Dietary Sources of Vitamin C and Limitations to Vitamin C

Ascorbate is found in many fruits and vegetables. Citrus fruits and juices are particularly rich sources of vitamin C but other fruits including cantaloupe, honeydew melon, cherries,

kiwi fruits, mangoes, papaya, strawberries, tangelo, watermelon, and tomatoes also contain variable amounts of vitamin C. Vegetables such as cabbage, broccoli, Brussels sprouts, bean sprouts, cauliflower, kale, mustard greens, red and green peppers, peas, tomatoes, and potatoes may be more important sources of vitamin C than fruits. This is particularly true because the vegetable supply often extends for longer periods during the year than does the fruit supply.

In many developing countries, limitations in the supply of vitamin C are often determined by seasonal factors (i.e., the availability of water, time, and labour for the management of household gardens and the short harvesting season of many fruits). For example, mean monthly ascorbate intakes ranged from 0 to 115 mg/day in one Gambian community in which peak intakes coincided with the seasonal duration of the mango crop and to a lesser extent with orange and grapefruit harvests. These fluctuations in dietary ascorbate intake were closely reflected by corresponding variations in plasma ascorbate (11.4–68.4 μmol/L) and human milk ascorbate (143–342 μmol/L).

Vitamin C is also very labile, and the loss of vitamin C on boiling milk provides one dramatic example of a cause of infantile scurvy. The vitamin C content of food is strongly influenced by season, transport to market, shelf life, time of storage, cooking practices, and chlorination of water. Cutting or bruising of produce releases ascorbate oxidase. Blanching techniques inactivate the oxidase enzyme and help to preserve ascorbate as also will low pH, as in the preparation of sauerkraut (pickled cabbage). In contrast, heating and exposure to copper or iron or to mildly alkaline conditions destroys the vitamin, and too much water can leach it from the tissues during cooking.

The use of citrus fruits by the British navy in the 18th century gave rise to the term ‘limey’, a colloquial term for British sailors. However, it is important to realise that the amount of vitamin C in a food is usually not the major determinant of a food’s importance for supply, but rather regularity of intake. For example, in countries where the potato is an important staple food and refrigeration facilities are limited, seasonal variations in plasma ascorbate are due to the considerable deterioration in the potato’s vitamin C content during storage; the content can decrease from 30 to 8 mg/100 g over 8–9 months. Such data can indicate the important contribution the potato can make to human vitamin C requirements even though the potato vitamin C concentration is low.

An extensive study has been made of losses of vitamin C during the packaging, storage, and cooking of blended foods (maize and soya-based relief foods). Data from a US Agency for International Development programme show that vitamin C losses from packaging and storage in polythene bags of such relief foods are much less significant than the 52–82 percent losses attributable to conventional cooking procedures.

References:

Haytowitz, D. 1995. Information from USDA's Nutrient Data Book. J. Nutr., 125:
1952-1955.

Bates, C.J., Prentice, A.M. & Paul, A.A. 1994. Seasonal variations in vitamins A, C, riboflavin and folate intakes and status of pregnant of lactating women in a rural Gambian community: some possible implications. Eur. J. Clin. Nutr., 48: 660-668.

Paul, A.A. & Southgate, D.A.T. 1979. McCance and Widdowson’s, The Composition of Foods. London: HMSO.

N.A.S. Institute of Medicine. 1997. Vitamin C fortification of food aid commodities: Final Report, Washington D.C., National Academy Press