What Are Alpha-Glucans and What Do They Do?

Alpha-glucans are a type of complex carbohydrate, known as a polysaccharide, built from repeating glucose units. These molecules are formed when glucose units link together through specific chemical connections called alpha-glycosidic bonds. They represent one of the most abundant forms of carbohydrates found in nature, present in organisms ranging from plants and fungi to animals. The most familiar examples of alpha-glucans are starch, the primary energy reserve in plants, and glycogen, the corresponding energy storage molecule in animals.

Dietary Sources and Common Forms

Alpha-glucans are readily available in the diet, with plant-based foods rich in starch being the most common source for humans. Staple foods consumed globally that are rich in starchy alpha-glucans include:

• Potatoes
• Corn
• Rice
• Wheat

The structure of starch itself consists of two types of alpha-glucan molecules: amylose, which is a mostly linear chain, and amylopectin, which is a highly branched structure.

While plants provide starch, animal tissues offer alpha-glucans in the form of glycogen. This molecule is the primary short-term energy reserve in animals and is stored mainly in the liver and muscle cells. Though not a major dietary source compared to plant starches, small amounts of glycogen are present in meat. A different category of alpha-glucans comes from fungi, such as certain types of mushrooms.

Role in Human Energy and Metabolism

When starchy foods are consumed, an enzyme in saliva called salivary amylase starts breaking down the long, complex alpha-glucan chains into smaller sugars. This process is a direct attack on the alpha-1,4 glycosidic bonds that hold the glucose units together. The breakdown continues in the small intestine, where pancreatic amylase further digests the starch into disaccharides and trisaccharides.

Other enzymes located on the intestinal lining then convert these smaller sugars into individual glucose molecules. This glucose is then absorbed into the bloodstream, becoming the body’s primary fuel source. Once in circulation, glucose is transported to cells throughout the body, where it is used in cellular respiration to produce adenosine triphosphate (ATP), the main energy currency of the cell. Any glucose not immediately needed for energy can be converted into glycogen, which is stored and ready to be broken back down into glucose whenever the body requires a rapid energy boost.

Distinguishing Alpha-Glucans from Beta-Glucans

Although their names are similar, alpha-glucans and beta-glucans are fundamentally different molecules due to a subtle variance in their chemical structure. The distinction lies in the geometry of the glycosidic bonds that link their glucose units. Alpha-glucans are characterized by alpha-1,4 and alpha-1,6 linkages, while beta-glucans are defined by beta-1,3 and beta-1,6 linkages.

This structural difference has profound implications for their function in the human body. Human digestive enzymes, such as amylase, are specifically shaped to recognize and break apart the alpha-linkages found in starch. Conversely, the human digestive system lacks the enzymes needed to break down the beta-linkages of beta-glucans.

Consequently, beta-glucans pass through the digestive tract largely intact, functioning as a type of dietary fiber. While not a source of energy, these fibers have their own set of biological activities, such as supporting gut health and influencing cholesterol levels, which are distinct from the metabolic role of their alpha-linked counterparts.

Specialized Alpha-Glucans and Immune Function

Beyond their role as an energy source, certain specialized alpha-glucans are investigated for their direct interactions with the immune system. These are not the large starch molecules found in food but are smaller, modified compounds. A prominent example is Active Hexose Correlated Compound (AHCC), an extract derived from the mycelia of shiitake mushrooms that is rich in low-molecular-weight alpha-glucans.

The manufacturing process for this compound results in alpha-glucan molecules that are significantly smaller than dietary starch. This smaller size is thought to allow them to be absorbed and to interact more directly with cells of the immune system. Research suggests these particular alpha-glucans can influence the activity of immune cells.

Studies have explored how these compounds may support the function of cells like Natural Killer (NK) cells and macrophages. NK cells are a type of immune cell that provides a rapid response to cellular threats, while macrophages are involved in engulfing pathogens and cellular debris.