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Letters to the Editor |
Department of Biochemistry, Biophysics and Molecular Biology 3252 Molecular Biology Building Iowa State University Ames, IA 50011 E-mail:jaolson{at}iastate.edu
Dear Sir:
After humans ingest a deuterated oral dose of vitamin A in oil, the absorbed deuterated vitamin A largely equilibrates with total body reserves of the endogenous vitamin in 16–25 d (1–5). During the equilibration period, deuterated vitamin A is being slowly catabolized in adults with a mean half-life of 
140 d (5, 6). Thus, the first-order rate constant k for the irreversible loss of vitamin A is 0.005/d, ie, k = 0.693/t1/2. In the formula devised to estimate the total body reserve of vitamin A (1), a factor, termed a, is multiplied by the other parameters of the equation to yield the total body reserve in millimoles. Factor a corrects for the turnover of deuterated vitamin A for the period between dosing and the drawing of the near-equilibrium blood sample.
The value of a can be determined in 2 ways: 1) by plotting graphically the log of the fraction of deuterated vitamin A remaining versus days, with 140 d plotted at 0.50 of deuterated vitamin A remaining, and 2) by calculating the value from the formula e-kt on a computer or calculator. The values of a at 20 and 40 d, determined by either procedure, are 0.905/d and 0.82/d, respectively. Because these procedures are straightforward, the reader may well inquire why I am writing this letter.
The reason is that a typographic error was published in 2 of the cited papers (1, 2): k is defined incorrectly as 1/140 d rather than as 0.693/140 d. This results in an incorrect k value of 0.0071/d rather than of 0.005/d. In both of these papers, happily, the total body reserves were calculated correctly, ie, by using a k value of 0.005. In the third cited study (3), a k value of 0.005 was also properly used but the formula for obtaining it was not given.
If the a values are obtained graphically, no harm will be done, inasmuch as the a values are determined without using a specific value of k. But if one uses the incorrect k value of 0.0071/d in the computation, the resultant a values at 20 and 40 d would be 0.87/d and 0.75/d, respectively. Thus, the total body reserves in adults would be underestimated by 4% or by 8% at 20 and 40 d, respectively. In view of the several assumptions made in calculating total body reserves, these differences are minor. Nonetheless, we all try not to introduce spurious factors into our studies.
I regret my own careless contribution to these misunderstandings.
REFERENCES
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