Simple’s guide to the science behind intermittent fasting

Written by Krista Scott-Dixon

Medically reviewed by Louise Bula, RD

Published on November 22, 2023 | Updated on June 17, 2024

Intermittent fasting has gained significant popularity as a dietary approach that goes beyond weight loss to other health benefits — such as more energy, lowering disease risk (such as type 2 diabetes or high blood pressure), and even improving overall nutrition when people eat during their eating windows. 

This dietary strategy, rooted in periods of eating and fasting, has intrigued both researchers and health enthusiasts alike. In this comprehensive guide, we’ll explore the science behind intermittent fasting, looking at what the research says about its historical origins, health benefits, potential drawbacks, and how it stacks up against traditional “calorie restriction” diets as an eating pattern.

Ready to explore the benefits of intermittent fasting for yourself? Take our Simple quiz to discover your ideal fasting routine, and get started on your wellness journey today!

History of intermittent fasting

What is intermittent fasting?

Intermittent fasting simply means periodically abstaining from food. (For a deep dive into this topic, see our article “What is intermittent fasting.”)

And early humans had plenty of opportunities to do it — probably mostly unwanted — when food was scarce.

Because our brain needs a constant supply of energy, we need a backup system to help when we don’t have food coming in.[1] Luckily, our metabolism is flexible enough to accommodate this situation.

After about 6–12 hours without food, our body shifts into a state where it can burn fat for energy instead of using energy (aka calories) from a meal we recently ate.[2] 

(If you’re using Simple’s fasting tracker, check the Metabolic Status indicator to watch your body shift into “fat-burning mode” in real time!)

This natural mechanism is one reason that intermittent fasting is safe for most people to try: our bodies already know how to do it!

Intermittent fasting through the ages

So, while many people are curious about how to start intermittent fasting, intermittent fasting is not a new concept. It has roots that extend deep into history. 

Over the last several centuries, various cultures and religions have practiced fasting for spiritual and health purposes.[3] 

For instance, in the 5th century CE, the Greek physician Hippocrates promoted it as a treatment for illness. 

Almost all world religions include fast days — for example,  the six fasting days in Judaism, Ramadan in Islam, Navratri fasting for Hindus, Lent for Catholics, and so on. The Buddha was said to have recommended fasting from noon until sunrise the next day — a practice of having a shorter “eating window” and longer “fasting window” that we now call time-restricted eating.

And many Indigenous groups worldwide continue the practice of fasting for spiritual journeys to this day.[4]

More recently, scientific studies have uncovered its potential as a practice that can promote health and weight loss.[5]

Intrigued by the history of intermittent fasting? Take our Simple quiz and discover how it can fit into your lifestyle!

Intermittent fasting vs. traditional diets

Many folks are familiar with the eating pattern of a traditional diet involving calorie restriction: every day, all day long, you try to eat less, watching your calorie intake. You might also try to avoid eating foods that are deemed “off limits.”

While traditional diets focus mainly on what and how much you eat, intermittent fasting focuses on when you eat (or don’t). This creates a unique approach to weight management and overall health.

Though research suggests that intermittent fasting and traditional diets show similar results for weight loss after several months,[6] what you often don’t see in the studies is how many people drop out of traditional diet programs. Traditional diets are typically rigid, hard to stick to, and might feel like an endless slog of self-restraint from day to day.

However, one key advantage of intermittent fasting is that it’s flexible and adaptable. You choose when to start eating and when to stop. 

While there are some basic intermittent fasting rules, like “Don’t eat food during your fasting window,” there are many types of intermittent fasting, and you can adjust your fasting windows around your lifestyle, schedule, daily routine, and preferences. 

Unlike traditional diets, intermittent fasting also doesn’t ask you to eliminate or restrict anything — aside, of course, from the period when you’re abstaining from all food and drinks with calories.

Let’s look at some other potential benefits.

Benefits and downsides of intermittent fasting

Benefits of intermittent fasting

Intermittent fasting research has highlighted a number of benefits.

• Facilitates weight loss: Intermittent fasting can help people shed excess body weight by using stored body fat for energy during fasting periods while also doing “calorie restriction that doesn’t feel like calorie restriction” — by virtue of having less time to eat during the day, people just naturally eat less.[7]
• Improves metabolic health: Fasting intervals enhance insulin sensitivity (i.e., the ability of our cells to properly respond to the signal of insulin), reduce blood sugar levels, and help regulate metabolism overall.[8]
• Enhances longevity: Studies suggest that intermittent fasting could potentially promote longevity, in part by protecting against age-related diseases.[9]
• Reduces inflammation: Fasting periods may reduce inflammation, a key factor in various chronic diseases.[9]
• Promotes cellular repair: Through the process of autophagy (aka the cleanout of cellular wastes and debris), fasting helps the body remove damaged cells and cell parts and promote tissue health.[10]
• Elevates mood: For some people, fasting seems to improve mood and mental clarity.[11,12] For instance, one intermittent fasting study in aging men found that the men experienced much less tension, anger, confusion, and total mood disturbance while enjoying more overall vigor.[13]

Downsides of intermittent fasting

Studies on intermittent fasting have also shown that there are some intermittent fasting side effects and challenges, however.

• Finding the right fasting timing: Some people may have to do some trial and error as they figure out what fasting routine is right for them. (If you haven’t already got your fasting habit solid, check out our article on intermittent fasting for beginners to get a jump-start on finding the fasting approach that might best suit your needs.)
• Managing irritability and mood swings: Some people may get grouchy when fasting, though others say they feel better.
• Addressing potential nutrient deficiencies: Good nutrition is important during the eating window.
• Navigating social challenges: Social situations and events centered around food can be difficult to navigate during fasting periods. However, if you’re flexible about your fasting timing, this isn’t much of a problem.

Many of these can be alleviated by avoiding the most common intermittent fasting mistakes.

Does intermittent fasting affect metabolism?

Bear with us as we get a little nerdy and dig deeper into the science behind intermittent fasting.

Metabolism is the sum total of all physiological and biochemical processes in the body. A lot of our metabolic activity involves how our bodies use fuel for energy or other jobs like repair and recovery.

By changing how our bodies use fuel for energy, intermittent fasting can significantly affect our metabolism both during and after fasting. 

Here’s how.

Flipping the metabolic switch

When you eat a meal, your body breaks down whatever was in that meal and uses it for energy and other physiological functions. Now, let’s say you stop eating — i.e., start fasting — after that meal.

About 8–12 hours after you start fasting, your body runs out of fuel and nutrients from the meal, as well as what it has stored in the liver. 

It needs to keep your body going, so it does something that scientists informally call “flipping the metabolic switch.”[14] 

With this switch, your body shifts from primarily using carbohydrate-based glycogen for energy to using stored fat from your fat tissues. This stored fat gets turned into substances called free fatty acids and then into ketones. 

Thus, you might know this state as being in ketosis.[2]

This is a normal process that evolved to make sure our brain always has fuel, even if we have to go a little longer without food.

You can see when your body switches to “fat-burning mode” by checking the Metabolic Status indicator in the Simple app while fasting.

Intermittent fasting research suggests that this metabolic shift is linked to many physiological changes that seem to have health benefits.

In particular, especially if we combine intermittent fasting with healthy eating, it seems to improve metabolism, such as how well and efficiently our bodies store and use energy.[8] 

For example, one intermittent fasting study looked at people who practiced intermittent fasting while also making sure to eat some protein at each meal during their eating window.[15] 

Compared to people who were on regular calorie-reducing “diets,” the intermittent fasting + protein group lost more weight and fat while also feeling less hungry — even though both groups ate the same amount of calories and got the same amount of exercise.

So:

• While fasting, your body is converting fuel to energy differently than it normally would. You’re using mainly stored body fat and helping your body get more efficient at using fat for fuel. Woot woot, you’re gonna be a fat-burning machine!
• But after fasting, especially if you build a fasting routine and healthy lifestyle, over time, your body may start getting better at handling fuel and nutrients even when you’re eating normally. For instance, your body may become better at regulating your blood sugar or blood lipids (e.g., cholesterol and triglycerides).[16]

Wondering how to add the healthy eating piece to your intermittent fasting routine and get the maximum benefits of intermittent fasting plus good nutrition? Check out our Nutrition Scores; log your food and get personalized insights!

Fasting and metabolic rate

When people say, “I have a slow metabolism” or “That person must have a fast metabolism — they can eat anything and stay lean!” they’re referring to metabolic rate (aka how fast our body burns calories). 

Many people worry that fasting might “slow down” metabolism. While it’s true that “crash diets” — i.e., stringent diets that restrict calories drastically — can slow down metabolic rate,[17] intermittent fasting doesn’t seem to have the same effect if you’re eating enough calories overall.[8] 

If you are doing intermittent fasting to lose weight, and thus eating less overall, you might see a slight slowdown—but generally very small, and not something that blocks weight loss.

This is likely because of the “intermittent” part: sometimes you eat normally, and sometimes you pause eating … rather than constantly restricting calories for days on end like a conventional “diet.”

Does intermittent fasting improve overall health?

A lot of people are curious: does intermittent fasting work? And what is the science behind intermittent fasting?

Intermittent fasting is supported by extensive evidence that highlights a wide range of potential health benefits.

Dozens of intermittent fasting studies and research reviews show that it positively impacts various aspects of health, including metabolic health, weight management, and even longevity.

The science behind intermittent fasting is promising. While many health benefits come from weight loss (which, on its own, tends to improve many health markers), research also supports the potential of intermittent fasting to improve:

• metabolism [8]
• blood sugar control and insulin sensitivity (factors in type 2 diabetes risk) [5]
• blood pressure [18]
• calorie intake through more mindful eating and an eating pattern that favors a shorter eating window [19]
• energy [20]
• inflammation [21]
• the risk of many chronic lifestyle diseases [10]

Written by Krista Scott-Dixon

Medically reviewed by Louise Bula, RD

Published on | Updated on June 17, 2024