A Guide to Vitamin C: Health Benefits and Best Sources

Last updated: December 1st, 2020

Vitamin C is commonly known for its role in supporting the immune system, particularly for preventing infections such as the common cold. But did you know that the health effects of vitamin C range from collagen formation to prevention of ocular diseases?

Continue reading to learn about vitamin C, including its functions in the body, health benefits, recommended intake levels, and the best sources of vitamin C.

What is vitamin C?

Vitamin C, also known as L-ascorbic acid, is a water-soluble vitamin and essential micronutrient. As humans, we cannot produce vitamin C in the body, so it must be obtained from dietary sources, primarily fruits and vegetables. Vitamin C is absorbed in the small intestine and high concentrations are stored in the brain, adrenal glands, pituitary gland, eyes, leukocytes (a type of white blood cell), (1) and skin. (12)

What does vitamin C do?

In the body, vitamin C supports immune health, acts as an antioxidant, and assists in collagen synthesis. (11) Vitamin C functions as a cofactor for enzymes involved in regulating gene transcription and synthesizing certain neurotransmitters and hormones. (3) In the immune system, vitamin C supports the function of the epithelial barrier, the cells that provide a physical barrier against pathogens. Vitamin C may enhance the function of various white blood cells, resulting in the destruction of microbes. (4)

As an antioxidant, vitamin C can neutralize free radicals that may be caused by certain environmental factors, such as exposure to environmental pollutants and ultraviolet (UV) radiation. (12) Vitamin C may further enhance antioxidant function in the body by regenerating (recycling and repairing) other antioxidants such as vitamin E. (2)(11) Vitamin C also enhances the absorption of non-heme iron, a form of iron that is less readily absorbed in the gastrointestinal tract. (8)

Health benefits of vitamin C

Research has examined the effects of vitamin C on various health conditions and processes in the body, including cardiovascular disease, cancer, ocular diseases, immune function, cognitive health, blood sugar management, and skin health.

Cardiovascular disease

A systematic review concluded that vitamin C deficiency is associated with a higher risk of cardiovascular disease (CVD) mortality. In certain groups, including individuals with low vitamin C levels, vitamin C supplementation may improve endothelial function and blood pressure levels, two indicators of cardiovascular health. However, the authors noted that there is insufficient evidence to support vitamin C supplementation for CVD risk reduction in all individuals. (10)

Cancer

Researchers have sought to understand the protective effects of vitamin C against cancer. Vitamin C supplementation may improve well-being, reduce pain, and increase survival in cancer patients. Specifically, high-dose intravenous (IV) administration of vitamin C was associated with increased survival in advanced cancer patients when compared to controls. (6)

Ocular diseases

Along with other antioxidant vitamins and minerals, vitamin C may slow the progression of advanced age-related macular degeneration and the loss of visual acuity (sharpness) in individuals showing signs of the condition. Vitamin C may also have beneficial effects on the development of cataracts and diabetic retinopathy, however, further studies investigating these effects are needed. (6)

Immune function

Vitamin C is known to support cellular functions of the innate and adaptive immune systems, and a deficiency in the vitamin may increase the risk of infections. Vitamin C supplementation is commonly used to prevent and treat upper respiratory infections. (4) Research suggests that with ongoing supplementation of vitamin C, the duration of the common cold may be shortened in both adults and children, and the severity of symptoms may be significantly reduced. (7) To learn about immune-supportive foods, visit the Fullscript blog.

Cognitive health

Studies have consistently found lower vitamin C levels in individuals who are cognitively impaired when compared to healthy individuals. (14) In the nervous system, vitamin C may be used to improve neurotransmission, a process involved in learning, memory, and movement. The use of vitamin C supplementation has been examined in neurodegenerative conditions, and animal studies suggest it may reduce the incidence of Alzheimer’s disease. (6)

Blood sugar management

Vitamin C supplementation may improve blood glucose (blood sugar) levels in individuals with diabetes. A meta-analysis of trials examining the use of vitamin C supplementation in individuals with type 2 diabetes found that vitamin C was associated with reduced levels of fasting blood glucose. Interestingly, the findings suggested that other antioxidants (with or without the use of vitamin C) had insignificant effects on blood sugar markers. (13)

Skin health

Vitamin C plays a variety of roles in supporting healthy skin, including promoting collagen formation and neutralizing damaging free radicals, particularly when used in combination with vitamin E. Vitamin C derivatives, such as magnesium ascorbyl phosphate, may decrease the synthesis of melanin (skin pigment), which may be helpful in addressing age spots and melasma, a condition characterized by dark, discolored skin patches. (12) Topical vitamin C serums and moisturizers are widely available and may be effective in reducing some of the visible signs of aging, including dark spots and fine lines. (12) If you have sensitive skin or other skin conditions, speak to a healthcare practitioner before adding a vitamin C serum to your skincare routine. Learn about the top supplements for skin health on the Fullscript blog.

Recommended daily intake of vitamin C

Individual requirements for vitamin C vary based on factors including age, gender, health status, and environmental exposures. The following table summarizes the daily recommended dietary allowances (RDAs) and adequate intake (AI) of vitamin C. RDAs are the average daily intake that will meet the nutrient requirement of most healthy individuals (over 97%). As an RDA has not been established for infants, the AI is listed for children below one year. (11)

Signs and symptoms of a vitamin C deficiency

Vitamin C deficiency may result in a number of health symptoms and complications, such as impaired immune health and increased vulnerability to infections. (4) A lack of vitamin C compromises the formation of collagen, which in turn impairs the integrity of collagen-containing structures in blood vessels, bones, mucous membranes, and skin. (1)

Within eight to 12 weeks of insufficient vitamin C intake, individuals may develop scurvy, a clinical syndrome of vitamin C deficiency. Scurvy is characterized by several symptoms including swollen gums, poor wound healing, hemorrhage (internal bleeding from ruptured blood vessels), and hyperkeratosis (skin thickening). (9)

Certain factors may increase the risk of vitamin C deficiency, including:

• Alcoholism or anorexia (1)
• Being elderly (1)
• Certain health conditions (e.g., malabsorption, certain forms of cancer, individuals with end-stage renal disease on chronic hemodialysis) (11)
• Certain medications (e.g., aspirin, corticosteroids, indomethacin, oral contraceptives, tetracyclines) (1)
• Infants fed boiled or evaporated milk (11)
• Liver transplant (1)
• Smoking and second-hand smoke (11)
• Unvaried or restricted diets (e.g., due to food fads, food allergies) (11)

Additionally, individual needs for vitamin C may increase due to factors such as air pollution, infections, and conditions characterized by inflammation and oxidative stress (e.g., type 2 diabetes, (4) arthritis, asthma). (1)

Sources of vitamin C

Regular dietary intake of vitamin C is required to maintain health and prevent deficiency of the nutrient. Whole food sources of vitamin C provide additional nutrients and phytochemicals, such as bioflavonoids, which may increase the nutrient’s bioavailability (proportion of the vitamin that is circulated for use). (3) When an individual’s need for vitamin C is increased or intake through dietary sources is insufficient, vitamin C supplementation may be considered.

Foods high in vitamin C

Vitamin C is abundant in several fruit and vegetables, including:

• Bell peppers
• Broccoli
• Brussels sprouts
• Cantaloupe
• Citrus fruits (e.g., orange, grapefruit)
• Kiwifruit
• Strawberries
• Tomatoes and tomato juice (11)

If you’re trying to get more of this vitamin in your diet, it’s important to note that heat may destroy vitamin C. In order to preserve vitamin C, consider consuming your produce raw or steamed, as opposed to broiled, grilled, or roasted. (11)

Vitamin C supplements

Scientific literature has proposed that the RDAs for vitamin C may not meet bodily needs and that optimal health may require vitamin C supplementation. Vitamin C can be supplemented orally or intravenously when higher doses are required. (5)

Research suggests that synthetic vitamin C supplements may have comparable bioavailability to vitamin C found in food. A review examined the effects of synthetic vitamin C, food-derived vitamin C, and the combination of vitamin C and bioflavonoids in human trials. The study findings suggest that the intake of vitamin C in tablets, capsules, and liquid solution was comparable to the food sources used in the trials (e.g., kiwi, orange juice, broccoli, raspberries). Additionally, the study included one relevant human trial involving supplementation of vitamin C with bioflavonoids which showed the combination supplement had a comparable bioavailability to vitamin C on its own. (3)

Additional considerations

While no serious adverse effects of vitamin C intake have been consistently observed, research suggests that the tolerable upper limit for vitamin C intake is approximately 2 g per day. With intravenous use, adverse effects may include flushing, headaches, nausea, and dizziness. (1) The most commonly reported adverse effect from oral vitamin C supplementation is gastrointestinal distress, such as diarrhea and abdominal cramps, which may occur at higher doses due to its osmotic effect in the large intestine. (5)(11)

The bottom line

It’s important to include vitamin C-rich foods or supplements in your diet to obtain this essential nutrient. The health benefits of vitamin C have been observed for cardiovascular disease, cancer, ocular diseases, immune function, cognitive health, blood sugar levels, and skin health. If you’re a patient, speak to your integrative healthcare practitioner prior to making any changes in your health or supplement plan.