I was asked this question at a recent seminar and my answer was “It’s not a question of either/or”. Like many micronutrients, vitamins C and D work together for a synergistic effect on immune function. Vitamin D is more about prevention while vitamin C is important for its antiviral and anti-inflammatory effects to help both prevent and clear infections. If you are deficient in one or the other then, when you encounter a cold virus, you are less likely to mount a response that protects against symptoms developing.
Diet always comes first
In an ideal world, following a healthy, varied Mediterranean-style diet should supply all the vitamins and minerals you need. In the real world, however, this is less easily achieved – especially for vitamin D. While you can make good amounts in your skin when the UV index is greater than 3, this is not possible during the colder months of the year leaving food as the primary source.
Vitamin D is mainly obtained from oily fish, fish liver oils, animal liver and meats, fortified margarine, eggs, butter and fortified milk. Vegan sources include certain marine algae and wild mushrooms that have been exposed to ultraviolet light (eg maitake, chanterelle, morel, oyster, shiitake, enoki, portobello and chestnut mushrooms). However, National Diet & Nutrition Surveys show that vitamin D intakes in all age groups within the UK are far too low to meet the current recommend needs. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7353432/
Vitamin C is easier to obtain at doses needed to prevent deficiency symptoms such as fatigue and easy bruising and, potentially, the diet might provide the higher doses needed for optimum immunity. Sources include berries, citrus fruit, kiwi, peppers and green leaves.
Vitamin D and immunity
It was recognised during Victorian times that cod liver oil and exposure to UV-rich sunlight was beneficial for those with respiratory infections – both of which are now known to raise circulating levels of vitamin D.
Researchers have found that immune cells such as B-lymphocytes (which make antibodies) and T-lymphocytes (which regulate immune responses) carry specific vitamin D receptors to stimulate their activity. Vitamin D also primes the scavenger, hunter-killer cells (macrophages) that seek out, engulf and destroy viruses and bacteria. Vitamin D also stimulates the production of natural antimicrobial factors such as antibiotic-like proteins (definsins) that form an important line of defence within the respiratory tract.
This may explain why studies involving over 19,000 adults found that having a low vitamin D status increased the likelihood of developing common cold symptoms by over a third (36%) compared with those with a good vitamin D status. A Spanish study, for example, found that 82% of people admitted to hospital with COVID-19 were deficient in vitamin D compared with 47% of the general population.
In fact, taking a vitamin D3 supplement can reduce the risk of developing respiratory tract infections such as the common cold, influenza and pneumonia by a third compared with placebo and results are even greater in those with a pre-existing vitamin D deficiency.
Official guidelines recommend that everyone takes a daily 10 mcg vitamin D supplement during the cold months of the year – a dose based on the role of vitamin D in promoting the absorption of calcium to maintain healthy bones. For immune support, a higher intake is recommended. Following the pandemic, researchers have suggested that adults take 20mcg to 50mcg supplements per day to help reduce the severity of infections. The higher dose of 50mcg is ideal for those aged over 50 years for who dietary absorption and synthesis within the skin tend to be reduced. One study found that people aged 62 to 80 years synthesised four times less vitamin D than those aged 20 to 30 years, for example, making supplements particularly important for older age groups.
Vitamin C and immunity
Vitamin C has a natural immune supporting action. It accumulates in immune cells to enhance their ability to detect (chemotaxis) and clear infections (phagocytosis). It enhances the differentiation and proliferation of B- and T-cells and primes macrophages to clear dead cells from sites of infection. It also stimulates the production of immune factors such as interferon and suppresses the activation of viral genes so infections are less likely to progress. As an antioxidant , vitamin C also mops up inflammatory chemicals to improve symptoms and hasten healing should a cold develop. Lack of vitamin C therefore impairs immunity and increases your susceptibility to infections. Conversely, infections significantly reduce vitamin C levels.
Preliminary studies with school children and students found that taking vitamin C reduced the risk of catching a cold by around 30%. For men involved in heavy physical exercise (military troops under training and participants in a 90 km running race) taking 600mg to 1g vitamin C per day halved the risk of developing cold symptoms.
When the results from 30 trials involving over 11,300 people were pooled, a consistent benefit was found from taking 1g to 2g vitamin C per day with an 8% reduction in the duration of colds for adults, rising to an 18% reduction for children. The severity of cold symptoms was also reduced.
If you hope to get through the year with minimal down time due to respiratory infections, take steps to ensure good intakes of all micronutrients, especially vitamins C and D. While I like to think my diet is good – full of home-made vegetable soups, fresh fruit and salads, I take a multivitamin plus additional vitamins C (500mg) and D (50 mcg) on a daily basis and can’t remember the last time I had a cold – although year round daily sea swims may have something to do with that, too!