Comment on the use of the term intrinsic labeling

Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, Norfolk, United Kingdom, E-mail: sue.fairweather-tait{at}bbsrc.ac.uk

Dear Sir:

Heaney et al (1) discussed problems encountered with isotopic labeling in their recent article, in which they compared the calcium bioavailability of fortified soy milk with that of cow milk. They added ⁴⁵CaCl₂ to cow milk and soy milk and left it for 14 h to equilibrate with the native calcium. This technique is referred to as extrinsic labeling and was first applied to study iron absorption from different foods (2). It was shown previously that added isotope with native calcium in milk exchanges fully (3), but this is not true for all foods. When there is uncertainty about the validity of extrinsic labeling, the isotope must be incorporated into the food in the same form as the native calcium, otherwise referred to as intrinsic labeling.

The soy milk tested by Heaney et al was fortified with tricalcium phosphate (TCP), and this was labeled by the addition of ⁴⁵CaCl₂ to a suspension of calcium hydroxide in water and precipitation of TCP with phosphoric acid. The [⁴⁵Ca]TCP was then used to replace unlabeled TCP in unfortified soy milk so that calcium absorption could be measured. Throughout the article the authors referred to the soy milk containing ⁴⁵Ca TCP as intrinsically labeled soy milk.

According to Weaver (4), "intrinsic mineral labeling of plants involves the biological incorporation of a form of the mineral of interest that can be distinguished analytically from the natural form of the element. The label may be substituted in tracer amounts or as a complete replacement of the naturally-occurring element .... The underlying assumption for use of intrinsic labeling techniques is that the label is deposited in the same manner and associated with the same constituents as occurs naturally."

It is generally accepted that the terms intrinsic and biosynthetic are interchangeable with respect to isotopic labeling of plant foods, including soy milk. The term intrinsic is associated with native calcium in the foodstuff, not added calcium salts. Thus, using the term intrinsically labeled soy milk to describe soy milk to which [⁴⁵Ca]TCP has been added is confusing or even misleading. The results section of the abstract refers to percentage calcium absorption from "intrinsically labeled soy milk," but the authors then conclude that "intrinsic labeling of the fortificant is required for such liquid suspensions." The lack of precision in terminology does not help crystallize the authors' arguments on method. We hope that this article will not set a precedent for misuse of the term intrinsic for describing isotopic labeling of foods.

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. Bjorn-Rasmussen E, Hallberg L, Walker RB. Food iron absorption in man. I. Isotopic exchange between food iron and inorganic iron salt added to food: studies on maize, wheat, and eggs. Am J Clin Nutr 1972;25:317–23.
3. Nickel KP, Martin BR, Smith DL, Smith JB, Miller GD, Weaver CM. Calcium bioavailability from bovine milk and dairy products in premenopausal women using intrinsic and extrinsic labeling techniques. J Nutr 1996;126:1406–11.
4. Weaver CM. Intrinsic mineral labeling of edible plants: methods and uses. CRC Crit Rev Food Sci Nutr 1985;23:75–101.