Vitamin C requirements
Vitamin C illustrates extremely well how different criteria of adequacy, and different interpretations of experimental data, can lead to different estimates of requirements, and to reference intakes ranging between 30 and 90 mg/day for adults.
The requirement for vitamin C to prevent clinical scurvy is less than 10 mg/day. However, at this level of intake wounds do not heal properly because of the requirement for vitamin C for the synthesis of colla-gen in connective tissue. An intake of 20 mg/day is required for optimum wound healing. Allowing for individual variation in requirements, this gives a ref-erence intake for adults of 30 mg/day, which was the British recommended daily allowance (RDA) until 1991.
The 1991 British reference nutrient intake (RNI) for vitamin C is based on the level of intake at which the plasma concentration rises sharply, showing that requirements have now been met, tissues are satu-rated and there is spare vitamin C being transported between tissues, available for excretion. This criterion of adequacy gives an RNI of 40 mg/day for adults.
The alternative approach to determining require-ments is to estimate the total body content of vitamin C, then measure the rate at which it is metabolized, by giving a test dose of radioactive vitamin. This is the basis of both the former US RDA of 60 mg/day for adults and the Netherlands RDA of 80 mg/day. Indeed, it also provides an alternative basis for the RNI of 40 mg/day.
The problem lies in deciding what is an appropriate body content of vitamin C. The studies were per-formed on subjects whose total body vitamin C was estimated to be 1500 mg at the beginning of a deple-tion study. However, there is no evidence that this is a necessary, or even a desirable, body content of the vitamin. It is simply the body content of the vitamin of a small group of people eating a self-selected diet rich in fruit. There is good evidence that a total body content of 900 mg is more than adequate. It is three times larger than the body content at which the first signs of deficiency are observed, and will protect against the development of any signs of deficiency for several months on a completely vitamin C-free diet.
There is a further problem in interpreting the results. The rate at which vitamin C is metabolized varies with the amount consumed. This means that as the experi-mental subjects become depleted, so the rate at which they metabolize the vitamin decreases. Thus, calcula-tion of the amount that is required to maintain the body content depends on both the way in which the results obtained during depletion studies are extrapo-lated to the rate in subjects consuming a normal diet and the amount of vitamin C in that diet.
An intake of 40 mg/day is more than adequate to maintain a total body content of 900 mg of vitamin C (the British RNI). At a higher level of habitual intake, 60 mg/day is adequate to maintain a total body content of 1500 mg (the former US RDA). Making allowances for changes in the rate of metabolism with different levels of intake, and allowing for incomplete absorption of the vitamin gives the Netherlands RDA of 80 mg/day.
The current US reference intake (75 mg for women and 90 mg for men) is based on intakes required to saturate leukocytes with vitamin C.
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