How your body burns fat and carbs during exercise

What is the best exercise for fat burning? How do you target “stubborn belly fat”? Can you turn fat into muscle? The short answers are: Over time, almost any exercise burns fat. You can’t “target” belly fat, but you can lose it. And no, you can’t turn fat into muscle. Many fad exercise programs claim you can “bust fat” in only a few minutes a day or target belly fat to shrink your waistline. The vast majority of these are marketing myths to get you to buy a product. The truth is, the way your body uses energy and burns fat largely depends on the type of exercise and the length of time you exercise. But how your body uses its different energy systems directly relates to how you exercise and maintain your weight. If you understand these energy systems, you can design your exercise routines to tap the different ways they help your body burn fat.

Your body’s energy systems

Adenosine triphosphate (ATP) is the molecule that allows your body to use and store energy. It is often described as your body’s “energy currency.” Without it, you aren’t able to create or use energy for activity, metabolism, or brain function. Your body has 3 systems to produce ATP: the phosphagen system, the glycolytic system, and the oxidative system. Which system you primarily use depends on your activity intensity. For extremely high-intensity activity, your body uses the phosphagen system. As intensity decreases from extremely high, you shift to the glycolytic system and, eventually, to the oxidative system for moderate- to low-intensity activity.

The phosphagen system is responsible for producing short-acting, rapidly available energy. When you start physical activity, you use existing stores of ATP quickly. Since you can only store a relatively small amount of ATP, the phosphagen system uses creatine phosphate (CP, also called phosphocreatine) to make more ATP. CP is a molecule your body uses primarily to create ATP very quickly, rather than carbs or fat. You can build your CP stores in two ways. First, you can get it through your diet by eating red meat and fish. Your body also makes CP from amino acids in your liver, kidneys, and pancreas. It’s stored in low levels in your muscles so it’s ready to be used within the first 10 seconds of high- to extremely high-intensity activity. For activity longer than 10 seconds, your body activates the glycolytic system to make more ATP.

The glycolytic system uses carbohydrates—blood glucose and muscle glycogen—to produce ATP. Glycolysis, the process of burning carbs, has two mechanisms, simply called “fast” and “slow.” Fast glycolysis uses no oxygen and can make ATP from glucose quickly, sustaining activity for 2–3 minutes until slow glycolysis has enough time to get started. Fast glycolysis is somewhat inefficient, producing only a small amount of ATP from glucose. Slow glycolysis requires oxygen and takes longer to make ATP because the process involves many more steps than fast glycolysis. While it lacks speed, it efficiently produces more ATP per molecule of glucose—or muscle glycogen—than fast glycolysis does. So, you need both fast and slow glycolysis to produce ATP: the former to meet your immediate need for energy, and the latter to sustain energy.

Your body primarily uses the oxidative system to make ATP when you’re not exercising, during low-intensity exercise, and during moderate-intensity exercise that lasts longer than 3 minutes. It’s also the only energy system that burns fat. It enables you to oxidize (burn using oxygen) all 3 macronutrients—carbs, fat, and protein. However, your system uses protein for energy only in times of starvation and during exercise sessions longer than 90 minutes. Even then, the amount of protein used to produce ATP is small compared to the amounts of carbs and fat used.

When you’re not exercising, about 70% of your ATP is produced from fat and 30% from carbs. When you start low- to moderate-intensity activity, fast and slow glycolysis kick in, and the ratio shifts to nearly 100% carbs. Then, when you exercise longer than 3 minutes, the ratio shifts back to using fat to produce ATP, especially as you reach 30 minutes or longer. During exercise, the highest ratio of fat to carbs you’ll use is about 50:50. Fat oxidation is the slowest but most efficient method of producing ATP. It produces the most ATP per molecule of fat burned.

Since it’s such a slow process, fat loss during exercise requires you to exercise for a long time. A quick 15-minute session will use some fat, but it won’t maximize fat loss. High-intensity exercise and short-duration, moderate-intensity exercise almost exclusively use CP and carbs to produce more ATP. However, fat oxidation continues at a relatively high rate after a long or intense workout to help your body recover and replace ATP.

Fatigue and work:rest ratios

When you use up your easily available ATP, CP, and glycogen stores, or when ATP production can’t keep up with energy demand, you become fatigued—meaning physically tired. If you have a normal amount of body fat, you won’t use up your fat stores to the point that it will affect your physical performance. Appropriate rest between exercise sets, and recovery between workout days, allows you to build back your energy stores.

When you exercise, it’s important to follow appropriate work:rest ratios because they dictate how much energy you can rebuild while you rest. For example, muscular-endurance training requires less rest between sets because you train your body in a relatively low energy state. It allows your muscles to perform endurance tasks for longer over time. Strength, hypertrophy, and power training all require longer rest periods between sets to build back ATP and CP stores, so you can apply maximum force to your lifts.

Weight management

How do your energy systems influence weight management? Managing your weight is largely a game of calories in versus calories out, with some other factors such as genetics and hormone levels. Exercise’s contribution to weight management is based on how you use each of the three energy systems.

Cardiorespiratory (cardio) training burns more calories per hour of exercise than resistance training because it gives your body more time to activate your slow glycolysis and oxidative systems. For cardio, as a general rule, you burn about 100 calories for every mile traveled by foot—walking or running. However, where those calories come from—carbs or fat—depends entirely on the intensity and duration of the exercise. For example, if you walk 2 miles, you’ll burn 200 calories, mostly from carbs but a little from fat because, as you reach the end of your walk, when you’re using mostly slow glycolysis and your oxidative system. If you run 2 miles, you’ll still burn 200 calories, but you will do so faster than with walking and almost exclusively from carbs. Since 2 miles is a fairly short distance, you will probably finish the run in less time than it takes for your body to start the fat-oxidation process. A cardio session should be at least 20–30 minutes and at an intensity of about 70% of your max heart rate in order to maximize fat loss during exercise.

Resistance training for muscular strength and hypertrophy primarily uses the phosphagen system and fast glycolysis for energy. When you’re strength training, most of your workout time is spent resting between the very short but intense periods when you’re actually lifting weight. Your body can burn about 120 calories per hour during intense weightlifting. However, most people will burn fewer calories during a lifting session because they don’t lift at a high enough intensity for a full hour.

Even though resistance training doesn’t get you much of a calorie deficit in the short term, it’s a major player in long-term weight management because it helps increase your resting metabolic rate. When you build muscle, you need to use more energy to keep that muscle. Remember: When you’re not exercising, your body produces most of its energy from fat stores, so resistance training will lead to fat loss over time rather than during a workout. That means you’re reducing fat while building muscle. This is probably what people mean when they say they’re “turning fat into muscle.” It takes at least 6–8 weeks of consistent resistance training to see improvements in muscle mass. By then, your resting metabolic rate will be going up to support the new muscle, and you might find it’s a little easier to shed some extra pounds. Don’t get frustrated if you don’t see immediate results.

Bottom line

When you start or change your exercise plan, consider how your body uses its different energy systems, especially if your goal is to lose weight, “burn fat,” and improve your body composition. Longer-duration cardio will help contribute to your daily calorie deficit because you need to burn a lot of carbs and fat to support the workout. Weight training, which mainly uses the phosphagen system, will help build muscle and increase your metabolism to use more fat for energy when you’re resting. Weight management is a long journey of many small changes that add up to meet your end goal. For long-term success, apply a holistic approach that includes proper nutrition, exercise, sleep, and stress management, as discussed in HPRC’s Fighting Weight Guide.

Published on: November 5, 2021

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