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A Doctor's Guide to Intermittent Fasting: Is It Right for You?

By Jeffrey J Kraft, MD, Board Certified Bariatric Surgeon HealthDay Reporter

Medically reviewed by Carmen Pope, BPharm. Last updated on Aug 12, 2025.

TUESDAY, Aug. 12, 2025 — As a bariatric surgeon, I talk with patients every day about weight loss and improving their overall health. One topic that comes up more and more is intermittent fasting. You’ve probably heard about it from a friend, seen it on social media or read about celebrities who swear by it.

But what is it, really? Does it work? And most importantly, is it safe? Let’s cut through the noise and look at the science behind this popular eating pattern.

What is intermittent fasting?

Intermittent fasting isn’t a diet in the traditional sense. It doesn’t tell you what to eat, but rather when to eat. It’s an eating pattern that cycles between periods of eating and periods of going without voluntary fasting.

Think of it this way: For most of human history, people didn’t have access to food 24/7. Our bodies are actually well-equipped to handle periods without food. Intermittent fasting simply builds a structured plan around this natural ability.

There are several popular ways to do intermittent fasting:

Intermittent fasting isn’t a diet in the traditional sense. It doesn’t tell you what to eat, but rather when to eat. It’s an eating pattern that cycles between periods of eating and periods of going without voluntary fasting.

Think of it this way: For most of human history, people didn’t have access to food 24/7. Our bodies are actually well-equipped to handle periods without food. Intermittent fasting simply builds a structured plan around this natural ability.

There are several popular ways to do intermittent fasting:

• The 16/8 Method: This is one of the most common. You fast for 16 hours a day and have an eight-hour window to eat your meals. For example, you might finish dinner at 7 p.m. and not eat again until 11 a.m. the next day.

• The 5:2 Diet: With this method, you eat normally for five days of the week. On the other two days, you significantly reduce your calorie intake, usually to about 500 to 600 calories.

• Eat-Stop-Eat: This involves a full 24-hour fast once or twice a week. For instance, you could eat dinner on Monday and then not eat again until dinner on Tuesday.

What does intermittent fasting feel like?

Starting any new routine takes some getting used to, and intermittent fasting is no different. It’s not a disease with symptoms, but your body will go through an adjustment period.

In the beginning, you might feel:

• Hunger pangs: This is the most obvious reaction, but it often passes after the first few days as your body adapts.

• Headaches: These can occur due to changes in your hormone levels and hydration. Drinking plenty of water is key.

• Irritability or “Hanger”: Feeling a bit grumpy is common as your body gets used to the new schedule.

• Low energy: You might feel a bit sluggish at first.

As your body adjusts, many people report feeling:

• More energetic: After the initial phase, many people experience a boost in energy and focus.

• Less bloated: Giving your digestive system a long break can help reduce bloating.

• More in control: Sticking to a schedule can make you feel more mindful and in control of your eating habits.

How does intermittent fasting work?

The magic of intermittent fasting happens on a cellular and hormonal level. When you go for an extended period without food, several things happen in your body.

After hours without food, your body exhausts its sugar stores and starts burning fat for energy, according to researchers from Johns Hopkins Medicine. This is called metabolic switching.

Here’s a simple breakdown:

• Insulin levels drop: When you eat, your body releases insulin to help your cells absorb sugar from your blood. When you fast, your insulin levels fall significantly. Lower insulin levels make it easier for your body to burn stored fat.

• Cellular repair (Autophagy): Fasting triggers a process called autophagy. Think of it as your body’s cellular “spring cleaning.” During autophagy, your cells get rid of old, damaged proteins and waste products. This process is essential for cellular health and may play a role in protecting against some diseases.

The potential benefits of intermittent fasting

The reason intermittent fasting has become so popular is that studies have linked it to a number of health benefits.

• Weight loss: This is the most common reason people try intermittent fasting. By limiting your eating window, you often naturally eat fewer calories. Furthermore, the hormonal changes, like lower insulin, can help your body burn fat more effectively. A 2022 review of studies found that intermittent fasting is an effective strategy for weight loss in people with overweight or obesity. A recent study in the Annals of Internal Medicine indicates that intermittent fasting, particularly the 4:3 method, may lead to greater weight loss over 12 months compared to daily calorie restriction. But this weight loss is still primarily driven by an overall reduction in calorie intake, meaning intermittent fasting is effective when it leads to a net calorie deficit. Furthermore, a review of 99 randomized clinical trials also found that various intermittent fasting strategies were generally as effective as, but not necessarily superior to, traditional calorie restriction for weight loss. The takeaway: Intermittent fasting is a tool for calorie reduction, not a way to bypass it.

• Heart health: Some research suggests intermittent fasting can improve several risk factors for heart disease. Studies in animals and humans have shown that fasting can help lower blood pressure, improve cholesterol levels and reduce inflammation, according to the National Institute on Aging.

• Brain health: Early research is showing promise for brain health. The metabolic switch from using sugar to using fat for energy may help protect brain cells. Studies suggest that fasting may improve memory and protect against age-related brain diseases.

• Insulin resistance: Intermittent fasting has been shown to be very effective in lowering insulin levels and reducing insulin resistance. This can significantly lower the risk of developing Type 2 diabetes.

Potential risks and who should be cautious

Intermittent fasting is a powerful tool, but it is not for everyone. It’s crucial to approach it safely and to understand the potential risks. As a doctor, I strongly advise against intermittent fasting for certain groups.

You should avoid intermittent fasting or speak with a doctor first if you:

• Are pregnant, breastfeeding, or trying to conceive. Your body needs consistent nutrients to support you and your baby.

• Have a history of eating disorders. The restrictive nature of fasting can be a trigger for unhealthy behaviors.

• Have Type 1 or Type 2 diabetes. While intermittent fasting can help with insulin resistance, it can also cause dangerous drops in blood sugar (hypoglycemia). It should only be done under close medical supervision.

• Are underweight or have a low body mass index (BMI, an estimate of body fat based on height and weight).

• Are a child or a teenager. Growing bodies have different nutritional needs.

• Take certain medications. Medications that need to be taken with food can be difficult to schedule with an intermittent fasting plan.

Making intermittent fasting a part of your life

If you and your doctor decide that intermittent fasting is a good option for you, the key to success is making it a sustainable part of your lifestyle.

Start slowly. You don’t have to jump into a 16-hour fast on day one. Try a 12-hour fast first (for example, from 8 p.m. to 8 a.m.) and gradually extend the window.

Most importantly, listen to your body. If you feel unwell, dizzy or overly weak, it’s a sign to stop and re-evaluate. And remember, the quality of your food still matters. During your eating window, focus on nutrient-dense foods like lean proteins, whole grains, fruits and vegetables. Intermittent fasting is not an excuse to binge on junk food.

Ultimately, intermittent fasting can be a safe and effective way to lose weight and improve your metabolic health, but it requires a thoughtful approach. It’s not a magic bullet, but for the right person, it can be a valuable tool on the journey to better health. Always consult with your health care provider before making any major changes to your diet or lifestyle.

About the expert

Dr. Jeffrey Kraft is a board-certified general surgeon specializing in minimally invasive procedures. He holds the position as the Chief of General Surgery at Palisades Medical Center. He prioritizes patient well-being and positive surgical outcomes, focusing on minimizing pain and maximizing results.

Disclaimer: Statistical data in medical articles provide general trends and do not pertain to individuals. Individual factors can vary greatly. Always seek personalized medical advice for individual healthcare decisions.

Posted August 2025

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