Vitamin C: Beyond Cold Prevention

What Is Vitamin C and How Does the Body Use It?

Vitamin C — chemically known as L-ascorbic acid — is a water-soluble vitamin and one of the most potent antioxidants in human biology. Unlike most mammals, humans lack the enzyme (L-gulonolactone oxidase) required to synthesize vitamin C endogenously and must obtain it entirely through diet. This makes vitamin C technically an essential nutrient for humans but not for most other animals.

Once absorbed, vitamin C participates in an extraordinary range of physiological processes:

• Collagen biosynthesis: Vitamin C is an essential cofactor for prolyl and lysyl hydroxylases, the enzymes that stabilize the collagen triple helix. Without vitamin C, collagen strands cannot form properly. This underlies the connective tissue breakdown seen in scurvy.
• Antioxidant defense: Vitamin C donates electrons to neutralize free radicals in aqueous environments. It also regenerates other antioxidants, including vitamin E (tocopheroxyl radical) and glutathione, from their oxidized forms.
• Iron absorption: Ascorbic acid reduces ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), the form absorbed by the duodenal enterocyte, dramatically enhancing non-heme iron bioavailability.
• Neurotransmitter synthesis: Dopamine β-hydroxylase — the enzyme converting dopamine to norepinephrine — requires vitamin C as a cofactor. Vitamin C also modulates glutamate receptor activity.
• Carnitine synthesis: Two vitamin C-dependent hydroxylation steps are required for L-carnitine biosynthesis. Carnitine transports long-chain fatty acids into mitochondria for β-oxidation. Vitamin C deficiency impairs fat metabolism and contributes to fatigue.
• Hormone synthesis: The adrenal cortex and medulla contain very high vitamin C concentrations. Vitamin C is required for the synthesis of cortisol and epinephrine, and is rapidly depleted during physiological stress.

The RDA for vitamin C is 90 mg/day for adult men and 75 mg/day for adult women. The UL is 2,000 mg/day. Smokers require 35 mg/day more than non-smokers because smoking increases oxidative stress and vitamin C turnover. Plasma vitamin C is saturated at approximately 400 mg/day in most people; absorption efficiency drops significantly above this intake.

Vitamin C and the Common Cold: Setting the Record Straight

The popular belief that vitamin C cures or prevents colds was largely propelled by Linus Pauling's 1970 book advocating gram-level daily supplementation. Decades of research have produced a more nuanced picture:

• Prevention in the general population: A Cochrane review (Hemilä & Chalker, updated 2013) analyzing 29 randomized controlled trials found that regular vitamin C supplementation (200 mg/day or more) did not reduce the incidence of colds in the general population.
• Prevention in athletes and physical laborers: In people undergoing extreme physical exertion (marathon runners, skiers, military personnel in subarctic conditions), regular vitamin C supplementation halved cold incidence. High-intensity exercise generates substantial oxidative stress that depletes vitamin C.
• Duration and severity: Regular vitamin C supplementation consistently reduces cold duration by approximately 8% in adults (about half a day) and 14% in children. Severity scores also modestly improve.
• Therapeutic dosing after cold onset: Starting vitamin C only after cold symptoms appear does not appear to reduce duration or severity, based on available evidence.

The conclusion: vitamin C is not a cold cure or reliable preventive for most people, but it may modestly shorten duration and is genuinely protective for those under high physical stress.

Food Sources of Vitamin C

Vitamin C is highly heat-sensitive and water-soluble — cooking, especially boiling, can destroy 15–55% of vegetable vitamin C content. Steaming or microwaving preserves significantly more than boiling. Fresh raw vegetables and fruits are the most reliable sources. Notably, red bell peppers contain nearly twice the vitamin C of oranges, making them an underappreciated source.

Vitamin C as an Antioxidant: The Oxidative Stress Connection

Oxidative stress occurs when reactive oxygen species (ROS) — including superoxide (O₂•⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (•OH) — overwhelm the body's antioxidant defenses, causing damage to lipids, proteins, and DNA. Vitamin C is the most abundant water-soluble antioxidant in plasma and the first line of defense against aqueous-phase oxidants.

Research links oxidative stress to the development of cardiovascular disease, cancer, neurodegenerative diseases, and accelerated aging. Higher dietary vitamin C intake and plasma vitamin C levels are consistently associated in epidemiological studies with lower rates of cardiovascular events, certain cancers, and cognitive decline. However, large antioxidant supplementation trials have failed to replicate these protective associations — suggesting that antioxidants work best in their natural food matrix and at physiological concentrations, not in megadose supplement form.

Scurvy and the Historical Context

Scurvy — vitamin C deficiency disease — was one of the most devastating nutritional diseases in history, killing tens of thousands of sailors on long ocean voyages from the 15th through 18th centuries. Symptoms appear after 4–12 weeks of vitamin C-free diet and progress through stages: fatigue and irritability → follicular hyperkeratosis and perifollicular hemorrhages → swollen, bleeding gums → petechiae (pinpoint skin hemorrhages) → joint pain and impaired wound healing → eventually death from infection or internal bleeding as collagen in blood vessel walls fails.

James Lind's 1747 controlled trial on HMS Salisbury — in which sailors given citrus fruit recovered from scurvy while those given other treatments did not — is often cited as one of the first clinical trials in history. The British Royal Navy's adoption of lime juice (hence "limeys") eliminated scurvy from the fleet.

Today scurvy is rare in industrialized countries but still occurs in individuals with extremely restricted diets (severe food insecurity, fad diets excluding fruits and vegetables, or alcoholism). Plasma vitamin C below 11 µmol/L indicates deficiency; below 23 µmol/L is considered insufficient.