Why do animals need carbohydrates to survive: The fundamental role of energy and metabolism

December 29, 2025 •

3 min read

Over 70% of a ruminant's energy supply can be provided by the fermentation of carbohydrates. This biological fact underscores the critical dependency on this macronutrient. So, why do animals need carbohydrates to survive, and how does this essential process function across different species?

Quick Summary

Carbohydrates provide animals with their primary source of fuel for energy, powering cellular function, growth, and activity. They are also stored as glycogen for quick energy reserves and assist in regulating the metabolism of other nutrients like fats and proteins.

Key Points

Primary Energy Source: Carbohydrates are the most efficient fuel for animal cells, broken down into glucose to power daily activities and growth.
Brain and Central Nervous System Fuel: Glucose from carbohydrates is the primary and preferred energy source for the brain, vital for proper neurological function.
Short-Term Energy Storage: Excess glucose is converted into glycogen, a branched polysaccharide stored in the liver and muscles for quick energy reserves.
Metabolic Regulation and Sparing: Carbohydrates help regulate the metabolism of fats and proteins, ensuring these are not broken down for energy and are instead used for essential functions like building tissue.
Digestive Health and Fiber: Complex carbohydrates, or fiber, are crucial for proper digestive function, providing bulk and stimulating intestinal activity, especially for herbivores.
Gluconeogenesis Is Inefficient: While the body can produce glucose from non-carbohydrate sources like protein, this process is metabolically taxing and less efficient than utilizing dietary carbs.

Carbohydrates as the Primary Energy Source

For most animals, carbohydrates are the most efficient and preferred source of energy. Through the process of digestion and metabolism, carbohydrates are broken down into simple sugars, primarily glucose, which is then absorbed into the bloodstream. This glucose is the universal cellular fuel, used immediately for metabolic work and activity, powering everything from muscle movement to organ function.

The Brain's Glucose Dependency

Certain parts of the animal body, notably the brain and central nervous system, have a particularly high demand for glucose. While the body can, as a last resort, convert protein or fat into glucose (a process called gluconeogenesis), this is far less efficient and more taxing. A consistent supply of dietary carbohydrates ensures that the brain has the energy it needs to function properly, preventing severe issues like hypoglycemia.

Glycogen: The Body's Emergency Reserve

When an animal consumes more carbohydrates than needed for immediate energy, the excess glucose is converted into glycogen and stored. This process, known as glycogenesis, primarily occurs in the liver and muscles. Glycogen acts as a short-term energy reserve, ready to be converted back into glucose through glycogenolysis when blood sugar levels drop or during periods of intense physical activity. For example, muscle glycogen provides the fuel for rapid, high-intensity exertion.

Structural and Metabolic Functions

Beyond energy, carbohydrates play other fundamental roles in animal physiology:

Nucleic Acid Components: The sugars ribose and deoxyribose are essential structural components of RNA and DNA, respectively.
Cellular Recognition: Carbohydrates on the surface of animal cells are crucial for cell-to-cell communication and recognition, which is vital for the immune system.
Structural Support: In some animal groups, such as arthropods (insects, crustaceans), the carbohydrate chitin forms the tough, protective exoskeleton.
Metabolic Regulation: Carbohydrates assist in the metabolic utilization of fats and proteins, preventing the body from unnecessarily breaking down muscle tissue for energy when fat and carbohydrate stores are available.

Specialized Digestion Across Animal Species

Not all animals process carbohydrates in the same way. The type of carbohydrates an animal can utilize and how it does so depends on its digestive system.


Ruminants vs. Monogastrics: A Comparison of Carbohydrate Metabolism	Feature	Monogastric Animals (e.g., dogs, pigs, poultry)	Ruminant Animals (e.g., cattle, sheep, goats)
Digestive System	Simple, single-chambered stomach.	Multi-chambered stomach (rumen, reticulum, omasum, abomasum).
Key Digestion Site	Primarily the small intestine.	The rumen, which acts as a large fermentation vat.
Digestible Carbs	Primarily starches and sugars (easily digestible).	Soluble carbohydrates, with the majority being fibrous, complex carbs like cellulose.
Indigestible Carbs	Fibers pass largely undigested, providing bulk for intestinal transit.	Broken down by a specialized microbial population (bacteria and protozoa).
Energy Product	Glucose is absorbed directly into the bloodstream from the small intestine.	Volatile Fatty Acids (VFAs) like acetate, propionate, and butyrate are absorbed through the rumen wall and are the primary energy source.
Key Enzyme(s)	Salivary and pancreatic amylase break down starches and sugars.	Microbial cellulase and other enzymes ferment complex fibers.

What Happens When Carbohydrates are Insufficient?

While some animals can survive on low-carbohydrate diets by converting other macronutrients to glucose, this is not ideal. A significant and prolonged lack of carbohydrates can lead to several health problems.

Lethargy and Weakness: With no easy-to-access glucose, the body's energy levels drop, leading to decreased stamina and overall lethargy.
Weight Loss and Poor Growth: When the body is forced to use its protein stores for energy, it can lead to muscle wasting and inhibited growth, which is particularly dangerous for young animals.
Hypoglycemia: Especially in young or highly active animals, insufficient carbohydrates can cause dangerously low blood sugar levels, leading to fainting, seizures, and other neurological issues.
Digestive Upset: A lack of dietary fiber (a complex carbohydrate) can disrupt the digestive tract, causing issues like constipation or improper stool formation.

Conclusion: The Unavoidable Need for Carbs

Carbohydrates are a fundamental and unavoidable component of animal nutrition. They provide the immediate and stored energy necessary for all bodily functions, from the brain's cognitive processes to muscle contractions. Furthermore, they contribute to the structural integrity of cells and larger organisms while regulating overall metabolism. While animals can adapt, relying on other nutrients for energy is less efficient and can lead to significant health complications. Understanding the role of carbohydrates and the differences in their metabolism across species is essential for providing balanced and healthy diets for all animals. To learn more about carbohydrate metabolism in dairy cattle, visit the National Center for Biotechnology Information (NCBI) for peer-reviewed research.

Frequently Asked Questions

The main function of carbohydrates for animals is to provide energy. They are broken down into glucose, which is used as the primary fuel source for all bodily cells, especially the brain.

Some animals, like strict carnivores, can survive on diets with minimal carbohydrates by producing glucose from other nutrients (gluconeogenesis). However, this is less efficient and can be taxing on the body over time.

Animals store excess glucose as a polysaccharide called glycogen. Glycogen is primarily stored in the liver and muscles and can be rapidly broken down into glucose when energy is needed.

If an animal lacks sufficient carbohydrates, it can experience lethargy, weakness, weight loss, and poor digestion. In severe cases, it may lead to hypoglycemia, which can cause serious neurological problems.

Simple carbohydrates like sugars and starches are digested and absorbed quickly for immediate energy. Complex carbohydrates, including fiber, are digested more slowly, providing a sustained energy release and promoting healthy digestion.

Ruminants, like cattle, have a specialized digestive system (the rumen) containing microbes that can ferment complex carbohydrates like cellulose, which is indigestible for monogastric animals. The microbes produce volatile fatty acids (VFAs) that serve as the ruminant's main energy source.

Common carbohydrate sources include starches from grains like corn, wheat, and rice, sugars from fruits, and fibers from vegetables, pasture, and hay.