Carbohydrates and Fats: What are the two major sources of energy for athletes?

January 11, 2026 •

4 min read

According to sports nutrition research, carbohydrates provide the most readily available fuel for high-intensity exercise, while fats are the body's primary energy source during low to moderate-intensity activities. Knowing what are the two major sources of energy for athletes is crucial for fueling performance and recovery.

Quick Summary

This guide explains how athletes use carbohydrates for high-intensity, short-duration activities and fats for prolonged, moderate-intensity exercise. The balance and timing of these two macronutrients are key to optimizing athletic performance and managing energy stores.

Key Points

Carbohydrates are high-intensity fuel: They provide the quickest energy for exercise and are stored as glycogen in the muscles and liver.
Fats are low-intensity fuel: They are the body's primary energy source for longer, less intense activities due to their extensive storage capacity.
Glycogen is crucial for peak performance: High muscle glycogen stores delay fatigue in high-intensity and endurance sports.
Fat metabolism spares glycogen: By training the body to use fat more efficiently, athletes can preserve their limited carbohydrate stores for critical moments.
Optimal fueling requires both: A balanced diet of complex carbs, simple carbs, and healthy fats, timed appropriately, is the best strategy for athletic performance and recovery.

The Dominance of Carbohydrates for High-Intensity Work

Carbohydrates are the most important fuel source for most athletes, especially for high-intensity exercise. When consumed, carbohydrates are broken down into glucose, which is then used immediately for energy or stored as glycogen in the muscles and liver. For any exercise requiring explosive power or sustained high effort, glycogen is the body's preferred and most readily available fuel.

Glycogen: The Athlete's High-Octane Fuel

Muscle glycogen is a critical energy reserve that can be quickly tapped for fuel. High muscle glycogen content has been shown to increase the time to fatigue in endurance athletes and improve performance during intermittent sprint sports. Conversely, low glycogen stores can lead to decreased performance, overtraining, and muscle damage as the body may resort to breaking down protein for energy. This is often what athletes refer to as 'hitting the wall' or 'bonking'.

Types of Carbohydrates for Athletes

Athletes should focus on both complex and simple carbohydrates to strategically fuel their bodies. Complex carbohydrates, found in whole grains, vegetables, and legumes, provide a slower, more sustained release of energy, which is ideal for daily training and maintaining overall energy levels. Simple carbohydrates, such as those found in fruits, sports drinks, and gels, offer a quick energy boost, making them valuable for immediate pre-exercise fuel or during long events.

Best Carbohydrate Sources:

Complex Carbs: Whole grain oats, quinoa, brown rice, potatoes, and sweet potatoes are excellent choices.
Simple Carbs: Bananas, dried fruit (in moderation), and sports drinks are effective for quick energy.
Recovery Carbs: Combining carbohydrates with protein after a workout helps to replenish muscle glycogen stores and promote muscle repair.

The Role of Fats in Endurance and Recovery

While carbohydrates fuel high-intensity efforts, fats are the primary energy source for low-to-moderate-intensity, long-duration exercise. Fat stores within the body are far more extensive than carbohydrate stores, making them an almost limitless fuel supply for extended activity. An athlete's body becomes more efficient at using fat for fuel, a process known as fat adaptation, particularly during lower-intensity training.

Benefits of Fat for Athletes

Endurance Fuel: By using fat as a fuel source during submaximal exercise, athletes can spare their limited glycogen stores for crucial high-intensity bursts, delaying fatigue.
Vitamin Absorption: Dietary fats are essential for the absorption of fat-soluble vitamins (A, D, E, and K), which play vital roles in overall health, immune function, and bone health.
Hormone Production: Fats are critical for the production of hormones, including testosterone, which is important for muscle building and repair.

Healthy Fat Sources

Athletes should prioritize healthy, unsaturated fats over saturated and trans fats. Excellent sources include:

Avocados: Rich in monounsaturated fats and vitamins.
Nuts and Seeds: Almonds, walnuts, and flaxseeds provide polyunsaturated fats, including omega-3 fatty acids, which help reduce inflammation.
Oils: Olive oil and canola oil are good sources of monounsaturated fats.
Fatty Fish: Oily fish like salmon are packed with omega-3s.

Comparison of Carbohydrates and Fats

To illustrate the difference in their roles, consider the following comparison of carbohydrates and fats as energy sources for athletes:


Feature	Carbohydrates	Fats
Primary Fuel For	High-intensity, short-duration exercise	Low-to-moderate-intensity, long-duration exercise
Storage Form	Glycogen in muscles and liver	Adipose tissue (body fat)
Energy Density	4 kilocalories per gram	9 kilocalories per gram
Energy Release	Fast, readily available	Slower, more complex to process
Storage Capacity	Very limited; can be depleted quickly	Vast; effectively unlimited for exercise
Performance Impact	Essential for peak performance and intensity	Crucial for endurance and sparing glycogen
Recovery Role	Primarily used for muscle glycogen replenishment	Supports hormone function and vitamin absorption

Optimizing Your Fueling Strategy

For athletes, the key is not to choose between carbohydrates and fats, but rather to use both strategically based on the training phase and intensity. During periods of intense training or before competition, a higher carbohydrate intake is recommended to maximize glycogen stores. For off-season training or lower-intensity, longer sessions, the body can be trained to rely more on fat metabolism, preserving glycogen. Nutrient timing also plays a significant role, with a focus on pre-exercise carbohydrates and post-exercise recovery meals containing both carbohydrates and protein.

Conclusion

Understanding what are the two major sources of energy for athletes—carbohydrates and fats—is foundational to a successful sports nutrition plan. Carbohydrates serve as the primary, quick-access fuel for high-intensity efforts, while fats provide a vast, sustained energy supply for longer, lower-intensity workouts. A well-planned diet that balances these two macronutrients based on training needs ensures peak performance, optimal recovery, and overall health. By fueling intelligently, athletes can maximize their energy output and achieve their performance goals.

Visit the Australian Sports Commission for more on sports nutrition

Frequently Asked Questions

Carbohydrates are the best source for quick energy bursts. They are rapidly converted into glucose, the body's preferred fuel for high-intensity, short-duration activities.

No, that is not true. Fat is a crucial energy source for athletes, especially during prolonged, low-to-moderate-intensity exercise, where it helps conserve the body's limited glycogen stores.

Glycogen is the stored form of carbohydrates in the muscles and liver. It is a vital and readily available energy source that allows athletes to perform at a higher intensity and for longer periods before experiencing fatigue.

Insufficient carbohydrate intake can lead to low glycogen stores, which results in decreased exercise performance and increased fatigue. In extreme cases, the body may break down muscle protein for energy.

Healthy fat sources for athletes include avocados, nuts, seeds, olive oil, and fatty fish. These provide essential fatty acids, aid in vitamin absorption, and support overall health.

At higher exercise intensities, the body relies more heavily on carbohydrates for quick fuel. At lower intensities, the body becomes more efficient at using fat for energy, preserving carbohydrate stores.

Some research suggests that 'fat adaptation' can benefit ultra-endurance athletes by increasing their reliance on fat stores. However, this strategy is debated and may not be beneficial for higher-intensity efforts. A balanced approach remains the safest and most effective method for most athletes.