The nutrient known chemically as L-ascorbic acid, or Vitamin C, is not generated by sun exposure. Vitamin C is an organic compound required by the human body for numerous processes. Unlike many other organisms, humans cannot manufacture this substance internally, meaning it must be acquired through external sources. This makes it an essential nutrient.
Vitamin C: A Required Dietary Intake
Humans must obtain Vitamin C solely through diet because our genetic makeup contains a non-functional gene for the final enzyme in the synthesis pathway. This is due to an evolutionary mutation in the gene responsible for creating L-gulonolactone oxidase (GULO). GULO is the enzyme necessary to convert glucose into L-ascorbic acid, a process most mammals still use to produce their own Vitamin C. Since this gene is inactive in humans, we lack the ability to complete the process.
This biological limitation means that Vitamin C must be consumed regularly, primarily through fruits and vegetables like citrus, bell peppers, and strawberries. Failure to consume adequate amounts leads to scurvy, a disease characterized by fragile connective tissues. While the body cannot produce it, a small amount of Vitamin C can be recycled by specialized processes involving red blood cells.
The Vitamin Synthesized By Sunlight
The common misconception that the sun produces Vitamin C likely stems from the process that creates a different essential nutrient: Vitamin D. This fat-soluble vitamin is unique because the human body initiates its synthesis upon exposure to ultraviolet B (UV-B) radiation from the sun. The process begins when UV-B photons penetrate the skin and strike 7-dehydrocholesterol (7-DHC), a compound naturally present in the epidermal layer. The UV-B energy causes the 7-DHC to transform into pre-vitamin D3, which spontaneously changes into Vitamin D3 (cholecalciferol). This cholecalciferol is then transported to the liver and kidneys for conversion into its active hormone form, confirming the sun’s role in Vitamin D synthesis, not Vitamin C.
Distinct Chemical Roles and Health Functions
Vitamin C and the sun-synthesized Vitamin D have completely different chemical structures and functional roles within the body. Vitamin C is a water-soluble molecule, meaning any excess is not stored and is instead excreted in the urine, requiring frequent replenishment. Functionally, it acts as an antioxidant, protecting cells from damage caused by unstable molecules called free radicals. It is also required for the biosynthesis of collagen, the main protein in connective tissues, and aids in the absorption of iron from plant-based foods.
In contrast, Vitamin D is a fat-soluble secosteroid, structurally similar to cholesterol, which allows the body to store it in fat tissues and the liver. Its main role is tied to mineral metabolism, acting as a precursor to a hormone that regulates calcium and phosphate levels in the blood. This regulation is important for maintaining bone density and skeletal health.