Which of the following nutrients produces energy but also provides materials that form structures and working parts of the body tissues? Protein Explained

October 19, 2025 •

4 min read

Protein, derived from the Greek word 'proteos' meaning 'primary' or 'first place,' accounts for approximately 50% of the dry weight of the human body. This critical macronutrient is the answer to the question: 'Which of the following nutrients produces energy but also provides materials that form structures and working parts of the body tissues?' and is composed of amino acids that serve as the body's fundamental building blocks.

Quick Summary

Protein is the macronutrient that yields energy while also providing the amino acids necessary for building and repairing body tissues, such as muscles, organs, and skin. It serves numerous functions, including acting as enzymes, hormones, and antibodies. Although it can be used for energy, its primary role is structural and functional.

Key Points

Protein's Dual Function: Unlike other macronutrients, protein can be used for energy but is primarily reserved for building and repairing body tissues.
Amino Acids are Building Blocks: Proteins are long chains of amino acids, and the specific sequence of these amino acids determines the protein's shape and function.
Structural Roles: Proteins are fundamental components of bodily structures like muscles, skin, hair, bone, and connective tissues, providing strength and elasticity.
Functional Roles: Beyond structure, proteins function as enzymes, hormones, and antibodies, regulating virtually all body processes and defending against illness.
Energy Reserve: The body breaks down protein for energy mainly when carbohydrate stores are low, a process that can lead to muscle loss if intake is insufficient.
Complete vs. Incomplete Proteins: Complete proteins contain all essential amino acids (e.g., animal products, soy), while incomplete proteins lack one or more (e.g., most plant sources).
Varied Sources are Best: Consuming a variety of protein sources, both animal and plant-based, ensures the body receives all the essential amino acids it needs.

The Dual Role of Protein: Energy and Structure

Proteins are complex macromolecules essential for virtually every process within the body. While carbohydrates and fats are the body's preferred and most efficient sources of energy, protein can also be broken down to provide calories. However, the body reserves this function for times of low carbohydrate intake or prolonged fasting, as proteins are far too valuable for their myriad other roles. The dual capacity of protein to function both as an energy source and a structural material is what sets it apart from other macronutrients like carbohydrates (primarily energy) and fats (primarily energy and storage).

The Building Blocks: Amino Acids

At its core, a protein is a chain of smaller organic compounds called amino acids. There are 20 common amino acids that combine in various sequences to create the thousands of different proteins found in the human body. The specific sequence of these amino acids dictates the protein's unique three-dimensional shape, which, in turn, determines its function.

Amino acids are classified into two main types based on how we obtain them:

Essential Amino Acids: These nine amino acids cannot be synthesized by the human body and must be acquired through diet. They are crucial for creating new proteins and other vital molecules.
Non-essential Amino Acids: The body can produce these 11 amino acids from other sources, so dietary intake is not strictly necessary.

The Structural Foundation of Tissues

Proteins are the workhorses of the body, providing the structural integrity for virtually all tissues. Key examples of structural proteins include:

Collagen: The most abundant protein in the body, providing strength and structure to bones, tendons, ligaments, and skin.
Keratin: The primary structural protein that makes up hair, nails, and the outer layer of skin.
Elastin: A highly elastic protein found in connective tissues, allowing them to return to their original shape after stretching or contracting.

Beyond simple structure, proteins form the working parts of tissues and cells. They make up enzymes that catalyze biochemical reactions, hormones that act as messengers between cells, and antibodies that defend against infection.

Protein as an Energy Source

While carbohydrates and fats are the body's primary fuel sources, protein can be used for energy when needed, providing 4 calories per gram. In a state of starvation or when carbohydrate stores are depleted (e.g., during intense exercise), the body can break down its own protein, particularly from muscle tissue, to convert amino acids into glucose for fuel. This process, known as gluconeogenesis, is an important survival mechanism but can also lead to muscle wasting if a person's dietary intake of both carbohydrates and protein is insufficient.

Comparing Macronutrient Functions

To understand the unique function of protein, it is helpful to compare it with the other macronutrients, carbohydrates and fats.


Feature	Protein	Carbohydrates	Fats
Primary Function	Structural building blocks, enzymes, hormones, antibodies	Primary and preferred energy source	Long-term energy storage, insulation, cell membranes, hormone production
Energy (kcal/g)	4	4	9
Composition	Chains of amino acids	Sugar molecules (monosaccharides)	Fatty acids and glycerol
Structural Role	Extensive: forms muscles, skin, hair, organs, etc.	Components of structural polysaccharides inside cells	Important for cellular membrane structure
Storage	Not stored for later energy use; excess is converted to fat or glucose	Stored as glycogen in muscles and liver for immediate energy needs	Stored in adipose tissue as triglycerides for long-term energy reserve

Dietary Protein and Body Needs

Ensuring adequate protein intake is crucial for maintaining bodily functions, repairing tissues, and supporting growth. The recommended daily intake for adults is typically 0.8 grams of protein per kilogram of body weight, though this can be higher for athletes, older adults, or during recovery from injury.

Sources of protein can be categorized as complete or incomplete based on their amino acid profile. Complete protein sources, such as meat, dairy, eggs, soy, quinoa, and buckwheat, contain all nine essential amino acids. Incomplete sources, like most legumes, nuts, and grains, lack one or more essential amino acids but can be combined throughout the day to meet all needs. A balanced and varied diet is the most effective way to provide the body with all the necessary protein building blocks.

Conclusion

In the intricate machinery of the human body, protein serves a unique and indispensable dual function. It can provide energy, much like carbohydrates and fats, but its most critical role is acting as the structural foundation and working components of body tissues. From the keratin in our hair to the enzymes that drive metabolism, protein's versatility makes it the most dynamic of the macronutrients. A consistent and sufficient intake of high-quality protein is therefore non-negotiable for anyone seeking optimal health and function.

For more in-depth information on protein metabolism and synthesis, consult resources from the National Institutes of Health.

Frequently Asked Questions

While all macronutrients provide energy, proteins also serve as the fundamental structural and working components for body tissues, a function not shared by carbohydrates or fats.

The body primarily uses protein for energy when carbohydrate and fat stores are insufficient. In this process, amino acids from protein are converted into glucose through gluconeogenesis to fuel the body.

The body cannot produce essential amino acids on its own. They must be obtained through diet to facilitate the synthesis of new proteins and other vital molecules for growth and tissue repair.

Complete protein sources, which contain all nine essential amino acids, include meat, poultry, fish, eggs, dairy products, soy, quinoa, and buckwheat.

Yes, by consuming a variety of plant-based foods, such as legumes, nuts, seeds, and whole grains, vegetarians and vegans can combine incomplete protein sources to obtain all essential amino acids.

Excess protein is not stored in the body like fat but is broken down and the nitrogen is excreted as urea. While a high-protein diet is generally safe for healthy individuals, long-term excessive intake could potentially stress the kidneys.

Exercise, particularly resistance training, increases the demand for protein to support muscle growth and repair. Athletes and highly active individuals often need more protein than sedentary people.