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<\/a><\/p>\nYour immune system relies on both vitamins.Credit: Design by Health; Getty Images<\/p>\n
Vitamin C and vitamin D both support immune function in different ways, but neither will prevent you from getting sick on their own.<\/p>\n
Vitamin C may slightly shorten colds, while vitamin D deficiency can increase infection risk\u2014what matters most is avoiding low levels of either.<\/p>\n
Most people can meet vitamin C needs through food, while vitamin D is harder to get and may require testing if you\u2019re at risk for deficiency.<\/p>\n
With cold and flu season approaching, there\u2019s lots of chatter about whether vitamin C or vitamin D is better for fighting off infections. The truth, however, is a little more complicated: Both vitamins are vital for immune health, but one isn\u2019t universally \u201cbetter\u201d than the other\u2014and they may not support your immune system in the ways you think they do.<\/p>\n
Vitamin C Helps the Immune System Function<\/p>\n
Vitamin C has many functions, including helping the immune system work well enough to protect against disease. The vitamin supports cellular functions of both the innate and adaptive immune systems (respectively, the immune defenses you\u2019re born with and those you build up over time, such as by responding to vaccinations and infections). It also acts as an antioxidant<\/a>, protecting cells from damage caused by free radicals, which can increase the risk of cancer and other diseases.<\/p>\n Vitamin C is \u201cessential for normal immune function,\u201d said Katharine DeGeorge, MD<\/a>, a family medicine physician with UVA Health who has studied cold remedies. But it\u2019s important to understand its limits. Vitamin C helps maintain regular immune health, but it isn\u2019t a shield from germs.<\/p>\n \u201cIn generally healthy adults, taking vitamin C doesn\u2019t keep you from getting sick,\u201d DeGeorge told HEALTH. \u201cAt best, if you start it within a day of feeling a cold come on, it may slightly reduce the duration of symptoms, but not by much.\u201d<\/p>\n Vitamin C may reduce the severity of a cold<\/a> by about 15% compared to a placebo, research suggests.<\/p>\n Vitamin D Regulates the Immune System<\/p>\n Vitamin D plays \u201ca regulatory role\u201d for the immune system, explained Adrian Martineau, MBBS, PhD<\/a>, a clinical professor of respiratory infection and immunity at Queen Mary University of London. Among other functions, it seems to keep inflammation in check<\/a> and prevent the immune system from attacking the body\u2019s own healthy tissues, Martineau explained.<\/p>\n People deficient in vitamin D also seem to be extra susceptible to infections<\/a>, which suggests it plays a part in maintaining immune health.<\/p>\n Based on that link, you might assume that popping vitamin D supplements will keep you from getting sick, particularly if your levels are low. But the data is complicated.<\/p>\n Some studies have found that when people deficient in vitamin D take supplements, their risk of infections goes down. But in 2025, Martineau and his colleagues reviewed more than 40 previously published studies on the topic. When looking at the whole body of evidence, they found that \u201cvitamin D was no better than a placebo at reducing the risk of acute respiratory infections<\/a>,\u201d he told HEALTH.<\/p>\n So, Which Is Better for Immune Health?<\/p>\n Neither vitamin is inherently \u201cbetter\u201d for immunity. Both are crucial for keeping your immune system functioning<\/a> as it should\u2014but neither is a magic bullet for preventing or treating common illnesses. Take a balanced approach: Don\u2019t neglect either vitamin<\/a>, but also don\u2019t rely on vitamins and vitamins alone to keep you healthy during cold and flu season.<\/p>\n \u201cIt\u2019s less about which one is more \u2018important\u2019 and more about avoiding deficiency in either,\u201d DeGeorge said.<\/p>\n Vitamin D deficiency is the more common of the two. By some estimates, about 25% of Americans don\u2019t get enough, and groups including older adults and people of color are at particular risk. By contrast, only about 7% of U.S. adults are deficient in vitamin C<\/a>, according to one 2023 study.<\/p>\n How to Get Enough Vitamin C and Vitamin D<\/p>\n Vitamin C is \u201cwidely found in fruits and vegetables,\u201d said Kristen Smith, MS, RDN<\/a>, a spokesperson for the Academy of Nutrition and Dietetics. Citrus fruits, peppers, kiwi, broccoli, and tomatoes are all good sources.<\/p>\n Most U.S. adults can meet their vitamin C targets\u201475 milligrams (mg) per day for women or 90 mg per day for men\u2014simply by eating a variety of fruits and vegetables daily. Eating a big mixed veggie salad for lunch, an orange with breakfast, and a handful of strawberries for a snack would get you there.<\/p>\n Meanwhile, federal dietary guidelines recommend getting 600 international units (IU) of vitamin D per day. \u201cBecause relatively few foods are naturally rich in vitamin D, it can be challenging to get adequate amounts through diet alone,\u201d Smith explained. Fatty fish, egg yolks, beef liver, and mushrooms are among the only natural food sources of vitamin D<\/a>, although other products are fortified with it, and the skin makes some vitamin D through sun exposure.<\/p>\n Even though it can be hard to consume enough through food, professional organizations such as the Endocrine Society generally do not recommend that adults without a known deficiency use vitamin D supplements to prevent disease. There\u2019s just not enough evidence to prove they work for that purpose.<\/p>\n When to Talk to a Doctor or Dietitian<\/p>\n Vitamin D supplements may be appropriate for those who are truly deficient, DeGeorge said. But it\u2019s not always easy to tell if you are. \u201cVitamin D deficiency usually has no symptoms<\/a> until it\u2019s severe enough to cause bone pain, muscle weakness, or fractures,\u201d she explained.<\/p>\n The U.S. Preventive Services Task Force says there\u2019s not enough evidence to say whether healthy adults should get screened for vitamin D deficiency. But some at-risk groups, including older adults, people who get little sun, people of color, and people with certain medical conditions, may want to discuss it with their doctors, DeGeorge said.<\/p>\n Taking a supplement when you don\u2019t have a deficiency may seem harmless, but that isn\u2019t always the case. \u201cMegadosing\u201d with more than 2,000 mg of vitamin C per day can upset the GI system and raise your risk of kidney stones, DeGeorge said. Excessive vitamin D (more than 4,000 IUs per day) is also linked to nausea, weakness, kidney stones, and other complications.<\/p>\n The bottom line: It\u2019s important to get enough vitamin C and vitamin D to support your immunity and overall health, but don\u2019t go overboard. \u201cTaking large supplemental doses \u2018just in case\u2019 isn\u2019t supported by evidence and can backfire,\u201d DeGeorge said.<\/p>\n