What are the two main nutrients used in energy production?

November 2, 2025 •

3 min read

Did you know that fats supply more than twice the energy per gram compared to carbohydrates? This article will explain what are the two main nutrients used in energy production, and how your body strategically utilizes each one for both daily function and intense physical activity.

Quick Summary

This guide examines the distinct metabolic roles of carbohydrates for quick fuel and fats for long-term energy reserves, explaining how the body utilizes these key macronutrients to power all bodily functions, from exercise to organ function.

Key Points

Carbohydrates for Quick Energy: The body's preferred and quickest source of energy comes from carbohydrates, which are broken down into glucose and used to fuel the brain and high-intensity exercise.
Fats for Long-Term Storage: Fats are the most concentrated energy source, providing more than twice the calories per gram of carbohydrates, and are primarily used for long-term energy reserves and low-to-moderate intensity activity.
Metabolic Flexibility: A key aspect of healthy metabolism is the ability to efficiently shift between using carbohydrates and fats as fuel, a process influenced by diet and exercise intensity.
Fueling for Performance: High-intensity workouts rely heavily on carbohydrate stores (glycogen), while endurance activities primarily draw from fat reserves after initial carb stores are depleted.
Excess Fuel Storage: When carbohydrate intake exceeds immediate needs and glycogen stores are full, the excess is converted and stored as body fat for future energy use.
Essential Roles Beyond Energy: Fats are essential for absorbing fat-soluble vitamins and producing hormones, while carbohydrates provide fiber for digestive health.

The Foundation of Your Body's Fuel

Our bodies are complex machines that require a continuous supply of energy to function. This energy, measured in calories, comes from the food we eat. While three macronutrients—carbohydrates, fats, and protein—all provide calories, the two main nutrients most efficiently utilized for energy production are carbohydrates and fats. Protein serves primarily as a building block for tissue and is only used for energy when carbohydrate and fat stores are insufficient. Understanding how your body processes and uses these two main fuel sources is key to optimizing your diet and overall health.

Carbohydrates: The Body's Quick-Access Fuel

Carbohydrates are the body's preferred and most readily available source of energy, making them the primary fuel for high-intensity activities and brain function.

Metabolism of Carbohydrates

Carbohydrates are digested and broken down into glucose, a simple sugar, which is then released into the bloodstream.
Insulin, a hormone, helps transport glucose from the blood into cells, where it is used to produce adenosine triphosphate (ATP), the body's main energy currency.
Excess glucose is converted into glycogen and stored in the liver and muscles for later use. This glycogen can be quickly mobilized when a rapid energy boost is needed, such as during a sprint.

Types of Carbohydrates

Simple Carbohydrates: Found in sugars, fruits, and dairy, these are quickly digested and provide a fast energy spike. While useful for immediate needs, excessive intake can lead to blood sugar fluctuations.
Complex Carbohydrates: Found in whole grains, legumes, and starchy vegetables, these are larger molecules that take longer to break down. They provide a more sustained and stable release of energy, helping to avoid energy crashes.

Fats: The Body's Concentrated Energy Reserve

Fats, or lipids, serve as the most energy-dense and efficient form of stored energy in the body. They are crucial for prolonged, lower-intensity activities when the body conserves its carbohydrate stores.

Metabolism of Fats

Dietary fats, primarily triglycerides, are broken down into fatty acids and glycerol in the digestive system.
These fatty acids are transported to cells and, when needed for energy, are broken down through a process called beta-oxidation to produce acetyl-CoA.
The acetyl-CoA then enters the Krebs cycle, similar to carbohydrate metabolism, to generate large quantities of ATP.
Because fats are so energy-efficient (9 calories per gram), the body stores any excess energy as fat in adipose tissue for long-term reserves.

Role in Exercise and Health

During prolonged, low-to-moderate intensity exercise, the body shifts its reliance to burning fat, sparing precious carbohydrate (glycogen) stores.
A key aspect of metabolism is metabolic flexibility, the body's ability to efficiently switch between burning carbs and fats based on energy demands.
Fats also play vital roles beyond energy, including aiding in the absorption of fat-soluble vitamins (A, D, E, and K), insulating organs, and producing hormones.

Comparison of Carbohydrates and Fats


Feature	Carbohydrates	Fats
Energy Release Rate	Quickest	Slowest
Energy Density per Gram	4 Calories	9 Calories
Primary Function	Immediate energy for cells and brain	Long-term energy storage, insulation
Preferred Activity	High-intensity, short-duration exercise	Low-to-moderate intensity, long-duration exercise
Storage Form	Glycogen in liver and muscles	Triglycerides in adipose (fat) tissue

Practical Application in Diet

For optimal energy and health, it is important to include both carbohydrates and fats in your diet. The balance depends on your activity level and goals. For instance, athletes engaged in high-intensity training may require a higher proportion of carbs for performance, while those performing low-intensity, longer-duration activities can utilize a higher fat intake.

Good sources of carbohydrates include:

Fruits and vegetables
Whole grains like oats, brown rice, and quinoa
Legumes such as beans and lentils
Starchy vegetables like sweet potatoes and corn

Good sources of healthy fats include:

Avocados and nuts
Seeds like chia and flaxseed
Fatty fish such as salmon and mackerel
Olive oil

Conclusion: A Balanced Approach to Nutrition

Carbohydrates and fats are the two fundamental nutrients fueling your body's energy production. Carbohydrates provide the fast-acting fuel necessary for high-intensity efforts and brain function, while fats offer a dense, long-lasting energy reserve for sustained activities. A balanced diet that incorporates quality sources of both is essential for maintaining optimal energy levels and supporting overall health. By understanding the distinct roles each nutrient plays in your body's metabolism, you can make informed dietary choices that align with your lifestyle and activity level, ultimately leading to better energy management throughout the day. For more on how these macronutrients influence health, read this resource on the roles of carbohydrates and fats from the MSD Manuals.

Frequently Asked Questions

Fats provide more energy per gram than carbohydrates. Each gram of fat contains about 9 calories, whereas each gram of carbohydrate contains only 4 calories.

Carbohydrates are the body's preferred source for immediate, rapid energy. They are broken down into glucose, which cells quickly convert into usable fuel (ATP) to power daily activities and high-intensity exercise.

The body uses fat for energy primarily during periods of rest or prolonged, low-to-moderate intensity exercise. When carbohydrate (glycogen) stores are low, the body increases its reliance on fat for sustained fuel.

Yes, protein can be used for energy, providing 4 calories per gram, the same as carbohydrates. However, the body prefers to use carbohydrates and fats for fuel and reserves protein for essential functions like building and repairing tissues.

When the body has more glucose than it needs, it first stores the excess as glycogen in the liver and muscles. Once glycogen stores are full, any remaining excess is converted into fat for long-term storage.

Metabolic flexibility is the body's ability to efficiently switch between using carbohydrates and fats for fuel based on energy demands. It is important for maintaining stable energy levels, optimal health, and enhancing athletic performance.

Healthy sources of carbohydrates include whole grains, fruits, and vegetables. Healthy sources of fats include avocados, nuts, seeds, and fatty fish.