Victor Forge
The human body uses two primary fuel sources during exercise: fat and sugar. During low-intensity exercise, such as walking, the body burns a higher percentage of fat. In contrast, during high-intensity exercise, the body burns more sugar, and the percentage of fat burned decreases as the heart rate increases. The body's preference for burning sugar during high-intensity exercise can be altered through interval training, which involves bringing the heart rate down to the fat-burning zone and then pushing it back up. While burning more fat may be desirable for weight loss, it is important to note that disrupting the natural cycle of glucose and fat burning can lead to diabetes, as indicated by mouse models.
| Characteristics | Values |
|---|---|
| Do muscles burn sugar? | Yes, muscles burn sugar (glucose) as fuel. |
| Type of muscle fiber used during a leisurely stroll | Slow-twitch fiber |
| What do slow-twitch fibers do? | They circulate the oxygen taken in and burn off blood glucose and the glycogen stored in the liver. |
| What happens when we start exercising? | We burn mostly sugar. The higher the heart rate, the more sugar we burn. |
| What happens when we burn a higher percentage of fat? | We feel more comfortable and that we could last for a long time. |
| What happens when we burn sugar as a fuel source? | We start feeling uncomfortable and that we don't want to keep going. |
| What happens when we exercise at high intensity? | We burn fat less efficiently. |
| What happens when we exercise at low intensity? | We burn fat more efficiently. |
| What happens when we exercise at low intensity and fast at night? | We lose body fat more easily. |
| What happens when we disrupt the natural cycle of burning glucose and fat? | It may lead to diabetes but can enhance exercise endurance. |
| What happens when we block the activity of a fat-regulating enzyme in muscles? | It leads to an increased capacity for endurance exercise. |
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What You'll Learn
Muscles burn more sugar during high-intensity exercise
During exercise, muscles burn a combination of fat and glucose (carbohydrates). The intensity of the exercise determines the ratio of fat to glucose that is burned. For example, during high-intensity exercises, such as sprinting or strenuous weight lifting, muscles burn a higher percentage of glucose than fat. This is because high-intensity exercises are anaerobic, meaning they are performed without oxygen. Glucose is a quick energy source that does not require oxygen to be metabolised, whereas fat requires oxygen to be metabolised. Therefore, during high-intensity exercises, the body relies more on glucose as a fuel source.
The ratio of fat to glucose burned during exercise can be manipulated by the intensity of the exercise. During low-intensity exercises, such as walking, the body burns a higher percentage of fat than glucose. This is because low-intensity exercises are aerobic, meaning they can be performed with oxygen. Since fat contains more than twice the amount of energy per gram compared to glucose, the body prefers to burn fat during low-intensity exercises when oxygen is readily available.
High-intensity interval training (HIIT) is a form of exercise that involves short bursts of intense activity followed by periods of rest. HIIT is an effective way to burn glucose as fuel. During the intense activity periods of HIIT, the body relies primarily on glucose as a fuel source due to the anaerobic nature of the exercise. The rest periods allow the body to recover and replenish its oxygen stores, preparing for the next burst of intense activity.
Overall, muscles burn more glucose during high-intensity exercises due to the anaerobic nature of these exercises. By manipulating the intensity of exercise, it is possible to target either fat or glucose as a fuel source. This knowledge can be applied to tailor exercise regimens to specific goals, whether it be burning fat or glucose.
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Muscles can burn sugar and fat simultaneously
The body's preference for burning glucose or fat depends on the intensity of the exercise. During high-intensity exercise, the body tends to burn mostly sugar. The higher the heart rate, the more sugar we burn. As our heart rate rises, the percentage of fat we burn decreases.
The body can be trained to burn more fat at higher intensities. This can be achieved by frequently bringing your heart rate down to where you burn a high percentage of fat, and then pushing it up again. This is known as "interval work".
According to a study by Baylor College of Medicine, skeletal muscles are important in controlling blood glucose in the body. The study found that mouse muscles use glucose as fuel when the animals are awake and active, and switch to burning fat when they are asleep. The switch is controlled by a molecule called histone deacetylase 3 or HDAC3.
While burning more fat and less glucose may increase exercise endurance, it could also increase the risk of diabetes.
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Muscles burning more fat and less sugar may cause diabetes
During a leisurely walk, our muscles use a type of muscle fibre called slow-twitch fibre. These fibres circulate the oxygen we take in, burning off blood glucose and the glycogen stored in our liver. If we walk briskly for 20 minutes or more, we begin to run out of glucose. To replace this, our body taps into our fat stores.
However, according to a study by Baylor College of Medicine, making muscles burn more fat and less glucose may increase exercise endurance but could simultaneously cause diabetes. Mouse muscles use glucose (carbohydrate) as fuel when the animals are awake and active and switch to fat (lipid) when they are asleep. The study found that disrupting this natural cycle may lead to diabetes. The switch is controlled by a molecule called histone deacetylase 3 or HDAC3.
To study the role of HDAC3, researchers genetically engineered laboratory mice to deplete HDAC3 only in the skeletal muscles. They found that when the knocked-out mice ate, their blood sugar increased, and insulin was released, but their muscles refused to take in and use glucose. Lacking HDAC3 made the mice insulin resistant and more prone to develop diabetes.
This finding opens up the possibility of selecting the right time to exercise for losing body fat but also raises the concern of using HDAC inhibitors as doping drugs for endurance exercise.
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Muscles use glucose as fuel when the body is awake and active
Mouse models have shown that muscles use glucose as fuel when the animal is awake and active. This is also likely to be the case for humans. When the mice are asleep, their muscles switch to using fat as fuel.
The process is as follows: when mice eat, their blood sugar increases, and insulin is released, which stimulates muscles to take in and use glucose as fuel. This is the same for humans. When we exercise, we burn mostly sugar. The higher our heart rate gets, the more intense the exercise, and the more sugar we burn.
There are two types of muscle fibre: slow-twitch fibres and fast-twitch fibres. During a leisurely stroll, slow-twitch fibres circulate the oxygen we take in, and with every breath, burn off blood glucose and the glycogen stored in our liver. If we walk briskly for 20 minutes or more, we will begin to run out of glucose. To replace that blood glucose, our body taps into our fat stores.
The soleus muscle in the calf, though only 1% of our body weight, can improve the metabolic health of the rest of our body if activated correctly. Professor Marc Hamilton of the University of Houston has pioneered the "soleus pushup" (SPU), which effectively elevates muscle metabolism for hours, even while sitting.
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The soleus muscle can promote glucose burning while sitting
Muscles burn sugar (glucose) and fat (lipids) as fuel. The body has a larger energy reservoir in the form of fat than glucose. However, when we start exercising, we burn mostly sugar. The higher the intensity of the exercise, the more sugar we burn.
The soleus muscle, located in the calf, is one of the 600 muscles in the human body. It is a posterior leg muscle that runs from just below the knee to the heel. Despite being only about 1% of our body weight, the soleus muscle can improve metabolic health in the rest of the body if activated correctly.
Marc Hamilton, a professor of Health and Human Performance at the University of Houston, has discovered an approach for optimal activation of the soleus muscle through the "soleus pushup" (SPU). This exercise can effectively elevate muscle metabolism for hours, even while sitting. The SPU's ability to sustain an elevated oxidative metabolism to improve the regulation of blood glucose is more effective than other popular methods such as exercise, weight loss, and intermittent fasting. Oxidative metabolism is the process by which oxygen is used to burn metabolites like blood glucose or fats, depending on the immediate energy needs of the muscle.
The soleus muscle has a lower-than-normal reliance on glycogen, which is the predominant type of carbohydrate that fuels muscular exercise. This lower reliance on glycogen allows the soleus muscle to work for hours without fatigue, as there is a definite limit to muscular endurance caused by glycogen depletion. When tested, the SPU showed a 52% improvement in blood glucose excursion and a 60% lower insulin requirement over three hours after ingesting a glucose drink.
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Frequently asked questions
Yes, muscles burn sugar (glucose). When we exercise, we burn mostly sugar. The higher our heart rate gets, the more sugar we burn.
When we are burning sugar, we start feeling uncomfortable and may not want to continue exercising.
You can teach your body to burn more fat at a higher intensity by frequently bringing your heart rate back down to where you burn a high percentage of fat, then pushing it up again. This is also known as "interval work".
According to a study, losing body fat would be easier by exercising lightly and fasting at night.
