16 consecutive hours a day, but you will have an 8-hour-a-day feeding window. For simplicity, many will follow a noon to 8:00 pm feeding window followed by an 8:00 pm to noon fasting window."
Some people take their intermittent fasting pattern to even greater extremes by using the "eat-stop-eat" approach. This involves choosing two non-consecutive days in which to fast for a full 24 hours. Many experts have pointed out that this can be dangerous, as you may experience low blood sugar, fatigue, irritability, extreme hunger, dizziness, or other harmful side effects. Most experts recommend against this approach in favor of the more moderate 5:2 or 16/8 models.
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Pros of Intermittent Fasting
You're likely to lose weight.
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If you're considering intermittent fasting for weight loss, you're likely to see results. "There are many benefits to intermittent fasting," says Rice. "First and foremost is that it will force the body to use fat for fuel," he says, noting that many of the weight loss benefits come from reducing one's overall calorie intake.
It may help you control your blood sugar.
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Intermittent fasting diets help control blood sugar by increasing insulin sensitivity and reducing insulin resistance. In fact, a 2023 study found that time-restricted eating (TRE) is a safe method for those with Type 2 diabetes (T2D) to lower their blood sugar levels.
In fact, this randomized controlled trial "found that a TRE diet strategy without calorie counting was effective for weight loss and lowering of HbA1c levels compared with daily calorie counting in a sample of adults with T2D."
You may lower your inflammation levels.
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Inflammation causes a range of age-related chronic illnesses, including heart disease, diabetes, Alzheimer's disease, cancer, and more. Intermittent fasting may improve your inflammation levels, thereby lowering your risk of those illnesses and related mortality.
"When the body is fasting and begins to utilize our fat storage as glucose is not readily available, our visceral fat levels may decrease. Less visceral fat, the unhealthy kind, means less inflammation in the body," explains Mattingly.
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Cons of Intermittent Fasting
It's not right for everyone.
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IF isn't right for everyone, experts caution. Seniors, people who are pregnant or breastfeeding, and children, teens, and young adults should avoid IF since they have different nutritional needs from the general adult population.
People with certain chronic illnesses, including diabetes or diseases of the heart, kidneys, or liver, are also at higher risk of adverse effects from IF. Avoid IF if you take certain medications, including blood thinners, blood pressure medications, or those that affect your blood sugar, says Mass General Brigham. Finally, IF can be harmful to anyone with a history of eating disorders.
You may experience a rough "detox" phase.
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Rice notes that many people may struggle most with an intermittent fasting diet when transitioning from their regular eating schedules.
"The only real drawback can be the first week or two," he says. "Many times, patients will go through a phase of detox. Our bodies initially do not like the change. It is important that our patients stick to the program, knowing they will feel much better after the first two weeks."
You may lose lean muscle mass.
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Some studies have suggested that people who lose weight by intermittent fasting may lose more lean muscle mass than people who eat for longer eating windows. However, this can occur when a person loses weight by any means and may not be specific to intermittent fasting.
To retain your lean muscle mass, it's important to do strength training exercises while intermittent fasting—no small feat when you're running on fewer calories on a fasting day.
It may come with certain heart risks.
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A recent study set off alarm bells when it found a correlation between intermittent fasting and cardiovascular and cancer-related deaths in those already diagnosed with those conditions. Most shockingly, the study found that people with heart disease who had an eating window of eight hours per day or less were 91 percent more likely to die from cardiovascular disease than those who had an eating window of at least 12 to 16 hours per day.
However, as The New York Times reports, the study didn't prove causation, only correlation—and the characteristics of the particular study subjects may have swayed the data. The group who ate for fewer than eight hours per day "tended to be younger and less educated; have lower income and less access to food; and be more likely to smoke than the other participants," the Times wrote.
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Final Thoughts
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Like many weight loss diet plans, intermittent fasting comes with potential risks and benefits, though the benefits appear to outweigh the risks. When losing weight, it's always best to do so under a doctor's supervision. That's why it's important to speak with your doctor before you decide to begin IF and to discuss any concerns you may have including underlying health conditions and medications you take.
It's also crucial to follow a healthy, whole foods-based diet while following an intermittent fasting plan. Eating ultra-processed foods during your eating window and fasting the rest of the time will not likely result in weight loss or any meaningful health benefits—and could, in fact, harm your health more than a well-balanced, non-time-restricted plan. Speak with a doctor or nutritionist to learn more about whether intermittent fasting may be right for you.
Best Life offers the most up-to-date information from top experts, new research, and health agencies, but our content is not meant to be a substitute for professional guidance. When it comes to the medication you're taking or any other health questions you have, always consult your healthcare provider directly.
