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Nutrition and Metabolism

Vitamin C

What is vitamin C?

Vitamin C is a water-soluble vitamin, also known as ascorbic acid. Vitamin C is essential for making skin, tendons, ligaments, and blood vessels; for healing wounds and forming scar tissue; and for repairing and maintaining cartilage, bones, and teeth.  Human body cannot synthesize vitamin C and has to obtain it from diet. The most common disease caused by vitamin C deficiency is scurvy, which is rare in modern society but was historically very common among sailors who had limited access to vegetables and fruits, the primary natural dietary vitamin C resources. 

How is vitamin C used in human body?

Vitamin C has two major functions in human body: reducing agent and anti-oxidant. The reducing power of vitamin C is responsible for maintaining the iron and copper ions at reduced states so they can function properly in a variety of reactions including collagen synthesis, carnitine synthesis, tyrosin metabolism, neurotransmitter and hormone (norepinephrine and serotonin) synthesis, and drug degradation. As an anti-oxidant molecule, vitamin C reacts with free radicals in blood or inside cells to prevent DNA, protein or long chain fatty acids from potential damages.

What is the normal vitamin C level in human body?

Vitamin C level is normally between 0.4 and 1.7 mg/dL in human blood stream. Since cells are saturated with vitamin C before plasma, vitamin C concentration in tissues are normally higher, although vary greatly in different tissues. For example, vitamin C in white blood cells can be as much as 80 times higher than in plasma. When plasma vitamin C concentration drops below 0.2 mg/dL, it is considered vitamin C deficient.

How much vitamin C do I need in my diet?

The Institute of Medicine of the National Academies recommends daily values in RDA (Recommended Dietary Allowances) for different human population based on age and gender (Table 1). Smokers need to consume 35 mg/day more dietary vitamin C since smoking depletes vitamin pool. Although high levels of vitamin C from supplements or medicine subscriptions are generally harmless to normal individuals, some people do have adverse reactions (see later).  Therefore, a Tolerable Upper Intake Level of 2,000 mg/day is recommended by the Institute of Medicine of the National Academies.

Table 1. Recommended Dietary Allowances (RDA) for vitamin C recommended by the Institute of Medicine of the National Academy of Sciences.

Age (years)  RDA (mg/day)
1–3 15
4–8 25
9–13 45
14-18 (male) 75
14-18 (female) 65
14-18 (pregnant women) 80
14-18 (lactating women) 115
19+ (male) 90
19+ (female) 75
19+ (pregnant women) 85
19+ (lactating women) 120



What are risks for having too much vitamin C?

High dose of vitamin C intake is unlikely to have side effects for healthy individuals. However, people carrying certain genetic mutations (such mutations in HFE or G6PD) can develop iron overload (hemochromatosis) when taking too much vitamin C.  People already have iron overload, thalassemia, and sickle cell anemia also develop adverse reactions to high vitamin C levels. Iron overload is a common genetic disorder among Caucasians in the United States, affecting approximately one in 240 to 300 Caucasians. Thalassemias are particularly associated with people of Mediterranean origin (16% in Cyprus), and Asians (3-8%). Sickle cell anemia is more prevalent in Africans (1 out of 500).

Why vitamin C is more important for people carrying certain gene variants?

Vitamin C from the diet is transported in human body by two transporter proteins.  One of them is encoded by the SLC23A2 gene. A genetic variant of this gene, carried by about 28% general population, has reduced capacity in the absorption and accumulation of vitamin C in many tissues, is associated with lower plasma vitamin C concentrations. The Hp gene encodes haptoglobin, a hemoglobin-binding protein that has antioxidant properties. A common polymorphism of the Hp gene consists of 2 structurally different alleles: Hp1 and Hp2. Individuals with the Hp2-2 genotype, distributed as high as 48% in Caucasians and 52% Asians, had an increased risk of deficiency if they did not meet the Recommended Dietary Allowance for vitamin C. For people with the SLC23A2 variant or with the Hp2-2 genotype, sufficient vitamin C intake is vital.

Vitamin C also interacts with three genetic variations in MTHFR genes to modulate the level of another vitamin folate.  For these variants carriers, adequate folate levels are also critically dependent on sufficient dietary vitamin C.

Which foods contain high level of vitamin C?

Vitamin C is generally rich in fruits and vegetables. The top 10 common foods that contain the highest content of vitamin C are shown in Table 2. In addition, other citrus (lemons, grapefruits, limes, tangerine), melons (cantaloupe, honeydew), crucifers (turnips greens, cauliflowers, watercress) are also good sources of vitamin C. Many fruit juices and fortified foods from grocery stores also have the amount of vitamin C listed in the Nutrition Facts on the label. They are not listed here.

Table 2. The top 10 common vitamin C-rich foods.

Fruits
Guava
Per 100 g (mg) 228
Per Serving (mg) 377
Serving Size 1 cup (165 g)
 
Kiwi
Per 100 g (mg) 93
Per Serving (mg) 167
Serving Size 1 cup (180 g)
 
Papayas
Per 100 g (mg) 61
Per Serving (mg) 88
Serving Size 1 cup (145 g)
 
Strawberries
Per 100 g (mg) 59
Per Serving (mg) 89
Serving Size 1 cup (152 g)
 
Oranges
Per 100 g (mg) 53
Per Serving (mg) 96
Serving Size 1 cup (180 g)
 
Vegetables
Kale
Per 100 g (mg) 120
Per Serving (mg) 80
Serving Size 1 cup (67 g)
 
Broccoli
Per 100 g (mg) 89
Per Serving (mg) 81
Serving Size 1 cup (91 g)
 
Brussels sprouts
Per 100 g (mg) 85
Per Serving (mg) 75
Serving Size 1 cup (88 g)
 
Peppers, sweet
Per 100 g (mg) 80
Per Serving (mg) 120
Serving Size 1 cup (149 g)
 
Mustard greens
Per 100 g (mg) 70
Per Serving (mg) 39
Serving Size 1 cup (56 g)
 

How to get sufficient dietary Vitamin C intake?

First you are recommended to use the GB Food Calorie and Nutrition Calculator to get the most accurate estimate of your current vitamin C intake.

To get sufficient dietary vitamin C from diet, the best way is to eat more fruits and vegetables.  Fresh vegetable juices, salads are also great sources of Vitamin C.

If you cannot get enough fresh vegetables or fruits, vitamin C supplement is an alternative.  Unless you have certain genetic predisposition (such as HFE or G6PD mutations) or healthy conditions such as iron overload, thalassemia, and sickle cell anemia, vitamin C supplement is an efficient way to ensure sufficient vitamin C to meet the need of your body.

 

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