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Dear Dr. Visek:
We should like to respond to the paper recently published by Rauma et al. (1995). We agree with the authors' conclusion, based on their data, that further study of the possible health risks of vegan diets is warranted. However, the sole use of plasma vitamin B-12 levels and mean corpuscular volume (MCV) as indicators of vitamin B-12 status is questionable, and the suggestion that eating seaweeds would prevent vitamin B-12 deficiency may, unfortunately, mislead many readers.
Firstly, it is not considered that, at an individual level, blood cell indices such as mean corpuscular volume (MCV) are rather insensitive indicators of vitamin B-12 deficiency. It has been observed that MCV becomes abnormal for an individual long before it exceeds the normal reference range of the laboratory (Herbert 1987 In fact, Rauma et al.'s observation that serum vitamin B-12 concentrations in vegans were not associated with MCV also questions the bioactivity of vitamin B-12 as measured in the plasma of these subjects. It is likely that the reported plasma vitamin B-12 levels (obtained by competitive protein binding; CPB) included vitamin B-12 analogs from plant foods, similar to our study in macrobiotic children (Dagnelie et al. 1991 In conclusion, the available evidence indicates that the alleged vitamin B-12 present in algae is not bioactive in humans. We are concerned that Rauma et al.'s paper could stimulate the popular vegetarian thought that vegan diets are safe as long as seaweeds are used, and stimulate the consumption of seaweeds in potentially harmful amounts. We are certain that this is not what the authors had in mind when they wrote their paper.
). In fact, the normal range of MCV is extremely wide, therefore the observation that in this small number of subjects (n = 9) MCV was "not significantly different" from omnivorous persons bears little meaning. Unfortunately, the authors do not present means ±SD of blood indices in vegans and controls. We have previously shown that MCV in children fed macrobiotic diets was higher than in omnivorous children (84.2 vs. 80.9 fL; P = 0.002) and the fact that distribution curves for MCV did not overlap between the two populations suggested consistent vitamin B-12 deficiency in the large majority of macrobiotic children despite MCV being within the normal range (Dagnelie et al. 1989 and 1994). It is unfortunate that the authors did not consider plasma methylmalonic acid and/or homocysteine concentrations which are now known to be far more sensitive indicators of vitamin B-12 deficiency than is MCV (Schneede et al. 1994). Secondly, this paper completely disregards the issue of bioavailability and bioactivity of vitamin B-12 from plant foods, which has been addressed by several authors quoted by Rauma et al. (Herbert 1987, Van den Berg et al. 1988). The classic Lactobacillus leichmannii assay includes not only true vitamin B-12 but also noncobalamin vitamin B-12 analogues that are unavailable to humans and may even block vitamin B-12 metabolism (Herbert 1987). Therefore, without information on the assay used, the quoted high vitamin B-12 content of chlorella, which was supplied to the authors by the Japan Dairy Technical Association, should be questioned. Moreover, we now know that even the modern competitive protein binding (CPB) assay for vitamin B-12 is likely not to be specific for true vitamin B-12. We regret that the authors, when taking figures on alleged vitamin B-12 content of nori from an earlier publication of ours (Van den Berg et al. 1988; based on CPB assay) have disregarded the important addition, in the same publication, that "the inclusion of nori and/or spirulina in these (i.e., in vitamin B-12 deficient macrobiotic) children's diets resulted in a further deterioration in mean cell volume despite increasing plasma vitamin B-12 levels." This observation was published in more detail, including a discussion of the pitfalls of vitamin B-12 in plant foods, in a more recent paper (Dagnelie et al. 1991) which is not cited by Rauma et al. In this paper, we concluded that high amounts of nori may actually block vitamin B-12 metabolism, in a similar fashion as described by Herbert for spirulina (1987).
). This implies that the use of plasma vitamin B-12 levels as an indicator of vitamin B-12 status may be misleading in vegetarian populations.
Pieter C. Dagnelie
Institute of Internal Medicine II
Erasmus University of Rotterdam
P. O. Box 1738, 3000 DR Rotterdam
The Netherlands
FOOTNOTES
Manuscript received 9 July 1996. Revision accepted 12 September 1996.
LITERATURE CITED
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