Why vitamin C matters especially during cold and flu season
Daryl Austin, USA TODAY
December 28, 2025 at 1:00 PM
0
As many of us battle colds, coughs, and the flu during another winter season, a familiar piece of advice is likely to recirculate: Make sure you're loading up on vitamin C. After all, the nutrient has long been associated with immune support, fewer sick days and faster recovery.
But while its reputation is sometimes dismissed as folklore or the efforts of orange juice marketing campaigns, vitamin C's benefits are firmly rooted in biology. "Vitamin C supports the body by working as a powerful antioxidant, helping lower inflammation, protecting cells from oxidative stress and by boosting iron absorption," says Yasi Ansari, a registered dietitian nutritionist and Los Angeles-based certified sports dietetics specialist.
Understanding what the vitamin is, where it's found and how it functions can help you make more informed choices about meeting your daily intake needs - especially during seasons when your body's immune system is under greater demand.
What is vitamin C?
Vitamin C is a water-soluble vitamin that the body cannot produce or store in large amounts. What's more, any excess is filtered through the kidneys and excreted in urine, which is why "it's important to regularly eat foods rich in vitamin C to help prevent deficiency," says Ansari.
Indeed, even modest shortfalls in intake can, over time, lead to symptoms such as fatigue, weakened immunity, slow wound healing, joint pain and bleeding gums. Severe deficiency - though rare in developed countries - causes scurvy.
"Those most at risk of deficiency are usually those who are not eating enough fruits and vegetables," says Ansari. But deficiency is also more common "among smokers (because they have higher oxidative stress and higher needs), individuals with gastrointestinal disorders and those with chronic inflammatory illnesses," explains Dr. Uma Naidoo, director of nutritional and lifestyle psychiatry at Massachusetts General Hospital and the Harvard-trained nutritional psychiatrist behind "Calm Your Mind with Food."
Vitamin C exists in two closely related forms: ascorbic acid - the active form used for antioxidant protection, collagen synthesis and immune support - and dehydroascorbic acid, its oxidized counterpart. Together, these forms help protect cells and tissues throughout the body.
In case you missed: IBS is more common than many realize. Here's how these treatments can help.
What does vitamin C do?
Vitamin C supports a wide range of essential functions, starting with immune health, primarily by helping immune cells function more effectively, by supporting white blood cell production and by "protecting cells from oxidative damage during infections," says Naidoo.
And while vitamin C doesn't reliably prevent colds in the general population, regular intake has been shown to modestly reduce the duration and severity of symptoms - particularly in people with low baseline intake or those under significant physical stress.
The vitamin "also plays a role in collagen synthesis, which supports joint and tissue health and is especially important for athletes recovering from training or injury," says Ansari.
Adequate intake may also support cardiovascular health. Ansari notes that research shows having higher vitamin C levels in the body "is associated with a lower risk of heart disease and stroke."
Another key role is iron absorption as vitamin C "significantly improves absorption of non-heme iron from plant foods," says Naidoo. This is an important consideration for people following vegetarian or plant-forward diets.
In the brain, vitamin C contributes to the synthesis of neurotransmitters, including norepinephrine, which helps regulate mood and alertness. "This is one reason vitamin C has been shown to support mood and cognitive function," adds Naidoo.
What is collagen? And what you should know about how to increase it.
What foods have vitamin C?
Although vitamin C supplements are widely available, the nutrient naturally occurs in many fruits and vegetables. Ansari notes that the daily value for vitamin C is 90 milligrams per day, "so the best sources are foods that provide at least 20% or more of that amount per serving."
Foods that meet or exceed that threshold include strawberries, oranges, grapefruit, broccoli, pineapple, cauliflower, tomatoes and spinach. "And all varieties of bell peppers provide 100% of the DV for vitamin C," adds Ansari, while "one cup of Brussels sprouts provides 75 mg of vitamin C."
Among some of the lesser-known sources of the vitamin, Naidoo points to guava, cabbage, papaya, acerola cherries, parsley, thyme and potatoes, "especially when eaten with the skin."
How these foods are prepared also matters because heat, light and oxygen can degrade vitamin C and other nutrients. This means cooking methods, food storage and processing can influence how much of the nutrient remains in food. For example, steaming and microwaving vegetables tend to preserve more of the nutrient than boiling or frying.
And while supplementation can help when dietary intake falls short, whole foods offer additional benefits, including dietary fiber and phytonutrients that work together to support health. "This is why I always recommend starting with a food-first approach," says Ansari. "A variety of fruits and vegetables can help you meet your vitamin C needs naturally."
This article originally appeared on USA TODAY: What does vitamin C do? And why it's important during cold, flu season