Fasting and Metabolic Health: What’s the Connection?

Written and medically reviewed by Rich LaFountain, PhD

Metabolic health has been gradually declining in recent decades across the world. According to recent estimates, less than 10% of adults are metabolically healthy. Luckily, there is an easy and straightforward way to increase your metabolic health through the power of fasting.

Read on to learn how this simple habit can transform your health and lifespan, for now and years to come.

What Is Metabolic Health and Why Is It Important?

The body is a complex system, so scientists, clinicians, and researchers do not (yet!) have a clear consensus definition for metabolic health. Instead, we take the inverse approach by defining metabolic syndrome, or a state of metabolic dysfunction. The diagnostic criteria are as follows:

  • Elevated waist circumference of ≥88 cm for women and ≥102 cm for men
  • Elevated triglycerides of ≥150 mg/dL, or drug treatment of elevated triglycerides
  • Reduced HDL cholesterol of <50 mg/dL in women and <40 mg/dL in men
  • Elevated blood pressure systolic of ≥130, and/or diastolic of ≥85 mm Hg
  • Elevated fasting blood glucose of ≥100 mg/dL

You’re considered metabolically healthy if you do not check off any of the boxes above. When you’re metabolically healthy, your cells maintain the ability to meet their energetic needs with precision and efficiency. Ultimately, this means that you have the energy to run a marathon, climb a hill, and chase after your kids. Conversely, in a metabolically unhealthy state, your cells may be limited in their flexibility and will have a harder time switching fuel sources. Metabolic syndrome can also lead to weight gain and an elevated risk of chronic disease.

Maintaining metabolic health means that you’re not exposing your cells to an unstable energy environment while they’re working hard to perform their necessary, life-sustaining functions. Think of energy production like a balance sheet — the energy production and demand for things like thinking, moving, and digesting must be balanced all day, every day, in each of your body’s cells. When this balance is disrupted, your cells may respond by becoming unnecessarily stingy, with a tendency towards much greater energy storage in the form of fat.

How Does Fasting Impact Metabolic Health?

Put simply, fasting trains your body to build metabolic flexibility and helps it better respond to being in a fasted or fed state. This, in turn, can engender other vital health benefits.

Fasting Promotes Metabolic Flexibility

When you fast for at least 12–16 hours, you’re essentially asking your body to diversify its energy production. This is because fasting is a mild, or hormetic, stressor, and your body responds to this stress by becoming more efficient at switching from carbs to fat for fuel. This metabolic flexibility makes fasting easier and provides you with more stable energy levels throughout the day.

Research also shows that people who are metabolically flexible (and, thus, metabolically healthy) are less likely to get sick with infectious and chronic disease. Similarly, metabolic flexibility makes you more resilient to a variety of physical, mental, and energetic stressors beyond fasting.

Fasting Promotes Healthy Body Weight Maintenance

Fasting is recommended as one of the best ways to simultaneously build metabolic health while progressing towards your own weight-loss or weight-management goals. In fact, accumulating evidence shows that TRF, or time-restricted feeding, can spontaneously decrease energy intake by 20–30%, even in ad libitum conditions.

Perhaps most importantly, fasting is a sustainable approach to both weight loss and weight management, which means that instead of keeping track of calories or giving up the foods you love, you can focus on eating high-quality, nutrient-dense whole foods during your eating window.

Fasting Promotes Slower Aging and Cell Rejuvenation

Fasting shifts your body’s metabolism towards fat burning, resulting in ketone production. Ketones are metabolites that serve as an excellent fuel source and contribute to your metabolic flexibility. Ketones also have many hormone-like effects throughout your body — according to the research, ketones can normalize nerve function, reduce inflammation, and even activate gene expression.

Fasting triggers ketone elevation, and fasts of 17 hours or longer have been shown to activate autophagy, a naturally-occurring process that repairs and rejuvenates your body’s cells, ultimately reducing senescence and age-related declines in function.

As you can imagine, aging impacts your entire body and all of your cells. Some cells in your body, like your skin cells, have a relatively short lifespan, and they’re replaced regularly. Other cells, such as those in your heart and liver, live years or decades, so they are rarely replaced. If autophagy is inhibited in these cells, they will not be able to maintain an orderly and clean cellular environment. This can lead to heart cell death or liver cell dealth, as well as clinical diseases of the cardiovascular system.

3 Ways to Leverage Fasting for Better Metabolic Health

Fasting allows your body to tap into powerful biological pathways that were relied upon for health and survival throughout the majority of human history. Even though our days of food scarcity and cave drawings are long behind us, we can still leverage the power of fasting today for optimal metabolic health and longevity.

#1. Make Fasting Foundational to Your Lifestyle

The science is clear: We weren’t made to eat food every couple of hours throughout all 24 hours of the day. Nonetheless, research shows that the average person eats for between 14–15 hours of their day, meaning that there’s rarely, if ever, an opportunity for the benefits of fasting to take place.

To make fasting a habit, you don’t need to fast every day, and you don’t need to fast for the same amount of time each day. However, if you’re a healthy adult, shorter-duration fasts of 12–24 hours will help your body make the metabolic switch from glucose to enhanced fat burning. And, as a bonus, you can also pair fasting with pretty much any diet under the sun.

#2. Complement Fasting with High-Quality Nutrition 

Like any avid faster knows, you still have to pay attention to what you’re eating in addition to when. As you condense your eating window, the nutritional quality of each meal becomes especially important, as you’ll have less time and fewer meals to hit your nutrient goals for the day.

A well-designed crossover study demonstrated that eating as much unprocessed food as desired improved health, promoted satiety, and reduced overconsumption compared to ultra-processed foods (think chips and pastries), which increased daily energy intake by over 500 calories. Combine the power of fasting with a high-quality diet that’s low in added sugar and has sufficient protein, fiber, and fat for a balanced, metabolically healthy approach to nutrition.

#3. Align Your Fasting Practice with Healthy Circadian Rhythm

Adopting a fasting practice that’s aligned with your natural circadian rhythm helps boost metabolic health and lowers the risk of chronic disease. Fortunately, this is easy to do — simply start your fast 2–4 hours before bedtime, and consume your calories during daylight hours. If possible, it’s a good idea to extend this to your workouts as well, so that you’re performing physical activity during the day. 

Fasting is an evidenced-based strategy that informs the “clock” genes in your body to regulate energy levels and hormones so that body weight and metabolic health are more easily optimized and maintained.


Metabolic health is fundamental to overall health and protection from chronic disease, and fasting is a foundational habit that connects your metabolic-health and longevity goals. If you want to be among the minority of metabolically healthy adults, then you should consider leveraging the power of fasting.

Rich LaFountain, PhD
Posted in Health & Science

A weekly digest with the latest science and motivation.