What is the only carbohydrate of animal origin?

December 26, 2025 •

3 min read

While most carbohydrates we consume are derived from plants, some animals produce carbohydrates naturally. The primary examples are glycogen, the storage form of glucose in animals, and lactose, the sugar found exclusively in mammalian milk.

Quick Summary

Glycogen is a glucose polymer stored in liver and muscle cells, providing a rapidly accessible energy reserve. Lactose, or milk sugar, is a disaccharide found in mammalian milk, synthesized in the mammary glands.

Key Points

Glycogen is a highly branched polysaccharide that serves as the primary energy reserve for animals and humans, stored mainly in the liver and muscles.
Glycogen is not typically consumed in meat, as it is broken down in the muscle shortly after slaughter.
Lactose, or 'milk sugar,' is the disaccharide found in mammalian milk and is produced exclusively by animals.
Lactose intolerance occurs when lactase enzyme production decreases, preventing the proper digestion of milk sugar.
Glycogen provides a quick-release energy source, whereas fat is a less compact but longer-term energy store.
The body synthesizes glycogen from excess glucose derived from dietary carbohydrates, a process regulated by insulin.
Liver glycogen maintains overall blood glucose levels for the body, while muscle glycogen fuels the muscles during exercise.

Glycogen: The Animal's Primary Energy Reserve

Glycogen is a multi-branched polysaccharide of glucose that serves as the main energy storage form in animals and humans. It is structurally similar to the branched part of starch (amylopectin) but is more extensively branched, allowing for quicker mobilization of glucose when needed. This intricate structure allows for the rapid breakdown of glycogen, providing a quick burst of energy during physical activity or when blood glucose levels fall.

Glycogen is stored primarily in two locations within the body:

Liver: The liver stores a significant amount of glycogen (approximately 5–6% of its weight). This hepatic glycogen acts as a glucose reservoir for the entire body, helping to maintain stable blood glucose levels for other organs, especially the brain.
Skeletal Muscles: Muscles store a higher total amount of glycogen (about 1–2% of muscle mass) because of their larger overall mass compared to the liver. Muscle glycogen serves as a readily available fuel source for the muscle cells themselves, powering physical activity.

How Glycogen is Stored and Used

The processes of glycogenesis (glycogen synthesis) and glycogenolysis (glycogen breakdown) are tightly regulated by hormones like insulin and glucagon.

Glycogenesis (Synthesis): After consuming a meal rich in carbohydrates, blood glucose levels rise, triggering the release of insulin. Insulin stimulates liver and muscle cells to convert the excess glucose into glycogen for storage.
Glycogenolysis (Breakdown): When blood glucose levels drop (e.g., during fasting or exercise), the hormone glucagon is released. Glucagon triggers the breakdown of glycogen back into glucose to be used for energy. This rapid mobilization is why glycogen is considered a short-term energy reserve, less compact than long-term fat stores.

Crucially, glycogen is not a dietary carbohydrate in the same way as plant starches. When animals are slaughtered, the glycogen stored in their muscles is quickly broken down. As a result, meat contains virtually no carbohydrates.

Lactose: The Other Animal Carbohydrate

Beyond glycogen, the only other carbohydrate produced by animals in significant, consumable quantities is lactose. Known as "milk sugar," lactose is a disaccharide composed of one glucose molecule and one galactose molecule. It is synthesized in the mammary glands of mammals during lactation and is the primary carbohydrate source in milk.

Lactose Digestion and Intolerance

To be used for energy, lactose must be broken down into its constituent simple sugars, glucose and galactose. This process is carried out by the enzyme lactase in the small intestine. Many mammals, including humans, produce high levels of lactase during infancy to digest mother's milk. However, lactase production can decrease with age, leading to a condition known as lactose intolerance. This results in digestive issues when consuming dairy products containing lactose.

A Comparison of Glycogen and Starch

Though both glycogen and starch are polysaccharides of glucose used for energy storage, they have significant differences that highlight their origins and function.


Feature	Glycogen	Starch
Origin	Animal	Plant
Storage Location	Liver and muscle cells	Plastids (e.g., chloroplasts)
Structure	Highly branched, more compact	Less branched (amylopectin) and linear (amylose)
Solubility	More water-soluble due to high branching	Less soluble in water
Primary Function	Short-term energy reserve for rapid glucose release	Longer-term energy reserve for plant growth
Dietary Source	Not a significant dietary source for animals	Major dietary energy source for animals (from plants)

Conclusion: A Limited but Crucial Contribution

In summary, while the vast majority of carbohydrates come from plant sources, the animal kingdom contributes two crucial examples: glycogen and lactose. Glycogen serves as the body's internal, readily accessible glucose reserve, stored in the liver and muscles for quick energy mobilization. Lactose is the unique sugar found in mammalian milk, providing a vital energy source for young offspring. The limited, specialized nature of these animal-derived carbohydrates contrasts sharply with the extensive range of carbohydrates found in plants, which form the foundation of most animals' diets. While meat itself offers little to no carbohydrates, the nutritional importance of glycogen and lactose to animal biology is profound.

Optional Link

For a deeper look into the regulation of glycogen metabolism, the National Institutes of Health (NIH) bookshelf provides extensive detail.

Frequently Asked Questions

Glycogen's main function is to store glucose for energy. It is stored in the liver to maintain stable blood sugar levels for the entire body, and in muscles to provide a quick energy source for physical activity.

No, meat is not a source of glycogen. The glycogen stored in an animal's muscles is broken down and consumed as energy during the process of converting muscle to meat after slaughter.

Lactose is a disaccharide, or 'milk sugar,' composed of glucose and galactose. It is found exclusively in the milk of mammals, including humans, cows, and goats.

Lactose intolerance is the inability to fully digest lactose due to a decline in the production of the lactase enzyme in the small intestine. This is a common condition that affects many adults.

Glycogen is the energy storage form in animals, while starch is the energy storage form in plants. Glycogen is more highly branched and water-soluble than starch, allowing for faster glucose mobilization.

Yes, the body can produce glucose from other non-carbohydrate sources, such as amino acids and glycerol, through a process called gluconeogenesis.

While trace amounts of other sugars might be present in animal tissue, glycogen and lactose are the only two carbohydrates considered to be of significant animal origin. Other carbohydrates like chitin are found in insects and fungi, not in animals as a primary dietary source.