Written and medically reviewed by Nicole Grant, RD
Everyone is born with the innate ability to shift between burning carbs and fat for energy. This “metabolic switching” has kept humans lean and energetic for millennia, without the need for intentional diets. Unfortunately, modern nutrition and lifestyle habits have destroyed this flexibility for the majority of people. Specifically, the foods people are eating and the timeframe in which they’re eating them have shifted in the wrong direction — and as a result, humans are now burning a lot more sugar and a lot less fat.
However, if you want to lose fat — which is what most people hope for when trying to lose weight — then fat is the fuel you need to be burning! So, let’s explore how to reignite your body’s natural ability to burn fat.
First, Let’s Talk Fuel
The human body is a complex biological machine that requires energy to power everything from cellular functions to whole-body movements. This energy is initially derived from the foods we consume, which are predominantly made up of carbohydrates, fats, and proteins.
While the body can technically convert all three of these fuels into energy, some are more preferable than others. Amino acids, for example, are primarily used for tissue repair and synthesis. In times of need, they can be modified and funneled into various parts of the metabolic pathway to produce energy; however, since the body would rather conserve these compounds for maintenance and growth of lean body mass to keep you healthy, fatty acids and glucose are what it most often burns for fuel.
Glucose vs. Fat: What Does Your Body Like Better?
Glucose, derived from dietary carbohydrates, is often dubbed the body’s “preferred” fuel source since it provides a quick and efficient source of energy. It is metabolized through a rapid process called glycolysis to eventually produce adenosine triphosphate (ATP), the molecule that provides energy for most cellular functions. Any excess glucose can be stored in the liver and muscles as glycogen or converted to fat and stored for later use.
Fat, on the other hand, supplies a slower, sustained energy release. Whether consumed in food or stored as adipose tissue, fatty acids take longer to process for energy because they undergo a more intensive breakdown process that ultimately feeds into the same cycle to produce ATP. The breakdown of fatty acids can also produce ketones, which, in the absence of sufficient glucose, serve as an alternate fuel source for the brain and other tissues.
Which fuel your body prefers will depend upon your diet as well as your physiological needs, such as the type or intensity of exercise you are performing. While glucose is the standard go-to in a fed state and when you need a quick energy burst, the body taps into fat reserves for sustained endurance and during periods of fasting and carbohydrate scarcity. In essence, our bodies have evolved to efficiently use both, depending on the situation and supply.
What Are the Advantages of Burning Fat?
Tapping into your fat reserves for energy provides a whole host of health benefits. One of the standout advantages of burning fat is enhanced metabolic flexibility. Simply put, this means your body can more easily switch between using glucose and fatty acids for fuel. This “metabolic superpower” ensures you remain energized when carbs are in short supply, during a fast, or even in your typical day-to-day grind.
When you increase your metabolic flexibility, you increase your ability to burn body fat — often a desired outcome when trying to lose weight. Instead of using fuel from food or glucose stored within the body, you are able to more readily tap into your fat stores. Healthy weight loss involves burning as much stored fat as possible for energy, thereby preserving lean body mass and improving body composition.
Using fat as a primary fuel source for energy and to decrease your total body fat can also have broader metabolic benefits and reduce your risk of chronic disease. As your body becomes adept at utilizing fat, it can lead to improved insulin sensitivity, decreased inflammation, and a reduction in visceral fat — all key players in type-2 diabetes, cardiovascular disease, cancer, and neurodegenerative disease. Overall, adopting a metabolic state where fat is used more readily for energy can pave the way for a healthier, more resilient body.
How Do I Burn Fat Instead of Glucose?
The human body regularly burns a mix of glucose and fat for energy, but the proportion of these fuels can be swayed by several factors, most notably fasting, nutrition, and exercise.
When glucose is less available, such as during periods of fasting, the body turns to stored fat reserves for energy. After your last meal for the day, your body will go to work processing, digesting, and using that consumed fuel to produce energy. Once the nutrients from your last meal are gone, the body then taps into its storage tanks. The breakdown of glycogen (stored glucose) tends to come first and is the preferred fuel source in a fasted state until about 12–36 hours into a fast, depending on how much your body has stored. After that, the metabolic switch “flips” and your body leans more heavily on fatty acids and ketones for energy production until you eat again.
To reduce the time it takes your body to start prioritizing fat, try eating fewer carbohydrates, especially in the meal immediately before you start your fast (generally before bed). Smaller, carbohydrate-conscious meals tend to keep glucose and insulin levels lower, speeding up the transition to burning fat. Maintaining a 2–4-hour gap between your last meal (or snack) and bedtime will also help you get quality sleep, which sets up your body for optimal fat burning. Studies have suggested that consistently getting 7–9 hours of sleep can result in better weight and fat loss and also reduces cravings or compensatory eating behavior, further supporting weight loss and maintenance.
Engaging in regular physical activity, especially in a fasted state, further boosts the body’s capacity to use fat as a primary fuel. There are two reasons: First, exercise depletes your glycogen stores faster, thus increasing the speed at which you will need an alternative fuel (fat!) for energy. As exercise intensity increases, so does the demand for glucose — so, engaging in moderate-to-vigorous exercise such as HIIT or weight training will help deplete your glycogen stores and have your body reaching for fat sooner. Second, low-to-moderate-intensity exercise, such as walking at a fast pace or taking an easy bike ride, amplifies fat-burn rate because your body shifts its fuel mix towards peak fat burning. Shoot for at least 20–30 minutes of daily fasted movement to boost your fat-burning potential. Regularly mix in low-, moderate-, and high-intensity exercise in order to speed up both your glycogen depletion and fat oxidation, leading to quicker results and broader health benefits.
Conclusion: Flip Your “Metabolic Switch” to Burn More Fat
The journey to effective weight loss is steered by your metabolic choices. Being intentional with your nutrition and incorporating strategies like fasted exercise can greatly amplify your fat-burning efforts. The more often you can employ these strategic choices to shift your fuel source from glucose to fat, the more achievable and sustainable your weight loss success will be.
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