Journal of the New Zealand Medical Association, 23-August-2002, Vol 115 No 1160
Glycohaemoglobin and ascorbic acid
Copplestone et al1 (
http://www.nzma.org....al/115-1157/25/) identified misleading glycohaemoglobin (GHb) results due to a haemoglobin variant (Hb D Punjab) and listed a number of other possible causes for such false results (ie, haemolytic anaemia, uraemia, lead poisoning, alcoholism, high-dose salicylates and hereditary persistence of foetal haemoglobin).
We have observed a significant "false" lowering of GHb in animals and humans supplementing ascorbic acid (AA) at multigram levels. Mice receiving ~7.5 mg/d (equivalent to > 10 g/day in a 70 kg human) exhibited no decrease in plasma glucose, but a 23% reduction in GHb.2 In humans, supplementation of AA for several months did not lower fasting plasma glucose.3,4 We studied 139 consecutive consenting non-diabetic patients in an oncology clinic. The patients had been encouraged as part of their treatment to supplement AA.
Self-reported daily intake varied from 0 to 20 g/day. The plasma AA levels ranged from 11.4 to 517 µmol/L and correlated well with the reported intake. Regression analysis of their GHb and plasma AA values showed a statistically significant inverse association (
eg, each 30 µmol/L increase in plasma AA concentration resulted in a decrease of 0.1 in GHb).
A 1 g oral dose of AA can raise plasma AA to 130 µmol/L within an hour and such doses at intervals of about two hours throughout the day can maintain ~230 µmol AA/L.5 Similar levels could also be achieved by use of sustained-release AA tablets. This AA concentration would induce an approximate 0.7 depression in GHb. The GHb assay used in our study, affinity chromatography, is not affected by the presence of AA.3 Thus, unlike the case with Hb D Punjab, our results were not caused by analytical method artifact.
More likely, the decreased GHb associated with AA supplementation appears related to an in vivo inhibition of glycation by the elevated plasma AA levels, and not a decrease in average plasma glucose.3
If this is true, the effect has implications not only for interpretation of GHb but also for human ageing, in which glycation of proteins plays a prominent role in age-related degenerative changes.