Find out what causes colds and other respiratory diseases, what you can do to support your immune system against them, and the essential role of vitamins C and D.
Up to 200 different viruses can infect the upper respiratory tract to cause symptoms of the common cold. Inflammation of the lining of the nose, throat and sinuses leads to the typical symptoms of sore throat, runny, stuffed-up nose, watering eyes, coughing and sneezing. These symptoms usually last from three to seven days, unless complications such as sinusitis or chest infection set in.
Cold viruses are highly infectious and spread from person to person through coughing, sneezing and contact with shed viruses, which can survive on door handles, tissues and other surfaces for at least 24 hours. Good hygiene is therefore vital to limit the transmission of cold viruses.
How long does a cold last?
After exposure to a cold virus, the incubation period is typically just under two days. If symptoms develop, they peak after 1 to 3 days and last 7 to 10 days, although in some cases, symptoms can persist for up to 3 weeks.
Supporting your immune system against colds
Maintaining a healthy immune system can mean that, when you encounter a cold virus, you mount a good response so that you either don't become ill, or develop only mild symptoms.
Good nutrition is key. Eating a healthy, varied diet optimises immune health to help prevent or minimise cold symptoms. Aim to eat a Mediterranean-style diet that supplies wholegrains, beans, 5-a-day fruit, vegetables, seafood, olive oil, onions and garlic. Fruit and vegetables are a rich source of antioxidant flavonoids with antiviral actions, for example.
Fruit and vegetables also provide other antioxidants and nutrients needed for good immune function such as vitamins A, B12, B6, C and D, plus minerals copper, folate, iron, selenium and zinc.
- A cold is the most common disease to affect humans.
- Respiratory tract infections are the most common cause of time off work.
- You are four times more likely to catch cold in winter than in summer.
- Children under 2 years have around 6 colds per year.
- Adults get an average of 2 - 3 colds per year.
- Older adults have fewer colds (1 per year) as they have built up antibodies against common viruses.
- However, cold viruses mutate regularly, creating new variants that may trigger symptoms.
What about vitamins C and D?
Vitamins C and D are especially important to support immunity against colds and respiratory diseases and work in different ways; vitamin D is more about prevention, while vitamin C is important for fighting off infections.
How vitamin D supports immunity
During Victorian times, respiratory infections were treated with cod liver oil and exposure to UV-rich sunlight, both of which are now known to help boost circulating levels of vitamin D.
Immune cells, including B lymphocytes (which make antibodies) and T lymphocytes (which regulate immune responses), all carry specific receptors that respond to vitamin D. Vitamin D helps to activate macrophages – hunter-killer cells that engulf and destroy viruses and bacteria – and stimulates our production of antibiotic-like proteins (definsins) that help protect the respiratory tract.
Studies involving over 19,000 adults show that having low levels of vitamin D increases the likelihood of developing common cold symptoms by over a third (36%) compared with people with high levels.
In one Spanish study, 82% of 216 people admitted to hospital with COVID-19 had a vitamin D deficiency, compared with 47% of the general population. This helps to explain why taking a vitamin D3 supplement was also found to reduce the risk of developing a respiratory tract infection (including the common cold, influenza and pneumonia) by a third compared with placebo.
What foods contain vitamin D?
Food sources of vitamin D include oily fish, fish liver oils, animal liver, fortified margarine, eggs, butter and fortified milk. Vegan sources of vitamin D include certain marine algae (the original source of vitamin D found in oily fish) and wild mushrooms that have been exposed to ultraviolet light.
Super Strength Vitamin D3
For healthy bones, teeth, muscles and immunity
- 25mcg vitamin D per tablet
- Helps keep bones, teeth and muscles healthy, and supports immunity
- Multiple health benefits from €0.06 a day
How much vitamin D do you need?
Although diet should always come first, it is difficult to get enough vitamin D unless you regularly eat oily fish and fortified foods. Also, in our northern climate, we are unable to make sufficient vitamin D in our skin in the autumn and winter. That's why official guidelines recommend that everyone takes a daily 10mcg vitamin D supplement during autumn and winter.
This recommended amount is based on the role of vitamin D in aiding absorption of calcium to maintain healthy bones. When it comes to immunity, however, higher amounts are needed. In light of the COVID-19 pandemic, experts have suggested that adults take 20mcg to 50mcg supplements per day to help reduce the severity of infection. The higher amount of 50mcg is ideally suited to people over the age of 50 years.
Gummies and sprays are available for those who prefer not to take tablets.
Is vitamin D3 the same as vitamin D2?
Vitamin D, or calciferol, is the collective term for five different but closely related fat-soluble vitamins. The most important for human health is vitamin D3 (cholecalciferol) which is obtained from animal sources. Vitamin D2 (ergocalciferol) is derived from plant foods such as algae and mushrooms that have been exposed to ultraviolet light.
These two forms have slightly different metabolic effects and interact differently with enzymes and vitamin D receptors. These differences make vitamin D3 more effective in maintaining vitamin D levels and more beneficial for human health than vitamin D2.
Can you have too much vitamin D?
Excess vitamin D can cause side effects due to disturbances in calcium metabolism, such as headache, loss of appetite, nausea, vomiting, diarrhoea or constipation, palpitations and fatigue, so do not exceed recommended doses except under medical advice.
How vitamin C supports immunity
Vitamin C (ascorbic acid) has a natural antiviral action through stimulating the production of immune factors such as interferon, and by suppressing the activation of viral genes. Researchers also believe the powerful antioxidant action of vitamin C mops up inflammatory chemicals produced during a viral infection, to improve symptoms and hasten healing if a cold should develop.
Early studies involving school children and students found that taking vitamin C reduced the risk of catching a cold by around 30%. Men doing heavy physical exercise are particularly prone to respiratory infections, and taking 600mg to 1g vitamin C per day was found to halve the risk of developing cold symptoms for military troops under training, as well as for participants in a 90km running race.
The combined results from 30 trials involving over 11,300 people showed a consistent benefit from treatment, with a reduction in the duration of a cold of 8% in adults and, in children taking 1g to 2g per day, a reduction of 18%. The severity of cold symptoms was also reduced.
Vitamin C is present in plenty of fresh fruit and veg, but for optimum immune health consider a supplement providing 500mg to 1g vitamin C per day.
How much vitamin C do you need?
A minimum daily intake of 10mg vitamin C is needed to prevent scurvy, a potentially fatal disease associated with bleeding gums and poor wound healing. The EU Nutrient reference value (NRV) for vitamin C is 80mg per day, but for optimum immune health supplements providing 500mg to 1g daily are often recommended.
What foods contain vitamin C?
Vitamin C is found in most fruit and vegetables, especially lemon, limes, oranges and other citrus fruit, berries, blackcurrants, capsicum peppers, kiwi fruit and green leafy vegetables. Unfortunately, it is one of the most unstable vitamins, and up to two thirds is lost during food processing, cooking and storage. Fruit juices also rapidly lose their vitamin C content when exposed to air, even if chilled, so consume your food sources as fresh as possible.
Can you overdose on vitamin C?
Because vitamin C is water soluble, any excess you don't need is readily lost from the body via the kidneys. However, doses above 1g per day may cause indigestion and have a laxative effect in some people. If you are sensitive to these acidity effects of vitamin C, this can usually be overcome by taking a so-called 'gentle' form, in which vitamin C is buffered by a mineral such as calcium to chemically neutralise the acid and form calcium ascorbate.
Another way to reduce potential side effects and to maximise absorption is to take a sustained release form of vitamin C that delivers the dose slowly rather than all in one go. Gummies and sprays are available for those who prefer not to take tablets.
Some urine test kits used to monitor blood glucose levels in diabetes are affected by high doses of vitamin C.
People with iron-storage disease (haemochromatosis) should only take vitamin C supplements under medical advice.