Vitamin C

Vitamin C, also known as ascorbic acid, is an essential nutrient, widely recognized for its antioxidant properties.

Even in small quantities vitamin C can protect critical molecules in the body such as proteins, lipids, carbohydrates, and nucleic acids (DNA and RNA) from damage by reactive oxygen species, which are generated during normal metabolism, by active immune cells, and through exposure to toxins and pollutants (e.g., certain chemotherapy drugs and cigarette smoke).

Vitamin C participates in immune function, wound healing, fatty acid metabolism, neurotransmitter production, and blood vessel formation, as well as other key processes and pathways.

Vitamin C's role in immune function, in particular, is crucial. It stimulates the production of white blood cells, especially neutrophils, lymphocytes, and phagocytes, and promotes the cells' normal functions, such as their ability to detect, move toward, and engulf pathogens.

Immune cells release large quantities of reactive oxygen species, often incurring damage. To protect themselves from this damage, immune cells accumulate large quantities of vitamin C, which serves as an antioxidant within the cells.

Immune cells also release interferons, a class of proteins produced as a defensive response to viruses. Some evidence indicates that vitamin C promotes the production of interferon, a protein that participates in antiviral activity.

Finally, vitamin C is involved in many other physiological processes. For example, it influences physiological levels of other vitamins. It regenerates vitamin E from its oxidized form and increases the bioavailability of iron from foods by enhancing gut absorption of nonheme iron. In addition, some evidence suggests that intravenous administration of vitamin C might be effective in treating certain types of viral infections and as adjunctive therapy in various types of cancers.

Although many members of the animal kingdom can synthesize vitamin C, humans cannot and must obtain it in their diets.

Dietary sources of vitamin C

Vitamin C is widely abundant in fruits and vegetables. Although citrus fruits are synonymous with vitamin C, the highest vitamin C content (per 100-gram serving) is found in guavas, kiwis, and bell peppers.

Vitamin C is easily destroyed by excessive heat and water, as well as exposure to air. Cooking foods destroys approximately 25 percent (or more) of the vitamin C present in foods.

Recommended intake of vitamin C for healthy people

The recommended dietary allowance, or RDA, for vitamin C varies according to age, sex, and life stage of healthy people. Needs are fairly high in infancy, decrease slightly during childhood, and peak in early adulthood. Pregnancy and breastfeeding increase a woman's vitamin C needs. Vitamin C is present in breastmilk, where it serves as an antioxidant for the mother's milk.

The goal of meeting the RDA is to maintain an optimal blood concentration of approximately 50 micromoles per liter of blood to prevent oxidation of low-density lipoproteins. The indicators used to estimate this requirement include the vitamins capacity to provide antioxidant protection in neutrophils against reactive oxygen species produced during phagocytosis; prevent DNA and chromosomal damage; enhance immune function; facilitate collagen metabolism and carnitine biosynthesis; and maintain periodontal health.

A tolerable upper intake limit of 2,000 milligrams per day has been established to reduce the risk of gastrointestinal complications commonly associated with higher doses of vitamin C, such as diarrhea. However, no scientific evidence demonstrates that vitamin C administered in doses up to 10 grams per day in adults is toxic or detrimental to health.

People who smoke, consume alcohol, or have specific medical conditions commonly have higher vitamin C needs than healthy people. Smoking increases oxidative stress, which increases antioxidant requirements. Alcohol consumption increases urinary vitamin C losses by nearly 50 percent, suggesting that higher intake might be required to prevent deficiency in regular or heavy drinkers.

People who have inflammatory bowel diseases, such as Crohn's disease or ulcerative colitis, might have reduced plasma vitamin C levels, even if their disease status is quiescent. In addition, people who have chronic renal failure commonly have lower plasma concentrations of vitamin C compared to healthy people, a condition that can be exacerbated by dialysis, which can trigger the production of reactive oxygen species.

A study involving 22 patients with renal failure and their matched healthy controls demonstrated that a single dialysis session decreased blood vitamin C levels even further. Prior to dialysis, average plasma vitamin C concentrations among the controls were approximately 42 micromoles per liter of blood versus 11 micromoles per liter of blood among the renal patients. After a dialysis session, however, the renal patients' average plasma vitamin C levels dropped markedly, to 6 micromoles per liter of blood, roughly half of their pre-dialysis levels.

Similarly, a study in nearly 140 men (average age, 61 years) undergoing dialysis treatment found that low total vitamin C plasma levels predicted adverse cardiovascular outcomes. Patients whose vitamin C levels were lower than 32 micromoles per liter had approximately 300 percent higher risk of cardiovascular disease-related death.

Patients with concentrations of 32 to 60 micromoles per liter had approximately 200 percent higher risk of cardiovascular-related deaths compared to patients whose vitamin C levels were greater than 60 micromoles per liter.

How much?

Supplementing 1 to 3 grams of vitamin C once per day produces transient peak plasma concentrations that are two- to three times higher than those from 200 milligrams of oral vitamin C. These plasma levels saturated at approximately 220 micromoles per liter and returned back to baseline levels after 24 hours.

Vitamin C for the use against the common cold

The common cold, a viral infection of the upper respiratory tract that typically affects the nose, throat, and sinuses, is probably the most-studied application for vitamin C, due to the vitamin's role in maintaining proper immune function. Many clinical studies have investigated the effects of vitamin C in preventing and treating symptoms of the common cold, with substantial variation in the magnitude of benefit. Factors such as age, dose, and duration of supplementation affect the degree to which vitamin C can prevent or decrease episodes of the common cold.

A meta-analysis of more than 6,000 participants across 23 clinical studies explored some of the factors that could explain the variation in the magnitude of benefit for taking vitamin C during a cold episode. The criteria used to evaluate efficacy in each trial included the mean duration of symptoms and mean absence from work or school, if applicable. The analysis found that supplementation with at least 2 grams per day of vitamin C during a cold had a greater benefit compared to a dose of 1 gram per day. A sub-analysis of five studies found that higher-dose vitamin C supplementation elicited a more robust effect against colds in children (younger than 16 years of age) compared to adults. Cold duration decreased 21 percent among adults taking 2 grams daily and decreased 6 percent among adults taking only 1 gram daily. However, cold duration decreased 26 percent among children taking 2 grams daily and 17 percent among children taking only 1 gram daily.

A meta-analysis of nine randomized controlled studies compared the administration of vitamin C as a prophylactic measure with or without therapeutic doses of vitamin C taken after the onset of symptoms to treat and prevent the common cold. The prophylactic regimens included doses between 1 and 3 grams of vitamin C taken per day over the course of several months along with a therapeutic dose of up to 6 grams at the onset of symptoms. The study found that prophylactic supplementation of vitamin C along with a therapeutic dose at the onset of common cold symptoms can reduce both the symptoms and the duration of a cold episode, while therapeutic doses given only at the onset of symptoms had no effect.

The analysis determined that taking vitamin C prophylactically and then increasing the dose when symptoms of a cold arise shortened the duration of a cold by half a day. If vitamin C was taken only at the onset of symptoms, there was no shortening of cold symptoms.

A meta-analysis of 29 trials involving more than 11,000 participants further evaluated the use of vitamin C as a prophylactic supplement at doses ranging from 200 milligrams to 2 grams per day for a time period of two weeks to six months. The analysis revealed that those who supplemented were only 4 percent less likely to develop a cold than those who did not supplement.

Some of the doses compared in this analysis were very low compared to other studies, so it is possible that higher doses could have prevented more colds, as demonstrated elsewhere.

Vitamin C and lung function

The innate immune system of the lungs plays an important role in the body's defense system as a means to protect the body from inhaled oxidants and pathogens. Vitamin C might help protect the lungs by boosting immune cell function and reducing oxidative stress. Some evidence demonstrates that certain bacteria (including pneumococci) release large quantities of hydrogen peroxide into cells, inactivating inflammasomes and weakening the immune response to lung infection.Inflammasomes are large, intracellular protein complexes that detect and respond to pathogens. Vitamin C's role as an antioxidant could have special relevance for lung pathogens by preventing them from inactivating the immune system. Multiple studies have investigated the use of vitamin C as a therapy to protect the lungs against oxidative stress caused by infections.

Vitamin C and the risk of respiratory diseases

Vitamin C might be protective against respiratory diseases, the fifth leading cause of death among people living in the United States. A population-based study analyzed the relationship between blood vitamin C concentrations and respiratory disease in more than 19,000 participants between the ages of 40 and 79 years without histories of respiratory diseases. To assess this relationship, the participants were separated into quartiles based on baseline blood vitamin C levels: less than 41, between 42 and 54, between 54 and 66, and greater than 66 micromoles per liter of blood. The study participants were followed for approximately 16.5 years. The analysis found that the participants with the highest blood vitamin C concentrations were 15 percent less likely to develop respiratory conditions and were 46 percent less likely to die of lung cancer, compared to the participants with the lowest blood vitamin C levels. The authors of the study noted that the participants with the highest baseline vitamin C levels also had lower risk factors for developing respiratory disease: they were generally younger and healthier, were less likely to smoke, consumed less alcohol, were more physically active, and had a lower prevalence of other chronic illnesses than those with lower vitamin C levels.

Vitamin C and mechanical ventilation

Vitamin C has proven to be effective at decreasing the duration for which patients are kept on mechanical ventilation, an important strategy used to treat people experiencing respiratory failure. A meta-analysis of six studies analyzed the duration of mechanical ventilation among patients who received supplemental vitamin C versus those who did not. The analysis found a significant difference in the effect of vitamin C on the duration of mechanical ventilation between patients who were ventilated for more than 24 hours compared to those on ventilation for less than 24 hours. In three of those trials (two of which utilized intravenous vitamin C) patients who were ventilated for more than 24 hours and given vitamin C spent approximately 18 percent less time on mechanical ventilation compared to controls. However, in three other trials where patients were on ventilators for less than 24 hours neither intravenous nor oral vitamin C administration had an effect on the duration of ventilation. These findings suggest that less than 24 hours on a ventilator is not sufficient time for vitamin C to exert protective effects.

Vitamin C and asthma

Asthma is a common long-term inflammatory disease of the airways of the lungs that affects a person's ability to breathe. Few studies have investigated the role of vitamin C in reducing the symptoms and severity of asthma attacks, and the findings have been mixed.

A study in 41 asthma patients (average age, 26 years) who took 1 gram of vitamin C or a placebo daily for 14 weeks found that the patients who took vitamin C had fewer, less severe asthma attacks compared to the placebo group. A separate study gave 300 hundred asthma patients aged 18 to 60 years 1 gram of vitamin C and 450 milligrams of magnesium chelate or a placebo daily for 16 weeks. Low blood magnesium levels have been implicated in the severity of asthma. The study identified no evidence of beneficial effects on any outcome measure of asthma control such as symptom scores, bronchodilator use, FEV1, and airway responsiveness to methacholine, a commonly used asthma drug. More randomized placebo-controlled studies are needed to determine whether vitamin C might improve symptoms and reduce the severity of asthma attacks. Additional studies should determine if outcomes of asthma are altered by different doses of vitamin C, age demographics, or varying baseline levels of vitamin C.

Vitamin C and exercise

Vitamin C might enhance exercise performance by reducing the potential negative consequences of excess reactive oxygen species, while also blunting the beneficial training adaptations that reactive oxygen species might mediate. Reactive oxygen species are highly reactive molecules produced during normal metabolic processes as well as during exercise, as a consequence of exercise-induced immune activation. Excessive exercise-induced reactive oxygen species can promote muscle damage, fatigue, and immune dysfunction, but the extent varies according to the duration and extent of exercise. For example, high-intensity exercise such as long-distance running is linked to an increased incidence of upper respiratory infection. However, reactive oxygen species might also mediate beneficial training adaptations as a part of a biologically useful signaling cascade.

The above was taken from the vast article by Dr Rhonda Patrick -


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