Vitamin C supplements are commonly used. Does more C equal better results? © NatchaS | Getty Images

As cold season gets into full swing, conversations over how to avoid becoming sick are a dime a dozen. Those dimes are adding up to big profit for vitamin and dietary supplement manufacturers. The National Institutes of Health’s Office of Dietary Supplements estimates that Americans spent approximately $55.7 billion dollars in 2020 on dietary supplements. In particular, vitamin C supplement sales grew dramatically in 2020 compared with the previous year. Vitamin C, also called ascorbic acid, has become nearly synonymous with immune health; but just how can we get the most benefit from this powerful nutrient? Research shows that consistent use may be more helpful than emergency use and that physiologic doses are more effective than “mega-doses.”

How Did Ascorbic Acid Become “Vital to Life?” James Lind, an eighteenth-century scientist, is credited with the discovery that some component in citrus fruits is essential in the human diet. Lind’s findings prompted future study of citrus fruits which eventually resulted in the isolation and identification of ascorbic acid as the essential component that is abundant in citrus fruits. We now know that vitamin C is an antioxidant and is involved in the synthesis of collagen, carnitine, and norepinephrine. Additionally, we now know many types of fruits and vegetables contain high amounts of vitamin C.

Nowadays, vitamin C deficiency (known as scurvy) is no longer a widespread issue due to easy access to foods high in vitamin C. However, people that do not frequently consume fresh produce could be at risk for low or even deficient vitamin C status. It is estimated that vitamin C deficiency affects approximately seven percent of people in the United States, but estimated rates are as high as 70 percent in developing areas such as northern India. Early stages of poor vitamin C status are linked to fatigue, depression, and irritability. If left untreated, vitamin C deficiency can result in impaired wound healing, joint pain, and swollen, bleeding gums.

Vitamin C Actions in the Body. Vitamin C is a water-soluble vitamin. This means that, unlike fat-soluble vitamins (A, D, E, and K), vitamin C is not stored in the body in large amounts but is excreted when there is excess of physiologic needs; excretion is primarily in urine. As previously discussed, vitamin C is not synthesized in the human body and must come from food or supplementation. Vitamin C absorption is most efficient at usual levels (usual to the amount that we could obtain from foods) and lower, or less efficient, at very high doses (such as more than 1 gram). Once absorbed, vitamin C is used in the body to create neurotransmitters, help build tissue, and enhance immune function. Vitamin C is thought to enhance the immune system through aiding in the movement and reproduction of some white blood cells and through neutralizing histamine. This functionality provides the basis for research into vitamin C’s potential benefit for fighting the common cold.

Are Vitamin C Supplements Useful? Studies from the 1970s have shown that, when taken consistently over several weeks, vitamin C may reduce the number of days of cold symptoms. Average reduction in symptom days is modest but statistically significant – a 14 percent decrease for children and an 8 percent decrease for adults. This reduction benefit was not observed when study participants began vitamin C supplementation after the onset of symptoms. The observed correlation of decreased symptom days and vitamin C supplementation held true only when vitamin C was used in a prophylactic manner—meaning supplementation was initiated and maintained prior to any signs of an oncoming cold.

What Does it all Mean? Maintaining adequate vitamin C intake year-round may be helpful for decreasing the amount of time with cold symptoms. The Recommended Dietary Allowance (RDA) is 90 milligrams for men over 19 years old and 75 milligrams for women over 19 years old. In countries such as the United States, access to vitamin-C rich foods is widespread and therefore rates of deficiency are low. Regular intake of a variety of fresh produce is typically sufficient for meeting the RDA. When working with clients, Caroline Kinzey MS, RDN, LD advocates for a “food-first approach for meeting daily micronutrient recommendations.” She explains that “this means we look to food as our first source to reach adequate nutrient intake by consuming a variety of whole, nutrient-dense foods with an emphasis on colorful fruits and vegetables.”

Groups at risk for deficiency include the elderly, people struggling with alcohol abuse, smokers, and low-income individuals. If you take supplemental vitamin C, remember that “your body is only able to absorb between 300-400 milligrams of ascorbic acid at one time,” says Kinzey. “If you are only able to find a higher dose,” Kinzey recommends “dividing up the dose throughout the day so that you can optimize absorption and not let that excess go to waste!” When it comes to supplementation, consistency is the key.