Vitamin C, in the form of ascorbic acid and dehydroascorbic acid, is widely available in foods of both plant and animal origin. Fruits, vegetables and organ meats, e.g. liver and kidney, are generally the best sources. Plants synthesize ascorbic acid from carbohydrates. Most seeds do not contain ascorbic acid, but start to synthesize it upon sprouting, and it is probably present in high concentrations in rapidly growing stems, root tips, green leaves and pods. Although most animals can synthesize their own supply there is relatively little stored in their tissues and muscle meat is therefore a poor source of the vitamin. Fruits, especially citrus fruits, are a good daily source of vitamin C as they are generally eaten raw and are thus not subjected to cooking that can destroy the vitamin. Green vegetables are also a useful source, although they contain highly variable amounts, much of which can be lost in preparation and cooking. Tubers such as potatoes, are not a rich source, but since large quantities are often eaten, they can be a major contributor to meeting requirements. The vitamin C content of some vegetables, fruits and tubers is listed in Table 8 (Combs, 1992).
Among fruits, guavas, rose hips and various berries are rich sources (>300 mg/100 g) while most other fruits provide 10-90 mg/100 g. Starchy roots, tubers and starchy fruits contain appreciable quantities of vitamin C with cassava containing 40 mg/100 g. However, Watson (1976) reported that cassava flour and cassava meal processed from fresh cassava contained only 9.9 mg/100 g and 6.3 mg/100 g, respectively. Sweet potato contains, on average, 37 mg vitamin C per 100 g edible portion (West et al., 1988) and can be an important source of the vitamin since it can be fairly easily cultivated in cooler or hotter climates. Among vegetables, the leaves of amaranthus and sweet potato contain 100-150 mg vitamin C/100 g and other vegetables such as tomatoes, peas and beans 10-30 mg/100 g. The cassava leaf, although rarely eaten in many African countries, is very popular in parts of central and West Africa; it is a particularly rich source of vitamin C with an average of 740 mg/100 g. Peppers are also rich sources with about 740 mg/100 g, depending on the variety (Watson, 1976). Dried pepper powder contains considerable vitamin C and therefore can increase the vitamin C content of cooked food when used as a spice.
• dried peppers: unspecified 180 mg/100 g
• dried red peppers 12 mg/100 g
Breast milk is a good source of vitamin C that covers an infant's needs when full lactation is maintained. Infantile scurvy is seen only in artificially fed infants.
Varieties of plant produce are excellent sources of vitamin C and should not be overlooked. Even though they are not eaten in large amounts they are used regularly where available (Nicol, 1958). The following are some examples of sylvan produce found in northern Nigeria and northern Ghana (Nicol, 1958 and Watson, 1976):
• Baobab fruit pulp (373 mg/100 g) is made into an emulsion with water and used as an adulterant of milk or in sorghum and millet gruels.
• Locust bean tree pulp (190 mg/100 g) is stirred into gruels and is made into a sweet syrup or eaten as it comes from the pad.
• Desert date pulp (140 mg/100 g) is eaten uncooked when the fruit is ripe.
Brand et al. (1982) found that a wild fruit (Terminalia ferdinandiana) used by Australian aboriginals, and eaten especially by children, may well be the richest natural source of vitamin C in the world with between 2300 mg and 3150 mg per 100 g edible fruit. Other rich sources are the amla fruit in India (600 mg/100 g) and the Barbados cherry (1000-2330 mg/100 g). Nicol (1958) reported the great importance of sylvan produce in the arid parts of Nigeria where sorghum and millet, which contain no vitamin C, are the staple foods. However, habitual yam-eaters, who may eat more than 1 kg of yam per day, can derive all their vitamin C requirements and more from this food alone. Watson (1976) also reported that in Ghana many of the starchy roots, tubers and starchy fruits are eaten in appreciable quantities in the coastal and forest zones and would probably contribute adequate amounts of vitamin C to the diet. However, in the northern regions, cereals form the bulk of the diet and sylvan produce may play an important role in covering the population's vitamin C requirements.
The report by a joint FAO/WHO expert group (1970) stated that people consume 100-350 mg per day of vitamin C where starchy roots and tubers are the staple foods and 10-70 mg per day where cereals are the staples. Besides the kind of staple food consumed, the season of the year is an important factor influencing vitamin C availability and intake. Clearly there are seasonal variations in the supply of fruits and vegetables but also variations in the amount of vitamin C contained especially in vegetables. Vitamin C in green vegetables is highest when the plants are growing rapidly (FAO/WHO, 1970).
Vitamin C intake levels in various parts of the world are found in annex. Vegetables contribute some 60-85% of the total intake in developing countries, but only 20-50% in Europe and the USA. Fruits contribute 1-20% of intakes in developing areas and 15-45% in the West (FAO/WHO, 1970).
Camel's milk, which is an important source of vitamin C for nomads in Somalia, contains approximately 6 mg/100 g, or about three times the amount in cow's milk. If not consumed fresh, the milk is traditionally soured and stored in opaque containers, which preserves the vitamin. The nomads traditionally consume up to 4 litres of camel's milk per day, thereby providing more than adequate quantities of the vitamin (Magan et al., 1983).