Intermittent Fasting Explained: How It Works, When to Eat, and Important Considerations

Fasting, or voluntarily going for a period of time without food, is a practice that has been around for thousands of years. Traditionally tied to spiritual, religious, and medical traditions, it remains studied today in evidence-based medicine and popularized within current health and wellness communities as a potential way to encourage weight loss and support metabolic health.

As a registered dietitian who offers nutrition counseling for a variety of health conditions, I am often approached by patients who have an interest in trying intermittent fasting because they read about it online or hope it can help them lose weight. Before making any decisions about diet, it’s important to understand how fasting works first since it isn’t right for everyone.

This article will walk you through many of the same points I share with my patients: the different types of intermittent fasting, how fasting affects the body, research associated with intermittent fasting health claims, and safety considerations.

What is intermittent fasting?

Intermittent fasting is a type of fasting that restricts food (and usually any calorie-containing beverages) to a specific time frame, meaning you are only allowed to eat during designated hours of the day or maybe certain days of the week. These are referred to as ‘eating windows.’

What makes intermittent fasting so popular is that some research suggests it may play a role in weight, cholesterol, and blood sugar management.

Popular ways to do intermittent fasting

There is no one-size-fits-all approach to doing intermittent fasting. Each method offers slightly different schedules and fasting lengths. Some even allow modified caloric intake instead of complete food restriction.

Below is a breakdown of a few of the most common methods for intermittent fasting:

The 16/8 method

This method involves fasting for 16 hours, then eating within an eight-hour window. It is popular as an introduction to intermittent fasting since it still allows for eating during ‘typical’ hours. I find that most of my patients who have tried this method tend to skip breakfast to prolong their overnight fast to 16 hours, then eat between the hours of something like 11 a.m. and 7 p.m.

You can also find variations of the 16/8 method, which adjusts for shorter or longer fasting and eating periods. Examples:

• 18/6 (fasting for 18 hours, eating within a six-hour window)
• 14/10 (14 hours fasting, 10-hour eating window)
• 12/12 (equal eating and fasting periods)

The 5:2 diet

This type of intermittent fasting allows for your typical diet five days out of the week. Then, for two days (but not two days in a row), you restrict your caloric intake to 500 or 600 calories.

Eat-Stop-Eat

Eat-Stop-Eat is a variation of the 5:2 diet. Five days out of the week, you eat normally as you are used to. One to two days a week, you fast for a full 24-hour period.

Interestingly enough, even though you fast for 24 hours at a time, it’s unlikely you will go a full day without eating. For example, the 24-hour period may start at 8 a.m. on Thursday and go until Friday at 8 a.m., meaning you would have had an opportunity to eat breakfast Thursday morning as well as have the opportunity to eat lunch and dinner the following day on Friday.

Alternate-day fasting

Alternate-day fasting is a type of intermittent fasting that involves fasting every other day. One day, you eat as usual; the next, you consume no more than 500 calories.

How intermittent fasting works in your body

While intermittent fasting restricts eating to a certain period of time without focusing on calorie restriction in most cases, in practice, you may end up eating fewer calories, which can result in a caloric deficit for weight loss.

However, in addition to time-restricted eating, intermittent fasting primarily works by encouraging the body to tap into fat stores for energy.

Glucose (the end product of carbohydrate digestion) is our body’s go-to source for energy. However, when glucose isn’t available because of a fast, the body finds alternatives:

• Without glucose from the diet, stored glucose (called glycogen) is released from the liver to support essential functions.
• When glycogen stores are gone (after about 12 hours of fasting or longer), the body turns to stored fat tissue for energy, converting fatty acids into ketones.

You should ease out of a fast with gentle, easy-to-digest foods and foods with branched-chain amino acids. Options like smoothies (with limited sugars), dried fruits, soups, cooked vegetables, fermented foods, and healthy fats such as eggs are recommended in particular.

What are the benefits of intermittent fasting?

Due to how intermittent fasting works to create metabolic shifts, here are a few of the results, as evidenced by research, it can have:

Weight loss

A systematic review of 27 clinical trials found that intermittent fasting can lead to weight loss ranging from 0.8% to 13.0% of a person’s initial body weight. Individual results may vary based on factors such as your current body mass index (BMI) and any underlying health conditions, such as type 2 diabetes.

There is conflicting research on whether an intermittent fasting diet is as effective at supporting weight loss as a diet that restricts calories alone. What’s more important — the timing of eating or the caloric deficit?

Truthfully, we don’t know for sure. A recent study does argue that intermittent fasting may be more successful, because of the flexibility it offers. However, another study states that intermittent fasting may not be superior to regular calorie restriction.

When I asked Nina Crowley, PhD, RD, nationally recognized nutrition expert in body composition and obesity care, to weigh in, she said the following, “Intermittent fasting may be effective for weight loss and can feel more manageable for some because it offers structure — clear boundaries between eating and fasting — rather than relying on constant moderation.”

In any case, what seems to matter most in weight management is the sustainability of the approach, the diet quality, and the consistency in achieving a caloric deficit.

Improved insulin resistance

​Several studies have investigated the potential intermittent fasting benefits for insulin resistance (a condition in which the body’s cells become less responsive to insulin, leading to blood sugar issues). They show that intermittent fasting may support better insulin sensitivity for improved blood sugar regulation.

However, it seems that insulin sensitivity is most strongly linked to weight loss itself, regardless of the eating method. For instance, the majority of available studies focus on how intermittent fasting affects insulin levels in those with prediabetes or type 2 diabetes versus healthy individuals — with some studies even showing a minimal change to insulin.

Heart health

Several studies suggest that intermittent fasting is beneficial in lowering heart health risks by reducing body fat and waist circumference, decreasing weight, and helping to manage cholesterol levels, which may contribute to blood pressure — especially when combined with exercise.

However, it's important to note that some recent observational studies have raised concerns about potential negative effects of time-restricted eating, including an actual higher risk of dying from cardiovascular disease when restricting eating duration to less than 8 hours per day.

Other benefits

Aside from these proposed benefits, conflicting information exists related to other effects of intermittent fasting, like reduced inflammation. And more research is needed to explore potential health benefits in the areas of gut health, blood pressure, cognitive function, and longevity.

Overall, we also don’t have rigorous and extensive data on the long-term effects of intermittent fasting, so while it may have some proposed benefits in the area of weight loss, we don’t fully understand the impact it may have on metabolic health (does it slow down metabolism over time?), nutrient balance (does it make it difficult to get adequate calcium or other nutrients?), and overall body composition (can you maintain muscle mass while fasting?).

As Nina Crowley reminds us, “Weight loss isn’t just about the number on the scale. Sustainable strategies should protect muscle, support energy and function, and fit into someone’s life long-term.”

Important things to consider before trying intermittent fasting

Especially for vulnerable populations, it’s important to weigh the potential benefits versus risks of intermittent fasting.

Potential side effects

Due to calorie restriction, potential side effects of intermittent fasting may include:

You may also notice an altered relationship with food if you start to excessively count calories and pay more attention to meal timing and limits.

Who should be cautious or avoid intermittent fasting

I always suggest to anyone who is considering a new eating plan to consult with their healthcare provider before starting, especially if they are managing any type of pre-existing condition like heart disease or type 2 diabetes or taking a prescribed medication.

The following groups are not recommended to participate in intermittent fasting due to increased risks:

• Individuals with a weakened immune system
• Those with a history of an eating disorder or unhealthy relationship with food
• People with type 1 diabetes (check with your healthcare provider if you have type 2 diabetes)
• Pregnant and breastfeeding individuals

Additionally, research shows that for female participants, fasting can alter and heighten cortisol (stress hormone) levels, especially if skipping breakfast. This can create hormonal changes and possibly impact reproductive function and menstrual cycles.

Choosing a method that fits your lifestyle

The best diet is one you can stick with, so if you are considering intermittent fasting as part of your health strategy, my advice is to ease into it and choose one that makes sense for your lifestyle. Remember, there is no one-size-fits-all way to do intermittent fasting, so do what works for you.

For example, you might start with the 12/12 method to introduce the idea of fasting and eating windows. Space your meals between the hours of 7 a.m. and 7 p.m. to help cut down on high-calorie or mindless late-night snacking. Then, during eating hours, focus on a healthy diet made up of a balance of whole grains, lean proteins, and healthy sources of fats.

Intermittent fasting can be a helpful tool for managing weight and supporting health when approached mindfully and with proper guidance. While research shows some proposed benefits, results can vary greatly and may not be suitable for everyone — especially those with underlying health conditions.

As with any lifestyle change, it’s important to choose a method that aligns with your individual health goals, preferences, and routine. If you’re considering intermittent fasting, start slow, stay informed, and consult a healthcare professional to ensure it’s a safe and sustainable choice for you.