Fats, Carbs, and Protein

Sandra Harrell, DPT, MS
May 22, 2025

Ever wonder what your body actually does with all that food you eat? It's not just about satisfying hunger- it's about providing the fuel your body needs to function. When it comes to energy, carbohydrates and fats are the undisputed champions, while protein plays a more supporting role.

Carbohydrates: Your Body's Preferred Quick Energy Source

Think of carbohydrates as your body's express lane to energy. When you consume carbs (like those found in fruits, vegetables, grains, and legumes), your digestive system breaks them down into glucose, a simple sugar. Glucose is then absorbed into your bloodstream, causing your blood sugar levels to rise.

Here's what happens next:

Immediate Energy: Glucose is readily available to be used by your cells for immediate energy.
Glycogen Storage: Excess glucose that isn't immediately needed is converted into glycogen and stored in your liver and muscles. These glycogen stores act as a readily accessible energy reserve, perfect for sustained activity or when you haven't eaten in a while.
High-Intensity Activities: Carbohydrates are the primary fuel source for high-intensity exercise. Your body can efficiently convert glucose into ATP (adenosine triphosphate), the energy currency of your cells, through processes that don't require as much oxygen as fat metabolism.

Fats: The Long-Burning Fuel Source

While carbohydrates offer quick energy, fats are your body's slow-burning, long-lasting fuel. Dietary fats (found in avocados, nuts, seeds, olive oil, and fatty fish) are broken down into fatty acids.

How fats provide energy:

Dense Energy Storage: Fats are the most energy-dense macronutrient, providing 9 calories per gram compared to 4 calories per gram for carbs and protein. This makes them incredibly efficient for storing large amounts of energy.
Rest and Low-Intensity Activity: At rest and during low-to-moderate intensity exercise, your body primarily relies on fat for fuel. This is an efficient way to conserve your more limited glycogen stores.
Sustained Energy: Because fat metabolism is a slower process, it provides a steady, prolonged supply of energy, making it crucial for endurance activities.
Essential Functions: Beyond energy, fats are vital for hormone production, vitamin absorption, and protecting your organs.

Protein: The Builder, Not the Primary Fuel

Protein, found in meats, fish, eggs, dairy, and legumes, is absolutely essential for your body. It's the building block for muscles, enzymes, hormones, and just about every cell in your body. When it comes to providing energy, protein is generally the last resort.

Here's why:

Prioritizing Building and Repair: Your body prefers to use protein for its primary functions: building and repairing tissues, synthesizing enzymes, and supporting immune function. Burning protein for energy is like using expensive bricks from your house to light a fire – it's inefficient and wasteful of a valuable resource.
Gluconeogenesis: In situations of extreme starvation or very low carbohydrate intake, your body can convert amino acids (the building blocks of protein) into glucose through a process called gluconeogenesis. While this ensures a minimal supply of glucose for critical functions (like brain activity, which primarily relies on glucose), it comes at the cost of breaking down muscle tissue.
Less Efficient Energy: Converting protein to energy is a more complex and less efficient process compared to using carbs or fats.

The Takeaway

Your body is incredibly adaptable and it can shift its fuel usage depending on your activity level, dietary intake, and physiological state.

For quick bursts of energy and high-intensity activities, carbohydrates are king.
For sustained energy during rest and lower-intensity exercise, fats are the preferred fuel.
Protein is primarily for building, repairing, and maintaining your body, with energy production being a secondary and less desirable role.

Talk soon,

Dr. Sandra and Team