Written and medically reviewed by Katya Meyers, RD
Here’s a sobering fact: Most adults consume calories for more than 15 hours every day. In this constantly fed state, your body burns more carbohydrates than fat, which causes its natural ability to transition between burning carbohydrates and fat for energy to get “stuck.” When that “metabolic switch” is stuck, your body stores excess body fat. This is why many people have difficulty losing weight and is a key factor behind steadily climbing obesity rates worldwide.
The solution is learning to “flip the metabolic switch.” And to do that, you need to give your body the right conditions and stimuli to get into Fat-Burning Mode faster and to stay there longer.
A Quick History Lesson
“Flipping the metabolic switch” describes the body’s process of switching between fuel sources from the fed to fasted state. More specifically, it refers to switching from using primarily glucose (which comes from food you’ve recently eaten) to primarily fatty acids and ketones (which come from your body’s stored fat) for fuel — something human bodies have done naturally for millennia.
Since the dawn of time, our ancestors were flipping the switch on which substrates they used for fuel in order to survive in periods of both feast and famine. These hunter-gatherers might have a successful hunt every 1–2 weeks, meaning they often had to go days without food. In order to have enough energy in their next hunt (even after stretches of not eating), these humans developed metabolic flexibility. It was a distinct evolutionary advantage to be able to burn incoming glucose and store extra calories as fat during times of feasting, and then switch to accessing the energy from these stores (in the form of fatty acid and fatty-acid derived ketones) during times of famine. And believe it or not, it’s still an evolutionary advantage — just for an entirely different reason.
What Happens If My Metabolic Switch Isn’t Activated?
When given the choice, your body will choose to burn glucose over fat for energy because glucose is more readily available and quicker to burn. In the short term and when food is plentiful, that’s not a bad thing — carbohydrates are an excellent source of energy. However, if your body never flips the metabolic switch to burn fat, you miss out on many health and metabolic benefits, including weight loss, improvements in body composition, lower inflammation, less insulin resistance, better cardiovascular health, and more stable energy.
Furthermore, the less often your body moves into fat burning, the more reliant on carbohydrates and less metabolically flexible it becomes. Being metabolically inflexible can lead to higher circulating glucose levels, resulting in the release of more insulin, the hormone responsible for ushering glucose into your cells. Over time, because your body has been subjected to so much insulin, it becomes less sensitive to this hormone (similar to the way you might tune out a loud sound if you hear it continuously), which can eventually lead to type-2 diabetes. Metabolic inflexibility can also negatively impact your ability to access fat stores for energy, which influences your weight, body composition, and energy levels. Conversely, being metabolically flexible allows you to efficiently manage glucose and tap into stored fat when glucose levels are low.
How Does Fasting Help Me Flip the Switch?
Modern eating patterns typically involve multiple meals spread across the day. The result is an average eating window spanning more than 14 hours, leaving most people with less than 10 hours in the fasted state. However, it takes the body 12–36 hours to burn through its last meal and also deplete its stored carbohydrates, known as glycogen and found in both liver and muscle. The metabolic switch begins to “flip” when liver glycogen stores are depleted and fatty acids are mobilized.
Put another way, glucose levels are continuously elevated in typical Western-style eating patterns. In contrast, intermittent fasting allows your body to lower its blood glucose, deplete its glycogen, and “flip the metabolic switch,” transitioning your metabolism away from utilizing primarily glucose and towards oxidizing fat, otherwise known as the Fat-Burning Mode.
Conclusion: Fast to Flip the Metabolic Switch
If you want to burn fat and lose weight, increasing your metabolic flexibility so that you can “flip the switch” between burning glucose and fat can aid you on your journey. Fasting is a proven approach that can help you flip the switch and spend more time in Fat-Burning Mode. In turn, this reduces glucose and insulin levels, lowers the stress on your cells, and improves your metabolic health. As an added bonus, the more adept your body becomes at metabolic switching, the fewer “hangry” episodes, insatiable cravings, and energy swings you’re likely to experience. Give it a try!
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