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Letters to the Editor

Reply to Russell et al.

Division of Human Nutrition Wageningen University Wageningen, The Netherlands

Dear Editor,

We admire scientists such as Dr. Russell and his colleagues who take on the daunting task of trying to establish recommendations on dietary intake and related matters such as the extent of conversion of food components to nutrients. The knowledge base to be evaluated is generally very poor and it is often difficult to make decisions on which data deserve the most credence. Thus it is not surprising that they take issue with our critique (1) of their report with respect to the extent of conversion to retinol of ß-carotene dissolved in oil (2).

Russell et al. place great reliance on the data of Sauberlich et al. (3) but we think that this is unjustified. They reject Wagner’s data (4) because of the lack of detail and standardization even though these issues are addressed, in contrast to Sauberlich et al. (3) who provide no information whatsoever on the methods used. They also state that the design of Wagner’s study was flawed but provide no arguments to support this view. Data from Booher et al. (5) were rejected because the period of 16–39 d in four of the five subjects was regarded as too short for loss of dark adaptation (in the fifth subject this required 124 d). However, the five subjects were depleted for a further period of 5–16 wk. With respect to the study by Hume & Krebs (6), it is interesting to note that the Institute of Medicine publication (IOM) (2) and the website (http://books.nap.edu/books/0309072794/html) accessed on 4 June 2003 report a retinol equivalency ratio of 3.8:1. When we accessed the IOM website on 29 January 2001, the ratio was 2:1. At that time, the committee was prepared to accept the Hume and Krebs’ data. Also when accessed on 29 January 2001, the IOM report quoted work from Russell’s group (7) saying that a value of 2.6:1 was obtained from one man and one woman while the publication quoted (7) presented values of 3.8:1 and 55:1 obtained from one woman. This paper is no longer referred to in the IOM report. The IOM now bases its findings entirely on the Sauberlich study (3) because of the purported larger number of subjects (n = 6 excluding two subjects who were repleted with high doses of retinol or ß-carotene). However, only two subjects received ß-carotene. Of the four subjects who received retinol, data from just one subject provide a valid value of 150 µg. Reversal of abnormal dark adaptation was observed with 150 µg/d in an additional two subjects but no lower dose was provided so it is conceivable that a lower dose would have also been effective. In a fourth subject, dark abnormal adaptation was reversed with 75 µg/d. Thus the evidence that 150 µg of retinol/d would be adequate is not as strong as that for 75 µg of retinol/d, which would indicate that 4 µg of ß-carotene in oil would have the same activity as 1 µg of retinol (conversion factor, 4:1).

Therefore, on the basis of the four studies that measured reversal of abnormal dark adaptation, we maintain our position that the conversion factor is higher than 2:1. We agree with Russell and colleagues that data from well-controlled studies using stable isotopes should be used. Based on two such studies involving 111 children in Indonesia, in contrast to relying on data from just one person in Sauberlich et al. ’s study (3), we found that 2.6 µg of ß-carotene in oil provides 1 µg of retinol (8). Taken with the 14% bioavailability that we observed for ß-carotene from mixed fruit and vegetables compared with that of ß-carotene in oil (9), a value of 19:1 is obtained which is very similar to the ratio of 21:1 that we reported for ß-carotene from mixed fruits and vegetables from our studies in Indonesia and Vietnam (1, 8). Thus there appears to be no doubt that >12 µg of ß-carotene in mixed vegetables and fruits, as suggested by the IOM (2), is required to produce 1 µg of retinol.

Manuscript received 13 June 2003.

LITERATURE CITED

1. West, C. E., Eilander, A. & van Lieshout, M. (2002) Consequences of revised estimates of carotenoid bioefficacy for the dietary control of vitamin A deficiency in developing countries. J. Nutr. 132:2920S-2926S.[Abstract/Free Full Text]

2. U. S. Institute of Medicine, Food and Nutrition Board, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes (2001) Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc 2001 National Academy Press Washington, DC.

3. Sauberlich, H. E., Hodges, R. E., Wallace, D. L., Kolder, H., Canham, J. E., Hood, J., Raica, W., Jr & Lowry, L. K. (1974) Vitamin A metabolism and requirements in the human studied with the use of labeled retinol. Vitam. Horm. 32:251-275.[Medline]

4. Wagner, K. H. (1940) Die experimentelle Avitaminose A beim Menschen. Ztschrf. Physiol. Chem. 264:153-188.

5. Booher, L. E., Calliston, E. C. & Hewston, E. M. (1939) An experimental determination of the minimum vitamin A requirements of normal adults. J. Nutr. 17:317-331.

6. Hume, E. M. Krebs, H. A. eds. Vitamin A requirements of human adults: an experimental study of vitamin A deprivation in man. Medical Research Council Special Report No. 264 1949 HMSO London, UK. .

7. Tang, G., Qin, J., Dolnikowski, G. G. & Russell, R. M. (2000) Vitamin A equivalence of ß-carotene in a women as determined by a stable isotope reference method. Eur. J. Nutr. 39:7-11.[Medline]

8. Van Lieshout, M., West, C. E. & van Breemen, R. B. (2003) Isotopic tracer techniques for studying the bioavailability and bioefficacy of dietary carotenoids, particularly ß-carotene, in humans: a review. Am. J. Clin. Nutr. 77:12-28.[Abstract/Free Full Text]

9. Van het Hof, K. H., Brouwer, I. A., West, C. E., Haddeman, E., Steegers-Theunissen, R.P.M., van Dusseldorp, M., Weststrate, J. A., Eskes, T.K.A.B. & Hautvast, J.G.A.J. (1999) Bioavailability of lutein from vegetables is 5 times higher than that of ß-carotene. Am. J. Clin. Nutr. 70:261-268.[Abstract/Free Full Text]

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