The food we consume contains complex chemical compounds, and among the most abundant are glycans. These molecules are carbohydrate compounds, ranging from simple sugars to long, intricate chains of starch and dietary fiber. Glycans are fundamental to human nutrition, acting both as the body’s primary fuel source and as a powerful influence on the digestive system. Understanding the fate of these compounds reveals a biological process that links diet directly to cellular function and overall health.
Defining Glycans in the Diet
Glycans are saccharides, classified based on the length of their sugar chain. The simplest form is the monosaccharide, a single sugar unit that cannot be broken down further during digestion. Examples include glucose, the body’s preferred source of immediate energy, and fructose, known for its sweetness in fruits.
Next are oligosaccharides, short chains composed of two to ten monosaccharide units. Disaccharides, like lactose (milk sugar) and sucrose (table sugar), are common examples. Oligosaccharides also include non-digestible types, such as inulin found in foods like onions and asparagus.
Polysaccharides represent the longest category, consisting of more than ten, often thousands, of monosaccharide units. This group includes starches, used by plants for energy storage in grains and potatoes, and non-starch compounds like cellulose. The structural differences determine if the body can convert them into energy or if they pass through to nourish the gut ecosystem.
The Role of Digestible Glycans in Human Energy
Most consumed glycans, such as starch and disaccharides like sucrose and lactose, must be broken down for energy use. Digestion begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase, which breaks starch chains into smaller units. These units and dietary disaccharides are then processed by enzymes on the brush border of the intestinal lining cells.
Enzymes like lactase and sucrase split disaccharides into monosaccharides, including glucose, fructose, and galactose. Only as monosaccharides are the glycans small enough to be absorbed across the intestinal wall and enter the bloodstream. Glucose is absorbed by enterocytes primarily through the sodium-glucose cotransporter 1 (SGLT1) protein.
The SGLT1 protein utilizes a sodium concentration gradient to move glucose into the cell. Once inside, glucose leaves the enterocyte via the GLUT2 transporter and enters the portal circulation, heading toward the liver. Glucose is the body’s primary energy currency, immediately available for cellular respiration to produce ATP, which powers cellular activities. Excess glucose is converted into glycogen, a storage polysaccharide packed primarily in the liver and muscle tissue for quick energy reserve.
Indigestible Glycans: Fueling Gut Health
Not all glycans are subject to human digestive enzymes; these indigestible carbohydrates are often referred to as dietary fiber. This group includes complex polysaccharides like cellulose and certain oligosaccharides, such as fructans. Since they resist breakdown in the stomach and small intestine, these glycans pass intact into the large intestine, where commensal bacteria await.
These indigestible glycans function as prebiotics, selectively feeding beneficial microbes and promoting their growth in the gut microbiota. Intestinal bacteria possess enzymes the human host lacks, allowing them to ferment these complex structures into important metabolites. This anaerobic fermentation is a symbiotic exchange, providing microbes with energy while producing compounds absorbed by the host.
The most significant metabolic end products of this fermentation are Short-Chain Fatty Acids (SCFAs), primarily acetate, propionate, and butyrate. Butyrate is notable because it serves as the main energy source for colonocytes, the cells lining the colon, meeting up to 70% of their energy needs. By nourishing these cells, butyrate helps maintain the integrity of the intestinal barrier, limiting the passage of harmful substances into the bloodstream.
Propionate and acetate are absorbed into the bloodstream and travel to the liver and other tissues, participating in metabolic pathways. Propionate can be used by the liver for gluconeogenesis, while acetate is utilized in the synthesis of lipids. Beyond providing energy, SCFAs act as signaling molecules, influencing the immune system and strengthening the tight junction proteins that seal intestinal cells. This process shows how fiber-rich foods we cannot digest become a powerful tool for supporting gut health and function.