Vyvienne Evans, of Dunedin, asks :-
I was told that humans are one of the few mammals who cannot produce vitamin C. However some people are hardly ever sick. Is it possible these people produce their own vitamin C?
Rana Ravindran, of Massey University's Institute of Food, Nutrition and Human Health, responded.
Vitamin C is a water-soluble vitamin which many mammals can synthesize from glucose via the uronic acid pathway. Many others, including humans, primates, guinea pigs and fruit bats, cannot due to a lack of L-gulonolactone oxidase, one of the four enzymes needed for the synthesis. This makes vitamin C required in the diet.
Vitamin C is readily absorbed and circulated in the blood; any excess is readily excreted and not stored by the human body. Hence we have a daily requirement. The level of Vitamin C in the blood fluctuates depending on the levels in the diet. Though it is speculated that some human can synthesise vitamin C, there is no research evidence yet to prove this conjecture. Why some people are hardly ever sick depends on genetics, life-style factors, and, importantly, a balanced diet. While some people may require more vitamin C than others, it is documented that the primary cause leading to an increased requirement for vitamin C is severe stress or trauma.
The recommended dietary intake (in mg/day) for vitamin C in NZ is 35 for children aged 1-8 years; 40 for boys and girls 9-18 years of age, 45 for men and women of 19-70 years, 55-60 for pregnancy and 80-85 mg/day for lactation. The suggested dietary target to reduce chronic disease risk is 220 and 190 mg/day for men and women, respectively. Tolerable upper intake by both is 2000mg/day. Though toxicity is remarkably low, mega doses may result in diarrhoea in healthy adults.
These levels can be attained by replacing nutrient-poor, energy-dense foods with plenty of fruits and vegetables. Fruits and vegetables pack large concentrations of vitamin C. Kiwi fruit (gold) has the highest vitamin C content. Fruits such as oranges, grapefruits, tangerines, lemons, limes, papaya and berries, and fresh vegetables including tomatoes, broccoli, green and red bell peppers, raw lettuce and other leafy greens are very rich in vitamin C. Five servings (two and a half cups) of these fruits and vegetables should provide ones daily requirement.
Fresh products contain more than cooked or processed ones. The amount of Vitamin C in these products decreases with time in relation to temperature of storage, type and length of cooking etc.