Vitamin C, also known as ascorbic acid, is a water-soluble vitamin that humans do not have the ability to make. It must be obtained through their diet. Vitamin C is widely distributed in fruits and vegetables. A sufficient intake of vitamin C (ascorbic acid) is important, as it helps the body to grow and repair tissues (e.g., through assisting collagen production and helping in the synthesis of neurotransmitters) and by working as an antioxidant, blocking damage caused by free radicals.Vitamin C
Signs of vitamin C deficiency include dry and splitting hair, inflammation of the gums, bleeding gums, rough, dry, scaly skin, decreased wound-healing rate, easy bruising, nosebleeds, and a decreased ability to ward off infection.
Whilst the recommended daily intake of vitamin C varies in individual countries, several European countries recommend a daily intake of 100 mg, and in the USA, an intake of 90 mg/day for men and 75 mg/day for women has been defined as adequate (1).
The common cold is an infectious disease of the upper respiratory tract which primarily affects the nose, and is caused by viruses (usually the rhino- or coronavirus). There are a variety of symptoms associated with colds, including headaches, tiredness, coughing, sneezing, nasal congestion, muscle aches, chills/shivers, sore throat, eye irritation and lack of appetite.
The common cold is the most common human illness. Whilst the condition is not usually serious, it is still the leading cause of visits to the doctor, and of time lost from work and school. Most people are subjected to repeated exposure to viruses causing colds, which in healthy individuals creates partial immunity. However, individuals who are under stress (mental, physical or biochemical), or who have a weakened immune system, have increased susceptibility to colds.
In the most serious cases, colds make way for a secondary bacterial infection of the upper respiratory system (e.g., bronchitis or pneumonia), which then requires antibiotic therapy (2).
Influenza or “flu” can be confused with a cold, but the symptoms of flu are much more severe and it is usually accompanied by a fever.
Whilst the viruses that cause colds are highly infectious, their spread can be reduced by good hygiene, good nutrition and adequate sleep.
There is no cure for the common cold, and being viral in origin, antibiotics are useless. However, most colds will be resolved without any intervention. Usually, “cold cure” products are designed to relieve cold symptoms, typically comprising of one or more of the following: vitamin C, antihistamines, decongestants, and/or pain relievers (3).
Vitamin C (or ascorbic acid) accumulates in human organs that have an intensive metabolism, e.g., the brain, liver, eye, spleen, pituitary and adrenal glands (4). Vitamin C has a biological half-life in the body of 8 to 40 days and a catabolic turnover of 10 to 45 mg/day. Biochemical stress, such as that imposed by smoking, rapidly reduces tissue levels of ascorbic acid (5).
Vitamin C is concentrated in cells that are important in our immune system (e.g., lymphocytes, neutrophils, monocytes, phagocytes and T cells), where the levels are 10- to 100-fold higher than in plasma. Thus vitamin C deficiency leaves an individual at increased risk of viral or bacterial infection (6).
Vitamin C is an effective water-soluble antioxidant, able to trap reactive oxygen species (ROS). It forms part of the antioxidant defense system of phagocytes.
In 2013, 70 years of clinical research on vitamin C and the common cold were systematically reviewed (7). The results showed that regular supplementation with 200 mg or more of vitamin C reduced the duration of colds, with a greater benefit in children than in adults. In addition, regular vitamin C supplementation with 250 to 2,000 mg/day reduced the incidence of colds in individuals under heavy physical stress (e.g., marathon runners, skiers, or soldiers in subarctic conditions). So, it is reasonable to conclude that regular daily supplementation appears to be very important in reducing the severity and duration of colds.
In 2014, Johnson et al. (8) performed a study into colds which was limited to men who all had plasma vitamin C levels below 45 μmol/L and who were considered to have an insufficient vitamin C status. Over eight weeks the men in the treatment arm of the study received 1,000 mg/day of vitamin C. This achieved elevated plasma ascorbic acid levels to near saturation. By the end of the study, the researchers found that subjects taking vitamin C supplements were 40% less likely to get a cold, and the duration of colds was decreased by nearly 60%. As is often the case, this study demonstrates that individuals with vitamin C deficiency benefit most from supplementation of the same.
Vitamin C functions to keep iron in a reduced state in two enzymes that are involved in carnitine production. Carnitine is a prerequisite for cellular mitochondrial energy production (9). However, more recent studies indicate that the presence of vitamin C is not essential for carnitine synthesis (10).
A new study by Schwager et al. (11) looked at the effect of ascorbic acid on neutrophils, a group of white blood cells (leukocytes) that protect the body from infection. They found that vitamin C increased neutrophil migration induced by chemo-attractants (fMLP and IL8). In humans, a plasma concentration of about 50 to 100 µmol/L of vitamin C is required to achieve this, which corresponds to an intake of about 200 mg vitamin C.
Neutrophils are attracted to the site of trauma or infection, so improving migration increases the elimination of pathogens in the body.
Vitamin C has an established role to play in the immune response and, more specifically, has been implicated in reducing the duration of the common cold. Vitamin C accumulates in white blood cells (which are part of the immune defense system). Vitamin C improves their mobility, enabling them to localize and attack invading organisms such as viruses, including those associated with the common cold.