Intermittent fasting (IF) has gained a lot of attention in recent years as a simple, flexible way to support weight loss and improve metabolic health. While traditional diets focus on what you eat, intermittent fasting focuses on when you eat during the day. Most people begin fasting 16 hours a day, leaving an 8-hour window for eating. If the goal is weight loss, the eating window often must be tightened to 6 hours or less, which is why we advise people to consult their physicians before starting.
Below, we’ll answer common questions, drawing on expert insights from Dr. Dan Monti’s recent conversation with Dr. Julita Mitchell. Dr. Mitchell is a board-certified physician in primary care and integrative medicine who leads an integrative weight management program at The Marcus Institute of Integrative Health in Philadelphia.
Video Transcript »
Dr. Monti:
There are a lot of diets out there, but do any of them work? There’s one that I use a lot in my practice called the intermittent fasting diet, so I’ve invited a top expert on weight management to help us do this diet in the healthiest way possible. I’m here with Dr. Julita Mitchell, a board-certified primary care physician who is also board-certified in integrative medicine. She runs the Integrative Weight Management Program at the Marcus Institute. Welcome, Dr. Mitchell.
Dr. Mitchell:
Thank you, happy to be here.
Dr. Monti:
So today, I really wanted to delve into diets that work because, so often in my practice, people say, “I’ve tried the low-carb diet, I’ve tried this diet, I’ve tried that diet.” What happens is they lose some weight, or they try a commercial weight loss program with membership fees or pre-packaged food. They lose weight but then gain it back, sometimes even more. I know, from my integrative medicine background, we want to get them to change their lifestyle. But sometimes people need more—they want a jump start and to try it without medication first. One diet I’d love to hear more about from you is the intermittent fasting diet. I’ve seen great data and great results with some of my patients, but as a weight loss expert, I’d love your opinion.
Dr. Mitchell:
Intermittent fasting is a great option for many people, though not for everyone. Essentially, intermittent fasting involves cutting out calories or food intake within a specific window of time. There are many ways to do this. The most popular approach is time-restricted eating, where you limit your eating to certain hours of the day. This can be as simple as an overnight fast with a 12-hour fasting window, or, as most medical research studies, a 16-hour fasting window and an 8-hour eating window. I typically aim for that 16:8 ratio with most of my patients.
Dr. Monti:
Do you find success with the 16:8 ratio? I’ve found that I sometimes have to restrict the window further.
Dr. Mitchell:
It depends on the patient, their starting metabolism, and their age.
Dr. Monti:
How does age affect this diet?
Dr. Mitchell:
I’ve found that the older the patient, the tighter the eating window often needs to be because they’re in a more metabolically slowed state. Sometimes, they may need a window as small as five or six hours to achieve the weight loss they’re looking for.
Dr. Monti:
That makes sense. Do you recommend starting gradually with intermittent fasting?
Dr. Mitchell:
Absolutely. One of the pitfalls I see is people starting too fast, too soon. I suggest beginning with an 8-hour eating window, then tightening it as needed. Gradual changes are more sustainable and avoid creating a restriction mindset. I want my patients to focus on nourishing their bodies, not restricting them.
Dr. Monti:
Why does intermittent fasting work, and is it healthy?
Dr. Mitchell:
Intermittent fasting increases metabolic flexibility. When you fast, your body shifts from primarily using glucose from carbohydrates as fuel to using ketones, which are derived from fat stores. Over time, this metabolic flexibility allows your body to efficiently alternate between fuel sources. During the fasting period, especially toward the end of the 16 hours, your body utilizes its glucose stores and then mobilizes fat stores, which produces ketone bodies.
Dr. Monti:
So, beyond mobilizing fat, are there additional health benefits?
Dr. Mitchell:
Yes. Intermittent fasting, when done correctly, is one of the most efficient ways to reverse insulin resistance, especially in the pre-diabetic range. There’s also evidence that calorie restriction and fasting can have positive effects on tumor markers in animal models and early human studies. This is linked to a process called autophagy, where your body clears out cellular debris, reducing inflammation and potentially decreasing risks like tumor growth.
Dr. Monti:
How long should someone be on an intermittent fasting diet?
Dr. Mitchell:
Research is ongoing, but current studies show it can be done safely for up to 12 months.
Dr. Monti:
What does weight loss typically look like for your patients on this diet?
Dr. Mitchell:
Many patients notice a reduction in visceral fat, particularly around their midsection. Their clothes fit better, even if the scale doesn’t show a dramatic drop. It’s more about reducing body fat than just pounds.
Dr. Monti:
Are there any pitfalls to watch out for?
Dr. Mitchell:
Yes. One common issue is neglecting quality nutrition during the eating window. I recommend high-protein, high-fiber, and high-quality fats. Poor nutrition during the eating phase can lead to micronutrient deficiencies and hinder progress.
Dr. Monti:
What dietary recommendations do you give your patients?
Dr. Mitchell:
I recommend starting with a Mediterranean-style diet. It’s the most researched anti-inflammatory, whole-foods diet and has profound benefits for cardiovascular health, metabolic health, brain health, and even cancer risk. Combining intermittent fasting with the Mediterranean diet maximizes not just weight loss but healthy weight loss.
Dr. Monti:
I’ve noticed that patients often feel overwhelmed at first, but most adjust to the eating window fairly quickly. Do you see that as well?
Dr. Mitchell:
Yes, and that’s likely due to hormonal shifts. Over time, fasting improves insulin sensitivity, reduces carb cravings, and allows people to stick to the diet for months as their hormonal balance improves.
Dr. Monti:
That explains why, once patients get into a rhythm, they naturally adhere to healthier eating habits, like the Mediterranean diet. It seems to all fall into place.
Dr. Mitchell:
Exactly. Intermittent fasting reduces the hunger hormone ghrelin and increases the satiety hormone leptin, which helps curb cravings and overeating. These hormonal changes also have positive immune and anti-inflammatory effects, which may contribute to long-term health and longevity.
Dr. Monti:
You and I have talked before about the importance of gut health. Does intermittent fasting have an impact on the gut microbiome?
Dr. Mitchell:
Yes, it does. Fasting helps regulate the digestive system, and when paired with a high-fiber Mediterranean diet, it promotes a healthy gut microbiome. A healthy gut improves metabolism and reduces cravings, making these changes more sustainable.
Dr. Monti:
That makes sense. I’ve noticed patients often report their tastes changing—they start craving healthier foods over time.
Dr. Mitchell:
Exactly. As they make small, positive changes, their taste buds adjust, and they begin to prefer foods that are lower in sugar, fat, and calories. This is the best-case scenario.
Dr. Monti:
One question I get often is whether people should combine intermittent fasting with newer weight loss medications. What are your thoughts?
Dr. Mitchell:
It’s not always recommended to combine them. While time-restricted eating and medications can work as standalone approaches, using them together can sometimes lead to difficulties in meeting nutrient requirements, like protein and fiber. This can result in deficiencies, so it’s something to approach with caution.
Dr. Monti:
I imagine combining Mediterranean diet principles with those medications would be a better approach?
Dr. Mitchell:
Absolutely. That’s a proven combination, but it’s a topic for another day.
Dr. Monti:
Thank you for such an insightful conversation, Dr. Mitchell. This was incredibly helpful for understanding the intermittent fasting diet.
Dr. Mitchell:
Thank you for having me.
How many hours a day should I fast vs. eat?
Your exact hours should be chosen in conversation with a physician who is well-versed in this diet and who has reviewed your medical history; these hours can vary depending on factors like your age, metabolism, and overall health goals. As we age, for example, it may be more important to tighten our eating window further (e.g., 5–6 hours) to see the desired results. During the fast there should be zero calories. Even creamer in your morning coffee ends the fast.
What time should I fast when on an intermittent fasting schedule?
There’s no single “right” time to fast; it’s all about fitting intermittent fasting into your lifestyle. Many people find it easiest to stop eating after dinner, fast overnight, and then push their first meal into mid-morning or early afternoon. If you’re new to fasting, start with a schedule that feels manageable—maybe stop eating at 8 p.m. and break your fast at 8 a.m. Over time, if needed, you can adjust this window to maximize benefits.
During the hours that you fast, there should be zero calories—even creamer in your morning coffee ends the fast.
Does it matter what kinds of foods I eat during my eating window?
While intermittent fasting primarily focuses on timing, what you eat during your eating window is still critically important. At The Marcus Institute of Integrative Health, we recommend a Mediterranean-style diet rich in high-quality protein, healthy fats, and ample fiber. This approach has well-documented benefits, including improved cardiovascular health, better metabolic markers, and anti-inflammatory effects. Think: whole, nutrient-dense foods like vegetables, legumes, whole grains, lean protein sources, and healthy fats from olive oil and nuts.
How long can someone safely practice intermittent fasting?
Current research suggests that intermittent fasting can be safely practiced for around 12 months. Of course, long-term studies are still underway, and it’s always wise to work closely with a healthcare provider if you plan to maintain IF beyond a year or if you have any pre-existing medical conditions.
At The Marcus Institute of Integrative Health, we recommend a Mediterranean-style diet rich in high-quality protein, healthy fats, and ample fiber.
What are the benefits of intermittent fasting?
Intermittent fasting isn’t just about shedding pounds. One key advantage is its ability to improve metabolic flexibility—your body learns to efficiently switch between using glucose and fat for fuel. Over time, this can lead to reduced visceral fat (thus, shrinking your waistline), improved insulin sensitivity, and potentially other health perks like lower inflammation and favorable changes in certain health markers. Early research even suggests positive effects on cellular cleanup processes (autophagy) that may influence long-term wellness.
Will I feel too hungry while practicing intermittent fasting?
Feeling hungry is a common fear, but most people adapt surprisingly quickly. As insulin sensitivity improves and hormones like ghrelin (the hunger hormone) decrease over time, cravings often lessen. Many individuals find that after a short adjustment period with intermittent fasting, going without food for longer stretches becomes easier, and their appetite naturally aligns with their eating window.
Can intermittent fasting help with insulin resistance and prediabetes?
Yes. Intermittent fasting is one of the most efficient ways to improve insulin sensitivity in those who have early insulin resistance (but not necessarily full diabetes). By giving the body an extended break from constant glucose intake, IF can help reset metabolic parameters and potentially reduce the risk of developing type 2 diabetes.
Can I combine intermittent fasting with GLP-1 weight-loss medications?
We suggest caution in combining intermittent fasting with certain newer weight-loss medications, such as GLP-1 agonists. We do not recommend combining the two, as these medications can suppress appetite, making it harder to get enough protein, fiber, and essential nutrients in a shortened eating window. If you’re on these medications, a more balanced approach—like incorporating Mediterranean diet principles without a strict fasting window—may be preferable.
Intermittent fasting can help reset metabolic parameters and potentially reduce the risk of developing type 2 diabetes.
Are there any drawbacks or pitfalls to watch out for when intermittent fasting?
One common pitfall is neglecting the quality of your food during the eating window. Intermittent fasting is not a green light to binge on nutrient-poor choices. Make each meal count by focusing on whole, high-quality foods.
Also, avoid tightening your window too quickly. Starting slowly and making gradual adjustments helps ensure long-term sustainability, helping you avoid a scarcity mindset and potential nutrient deficiencies.
Can I do intermittent fasting if I’m vegan?
Yes, you can practice intermittent fasting on a vegan diet. The key principle of IF—controlling when you eat—applies to any eating style, including plant-based diets. However, it’s a good idea to talk with your doctor or a nutritionist about any supplements or dietary adjustments needed to ensure you get enough of certain key nutrients while on a vegan diet—whether or not you are practicing intermittent fasting!
During your eating window, focus on nutrient-dense, whole-food choices like legumes, vegetables, fruits, whole grains, nuts, and seeds. These foods can help you get enough protein, healthy fats, fiber, and essential micronutrients.
As always, consult with a healthcare provider or a registered dietitian to tailor the approach to your individual needs, especially if you’re new to fasting or have any underlying health conditions.
Is intermittent fasting different for men and women?
Intermittent fasting can be practiced by both men and women, but women may need to be more mindful about timing and intensity—particularly before their menstrual cycles. Some women may experience greater sensitivity to changes in eating patterns due to hormonal fluctuations.
If you notice disruptions in your cycle, mood changes, or lower energy when starting IF, consider a more moderate approach. For instance, women can begin with a shorter fasting window, gradually adjust, and pay close attention to how their bodies respond throughout the month. Consultation with a healthcare professional knowledgeable in women’s health can help fine-tune your approach to ensure it supports both metabolic goals and overall well-being.
Intermittent fasting offers a flexible, research-backed approach to weight management and metabolic health. By choosing a fasting window that works for you, focusing on nutrient-dense meals, and giving your body time to adjust, you can reap the benefits without feeling overly hungry or deprived. If you have underlying health conditions or questions about combining IF with medications, consult a qualified healthcare professional for personalized guidance.
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