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Ultimate Guide to Intermittent Fasting for Metabolic Fitness

Ultimate Guide to Intermittent Fasting for Metabolic Fitness

Fasting has many benefits for metabolic health, and it can help train our bodies to be more metabolically flexible. Research also suggests it can help improve cardiovascular health, reduce inflammation, and combat obesity. There’s no firm consensus on the best type of fast, but there are several options that may fit your preferences and lifestyle. Here’s how fasting works and the research behind it.

What is Fasting?

Essentially, fasting is when you do not eat or eat a very low number of calories for a certain period of time. Most everyone does some sort of fast every day after going to sleep. The next time you eat—breakfast, snack, or lunch—is what breaks your fast.

Fasting doesn’t have a strict definition or way to do it. The most common practices are:

  • Time-restricted eating or intermittent fasting: You follow a predetermined short-term “eating window.” For example, in the popular 16/8 model, you fast for 16 hours and then feed for 8 hours during that day. You decide your eating window, but 10 a.m. to 6 p.m. or noon to 8 p.m. are common. It’s best to leave at least a few hours between eating and sleeping.
  • Periodic fasting: In this model, you eat normally for five or six days. One or two days per week, you eat zero or very few calories. Popular in this group is 5:2 fasting, which involves eating normally for five days and then consuming about 500 calories on two nonconsecutive days.
  • Alternate-day fasting: You consume nothing one day and then eat the next. Modified alternate-day fasting is when you eat very few calories on fasting days.
  • 24-hour fast: This could follow an entire calendar day or have you stop eating at noon on the first day and then start eating again at noon the following day, so you are still eating each day.
  • Extended fasts: Very low or no calorie intake for 48 or 72 hours or longer.


Guidelines around what you consume during fasting periods vary. Some people only drink water or bone broth, while others take in very few calories. Your choice depends on how your body reacts to fasting and your goals.

“With metabolic flexibility, your body can flip—like a switch—to metabolize carbohydrates and fat efficiently in response to what you eat, when you exercise, and during rest.”

It’s always good to speak with your doctor before embarking on a fasting regimen, particularly extended fasts. Generally, you may want to avoid fasting if you are pregnant, breastfeeding, have hormonal imbalances, have an eating disorder, or are underweight.

The Benefits of Fasting

Metabolic flexibility

The two primary fuel sources for your cells are glucose (usually delivered through carbohydrates you eat) and fat, which can also be stored and later converted to fuel in the form of ketones. After a typical meal, when glucose concentrations in your blood rise, your body releases insulin, which tells the cells to take up that extra glucose as fuel. The rest gets stored in the liver or muscles as glycogen or converted to fat.

For most people, the metabolic benefits of fasting don’t start until around 12 hours into it. At that point, your body begins to switch to relying on fat for fuel. That’s when your body has used up the excess glucose circulating in your bloodstream and has depleted your liver’s stored glycogen. When both are spent, lipids in fat cells become free fatty acids that eventually get metabolized into ketones. Several studies show that using ketones for energy benefits muscle development, cell, organ function, and even brain health.

Metabolic flexibility refers to your body’s ability to toggle between the fed and fasted state. With this flexibility, your body can flip—like a switch—to metabolize carbohydrates and fat efficiently in response to what you eat, when you exercise, and during rest. Making that switch more quickly helps reduce some of the negative physical responses we feel when our body runs low on glucose. What’s more, that flexibility can also help you burn more fat following a high-fat meal. By contrast, when your body gets accustomed to persistent glucose intake, it is less efficient at burning fat and is more metabolically inflexible. Metabolic syndrome, type 2 diabetes, and chronic inflammation are associated with metabolic inflexibility.

So, where does fasting come in? Fasting essentially forces your body to practice—and over time, improve at—this process of switching between burning available carbohydrates (when you eat) and burning fat for energy (when you’re not eating). (Exercise is another way to boost your body’s ability to do this.) Weight loss alone will also help improve this process, but traditional dieting—or cutting calories every day—is difficult for many people to adhere to long term.

Too much glucose—either from frequent eating or from loading up on carb-heavy foods during your feeding period—interferes with the process of building metabolic flexibility in two ways.

First, if we give the body lots of energy in the form of glucose, the body will use that first and not need to make the metabolic switch to burning fat to produce ketones. Second, insulin prevents our bodies from burning fat, so high insulin levels make it very difficult to be metabolically flexible and make us more reliant on glucose for energy.

Insulin resistance and glucose processing

Research suggests that each type of fasting can help improve insulin sensitivity quickly. (This means cells can take in glucose efficiently, allowing blood sugar to return to normal after eating.) Data shows that following a two-day fast, insulin sensitivity remained improved through the “feeding” days.

A standard eating pattern for people not fasting is to eat for more than 15 hours per day and calorie-load at night. Instead, shrinking that window and shifting calorie consumption to the daytime can put your feeding more in line with circadian rhythms. This shift allows your body to process glucose better.

However, the effects on fasting glucose levels and insulin resistance tend to vary across studies, so it’s unclear exactly what type of fasting or time-restricted eating might be best. Other factors, like when people choose to fast (consuming most calories earlier in the day versus in the late afternoon/evening), may also factor into the results.

Weight Loss

Metabolic dysfunction is linked to obesity, which is also associated with diabetes, cardiovascular disease, and certain cancers, notes a 2021 perspective in Advances in Nutrition.

The simple reasons fasting leads to weight loss are that first, it often simply means eating fewer calories. Second, it gives the body more time with insulin low so that it can stay in fat-burning mode. But there’s more at play here.

With traditional dieting, daily calorie restriction results in both muscle and fat loss. Ultimately, the combination of losing weight and a reduction in muscle mass dampens your metabolism, such that people have to consume fewer and fewer calories to maintain weight loss. However, fasting appears to better preserve this muscle tissue while shedding fat, which benefits body composition and helps support your metabolism. Fasting may also stabilize your appetite by reducing ghrelin levels (the “hunger” hormone), so fewer biological signals tell you to eat.

A recent study called into question the effects of time-restricted eating—the 16/8 model in particular—for weight loss, finding that both a fasting group and a control group not following a fasting protocol lost about the same amount of weight over 12 weeks. However, others pointed out that the results are not necessarily a clear indictment of fasting, as the study only recommended that the fasting group follow the protocol, and included their results whether they did or not (called an intention-to-treat study).

How to Think About Glucose Levels While Fasting

Paying attention to your glucose intake—either by wearing a continuous glucose monitor or merely aiming for a low glycemic diet—can help make your fast more successful.

Break the fast right. Your body may be accustomed to getting fed at certain times of the day, triggering hunger hormones and readying the digestive process for incoming food. That hunger can tempt you to break your fast with foods that will send your blood-sugar levels soaring. Intense swings in blood glucose can leave you with side effects like irritability and food cravings. You might attribute these feelings to fasting when they are actually a reflection of your food choices. Instead, ease into eating with a meal that doesn’t send glucose soaring.

Eat well when you do. Similarly, try to avoid high-carbohydrate meals before your fasting window, so you’re not packing your body with more glucose to clear before it can switch to fat burning. Also, since fasting may limit the number of calories you take in, it’s even more critical to maximize the health benefits of fasting by eating a diet rich in micronutrients during your feeding window. Although studies about fasting typically do not restrict what people eat during the feeding periods, consuming a thoughtful, clean, unrefined diet filled with fiber, protein, healthy fats, and micronutrients will help you feel your best during eating windows and set you up for success.

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