Relationship between vitamin C intake and plasma concentration

 

How much vitamin C do we need to take?

Many biochemical functions which occur in different tissues of the body rely on vitamin C. To determine optimum vitamin requirements, it is necessary to know the relationship between intake and tissue concentrations. Since blood is the vehicle for vitamin C transport, plasma concentration is a good indicator of an adequate supply. It has been suggested that the optimum plasma concentration of vitamin C is 50 µmol/L. The aims of this meta-analysis are to (i) describe the relationship between intake and plasma concentration of vitamin C and (ii) ascertain how much vitamin C needs to be taken to achieve a plasma concentration of 50 µmol/L.

Message

In the general population, the assumed optimum plasma concentration of 50 µmol/L can be achieved by taking 100 mg of vitamin C a day. This is more than double the current UK recommendation of 40 mg/d. Smokers and people aged 60-96 years need a higher daily intake to reach the same plasma levels as non-smokers and people aged 15-65 years.

Reference

D Brubacher et al. Vitamin C concentrations in plasma as a function of intake: a meta-analysis. Int J Vitam Nutr Research 2000 70: 226-237.

Search

The literature was searched using MEDLINE until 1998 with no language restrictions. The inclusion criteria for papers were as follows: participants had to be healthy adults; vitamin C derived from foods had to be reported if supplements were given; and the plasma concentration of vitamin C, with its respective intake, had to be reported.

Thirty-six papers fulfilled these criteria. Participant numbers ranged from fewer than 50 to 3000. Both men and women were represented. Vitamin C was derived from either foods and/or supplements. The methods used to assess vitamin C intake varied across studies, including recalls, food frequency questionnaires and food records covering 24 hours to seven days. The methods used to assess plasma concentrations of vitamin C also varied (references given, but no further details).

Estimations were performed for: (i) adults aged 15-65 years; (ii) older people aged 60-96 years; (iii) non-smokers; and (iv) smokers.

Results

For a daily vitamin C intake of 60 mg, the median plasma concentrations were 42 µmol/L for all participants, 44 µmol/L for adults, 31 µmol/L for older adults, 42 µmol/L for non-smokers and 34 µmol/L for smokers.

For a daily vitamin C intake of 100 mg, the median plasma concentrations were 54 µmol/L for all participants, 56 µmol/L for adults, 42 µmol/L for older adults, 53 µmol/L for non-smokers and 42 µmol/L for smokers. The increased intake of 40 mg led to a rise of over 10 µmol/L in plasma concentrations except in smokers.

To reach a vitamin C plasma level of 50 µmol/L, the calculated daily intakes of vitamin C were 83 mg for all participants, 78 mg for adults, 150 mg for older adults, 89 mg for non-smokers and 207 mg for smokers. These figures are the medians or the estimated average requirements.

Comment

Despite differences in studies, this meta-analysis usefully and successfully determines optimum vitamin C requirements, not only in the general population, but also in selected groups. It confirms previous results that vitamin C requirements are higher in smokers and older adults. The difference between smokers and non-smokers is interesting and important.

Furthermore, the estimated average requirement of 83 mg/day translates to a recommended intake of 100 mg/day, which is the new recommendation of some European countries. The current UK recommendation is 40 mg/day.

(To explain the translation of the estimated average requirement (83 mg) and the recommendation (100 mg): recommended intakes are based on two standard deviations above estimated average requirements; and are higher to ensure more of the population have an adequate intake.)