Why are carbohydrates and fats known as energy-giving foods?

November 18, 2025 •

4 min read

According to the U.S. Dietary Guidelines, carbohydrates should make up 45–65% of your total daily calories, and fats 20–35%, because these macronutrients are the primary fuel sources for the body. Carbohydrates and fats are known as energy-giving foods because the body efficiently breaks them down to produce ATP, the main energy currency for all cellular activities.

Quick Summary

Both carbohydrates and fats are macronutrients that the body converts into adenosine triphosphate (ATP) for cellular energy. Carbohydrates offer a fast, preferred energy source, while fats provide a more concentrated, slow-release energy reserve that can be stored long-term.

Key Points

ATP Production: Both carbohydrates and fats are broken down by the body to produce ATP, the molecule that provides energy for cellular functions.
Quick vs. Slow Energy: Carbohydrates offer a faster source of energy, converted quickly to glucose, while fats provide a more sustained, slower-releasing energy.
Storage Mechanisms: Excess carbohydrates are stored as glycogen in the liver and muscles for short-term use, while excess energy from both is stored long-term as fat in adipose tissue.
Density of Energy: Fats are the most energy-dense macronutrient, containing 9 calories per gram compared to the 4 calories per gram in carbohydrates.
Fueling the Brain: The brain primarily uses glucose from carbohydrates for energy, though it can adapt to use ketone bodies from fat during low-carb states.
Performance Factors: The body's mix of carbohydrate and fat usage depends on exercise intensity; carbs fuel higher intensity, while fats are dominant during lower intensity and rest.

The Scientific Breakdown: How Carbohydrates Fuel the Body

When you consume food rich in carbohydrates, your digestive system gets to work breaking them down into simpler sugars, primarily glucose. This glucose is then absorbed into the bloodstream. The hormone insulin helps transport glucose from the blood into your body's cells, where it is used to create energy.

The cellular respiration process

The cellular process that converts glucose into usable energy is called cellular respiration. It is a multi-stage process where one molecule of glucose is gradually broken down to generate approximately 30 molecules of adenosine triphosphate (ATP), the universal energy molecule used by all cells.

Short-term energy storage

If your body has enough glucose for its immediate energy needs, the excess is converted into glycogen, a storage form of glucose. Glycogen is primarily stored in the liver and muscles. During periods of increased activity or between meals, this stored glycogen can be quickly converted back to glucose to provide energy. This makes carbohydrates an ideal source of fuel for high-intensity, short-duration exercise, as the energy is readily available from muscle glycogen stores.

The Role of Fats in Energy Production

Fats, or lipids, serve as a dense and long-term energy source for the body. A single gram of fat contains about 9 calories, more than double the 4 calories found in a gram of carbohydrate or protein. This makes fats the most energy-efficient form of food, allowing the body to store a significant amount of energy in a compact form.

Long-term energy reserves

After digestion, fats are broken down into fatty acids and glycerol. These are then used by cells for energy or packaged into triglycerides and stored in adipose (fat) tissue for future use. This stored fat represents the body's largest energy reserve, providing a substantial fuel source for prolonged activities or during times of limited food intake. When energy is needed, the body breaks down these stored triglycerides through a process called lipolysis. The released fatty acids are then transported to cells to be metabolized for ATP production.

The brain's fuel preference

While most cells can use fatty acids for fuel, the brain typically relies almost exclusively on glucose for energy. In situations of prolonged starvation or very low-carbohydrate intake, the body can adapt by producing ketone bodies from the breakdown of fats to fuel the brain.

The dual-fuel advantage

Your body constantly uses a mix of carbohydrates and fats for fuel, with the ratio depending on factors like activity level and diet. For instance, during low-to-moderate intensity exercise, the body primarily burns fat. At higher intensities, it relies more on carbohydrates because glucose can be converted into ATP more quickly and with less oxygen.

A deeper look at energy conversion

Carbohydrate-to-Energy Conversion (Aerobic Respiration)

Initial Breakdown: Begins with digestion in the mouth and small intestine.
Pathway: Glucose undergoes glycolysis in the cytoplasm, producing pyruvate and a small amount of ATP.
Further Oxidation: Pyruvate enters the mitochondria and is converted to acetyl-CoA, which enters the Krebs cycle.
Final Stage: The electron transport chain generates the bulk of ATP.
Speed: Rapid conversion, ideal for immediate energy needs.
Oxygen Requirement: Requires oxygen for complete aerobic respiration.

Fat-to-Energy Conversion (Beta-Oxidation)

Initial Breakdown: Occurs in the small intestine with the help of bile and lipase.
Pathway: Fatty acids enter the mitochondria and undergo beta-oxidation, which also produces acetyl-CoA to enter the Krebs cycle.
Final Stage: Similar to carbs, the electron transport chain produces ATP.
Speed: Slower conversion, suited for sustained energy.
Oxygen Requirement: Requires more oxygen per unit of energy compared to carbohydrates.

Comparison: Carbohydrates vs. Fats for Energy


Feature	Carbohydrates (Glucose)	Fats (Fatty Acids)
Primary Function	Immediate and readily available energy source.	Concentrated, long-term energy storage.
Energy Yield	~4 calories per gram.	~9 calories per gram.
Energy Release	Fast and efficient, especially for high-intensity activity.	Slow and sustained, ideal for low-to-moderate intensity exercise.
Storage Form	Glycogen, primarily in the liver and muscles.	Triglycerides in adipose (fat) tissue, with much larger capacity.
Oxygen Efficiency	Requires less oxygen per unit of ATP produced.	Requires more oxygen per unit of ATP produced.
Brain Fuel	Preferred and primary fuel source.	Can be converted to ketone bodies for brain fuel during prolonged low-carb intake.

Conclusion: A Balanced Approach to Fueling Your Body

Both carbohydrates and fats are essential energy-giving foods, each playing a unique and complementary role in powering the body. Carbohydrates offer a quick, accessible fuel source for high-intensity activities and are the brain's preferred energy. Meanwhile, fats provide a dense, long-lasting energy reserve for endurance and rest. A healthy, balanced diet includes both macronutrients, allowing your body to draw from the most appropriate fuel source depending on its needs. Instead of demonizing one or the other, understanding their distinct functions is key to optimizing energy levels and overall health. The Dietary Guidelines for Americans generally recommend getting 45–65% of daily calories from carbs and 20–35% from fats, highlighting the importance of balancing these macronutrients based on individual activity levels and health goals.

Frequently Asked Questions

Carbohydrates provide a fast, readily available energy source by breaking down into glucose, which is the body's preferred fuel. Fats offer a slower, more concentrated energy source and serve as the body's primary long-term energy reserve.

The body uses glucose from carbohydrates as its primary and preferred fuel source for immediate energy needs. It then turns to stored glycogen and, finally, to fats for energy when carbohydrate stores are low or during prolonged, low-intensity activity.

Fats are more energy-dense because they are more chemically reduced than carbohydrates and do not bind to water. This means they contain more potential energy per unit of weight. Fats yield 9 calories per gram, while carbs yield 4 calories per gram.

While the brain's preferred fuel is glucose from carbohydrates, it can adapt to use ketone bodies, which are derived from fats, during periods of very low carbohydrate intake, such as during prolonged starvation or a ketogenic diet.

Fat storage is more energy-efficient and has a far greater capacity. The body's fat tissue can store an almost unlimited amount of energy, whereas carbohydrate storage as glycogen in the liver and muscles is limited.

If you consume more calories from either carbohydrates or fats than your body needs, the excess energy is converted into triglycerides and stored as body fat. This long-term storage is how the body manages energy surplus.

For endurance exercise, both carbohydrates and fats are important. Carbs provide the high-octane fuel for intense bursts, while fats supply the sustained, long-duration energy. The body uses a blend of both, adapting the ratio to the exercise intensity.