Reply to B Teucher and SJ Fairweather-Tait

Creighton University Medical Center Osteoporosis Research Center 601 North 30th Street, Suite 4841 Omaha, NE 68131 E-mail: rheaney{at}creighton.edu

Dear Sir:

We thank Teucher and Fairweather-Tait for the opportunity to clarify a terminologic issue and at the same time to make a point that we perhaps did not emphasize sufficiently in our article (1). Teucher and Fairweather-Tait are, of course, correct in stressing the importance of uniform distribution of tracer through all of the native ionic species. Otherwise, isotope movement could not be used validly as a quantitative marker, or tracer, for the native species, whether calcium, iron, or any other mineral or compound.

Moreover, we agree that the distinction between intrinsic and extrinsic labeling lies in whether the tracer is incorporated into the various native calcium species during their own synthesis, or whether it is added after the fact. We believe that, thus far, we are of one mind with Teucher and Fairweather-Tait. However, for use of the term intrinsic, they confine the notion of synthesis to biosynthesis, which, in the case of a calcium-enriched food, is not relevant. The article by Weaver (2) to which Teucher and Fairweather-Tait refer related not to engineered foods but to edible plants, in which synthesis and biosynthesis are one and the same.

Heaney et al (3) reported previously in the Journal on the bioavailability of soybean calcium, intrinsically labeled in exactly the sense preferred by Teucher and Fairweather-Tait. However, such intrinsic labeling of the plant calcium would not be useful in the case of fortified soy "milk," because virtually none of the native bean calcium ends up in the milk. Soy milk, as it is commercially prepared, is a deficient food—that is why it is fortified. Consumers and the nutritional community need to know the bioavailability of the calcium in the end product, as consumed. The vast bulk of the calcium in fortified soy milk comes from the fortificant, and it is the fortificant that we have shown must be intrinsically labeled. The fact that the fortificant is extrinsic to the soy milk is beside the point. Moreover, the bioavailability of the fortificant must be tested, not in isolation but in the final food matrix. In our article, we used the terms intrinsic and extrinsic to refer to the labeling of the fortificant. We are sorry if we failed to make that point sufficiently clearly.

REFERENCES

1. Heaney RP, Dowell MS, Rafferty K, Bierman J. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. Am J Clin Nutr 2000;71:1166–9.
2. Weaver CM. Intrinsic mineral labeling of edible plants: methods and uses. CRC Crit Rev Food Sci Nutr 1985;23:75–101.
3. Heaney RP, Weaver CM, Fitzsimmons ML. Soybean phytate content: effect on calcium absorption. Am J Clin Nutr 1991;53:745–7.