Sugars are fundamental to life, playing diverse roles within living organisms. These organic compounds provide much of the energy that powers biological processes, from cellular functions to complex bodily movements. Beyond energy, sugars also serve as building blocks for larger molecules that make up cellular structures and facilitate cell communication. Understanding sugars helps explain how our bodies function and interact with food.
Defining Hexose Sugars
Hexose sugars are simple sugars, also known as monosaccharides, characterized by their six-carbon atomic structure. The “hex-” prefix indicates six carbon atoms, and the “-ose” suffix is common for sugars. Each hexose molecule has a chemical formula of C6H12O6. These sugars exist in both open-chain and cyclic forms, which readily convert between each other in aqueous solutions.
In their open-chain form, hexoses feature five carbon atoms bonded to a hydroxyl group and one carbon atom double-bonded to an oxygen, forming a carbonyl group. Hexoses are the foundational units from which more complex carbohydrates, like disaccharides and polysaccharides, are constructed. The placement of the carbonyl group determines whether a hexose is classified as an aldohexose (carbonyl at position 1) or a ketohexose (carbonyl at position 2).
Key Hexose Sugars We Encounter
Among hexose sugars, glucose, fructose, and galactose are the most commonly encountered and biologically significant for humans. Glucose, an aldohexose, functions as the body’s primary energy source, readily utilized by nearly all cells. It is a component of many larger polysaccharides. Fructose, a ketohexose, is known as the sweetest natural sugar and is found abundantly in fruits and honey.
Galactose, another aldohexose, rarely exists freely in foods but combines with glucose to form lactose, the sugar in milk. Mannose is also a hexose sugar, an aldohexose like glucose and galactose, though it is less prominent in the human diet.
How Our Bodies Use Hexose Sugars
Hexose sugars undergo specific processes in the body to provide energy and support physiological functions. After absorption from the small intestine, glucose is immediately available for cellular use. It serves as the main fuel for all body cells, with organs like the brain and nervous system relying almost exclusively on glucose for their energy needs.
Fructose and galactose, upon absorption, are transported to the liver where they are converted into glucose or other metabolic intermediates before being utilized by cells. This conversion ensures glucose remains the central molecule for energy production. Excess glucose can be stored as glycogen in the liver and muscles through glycogenesis. Liver glycogen can be broken down to release glucose back into the bloodstream when blood sugar levels drop, while muscle glycogen is primarily used for energy within the muscle itself.
Where Hexose Sugars Are Found in Our Diet
Hexose sugars are widely distributed in various foods, appearing in both naturally occurring forms and as added sweeteners. Glucose is naturally present in many fruits and vegetables, and it is also a component of starches found in grains, potatoes, and other plant-based foods. Honey and certain processed foods also contain glucose, often from the breakdown of complex carbohydrates.
Fructose is abundant in fruits, honey, and high-fructose corn syrup, commonly used in sweetened beverages and processed foods. Its presence contributes to the sweet taste of many natural and manufactured products. Galactose is primarily found in dairy products, where it is part of lactose, the sugar in milk. While free galactose is rare in most foods, it becomes available when lactose is broken down, such as in fermented dairy products or lactase-hydrolyzed milks.