Soy Supplement: Why Is the Effect So Elusive?

Paola Albertazzi

Centre for Metabolic Bone Disease, Hull Royal Infirmary Hull, HU3 2RW, United Kingdom

Address correspondence to: Paola Albertazzi, M.D., Centre for Metabolic Bone Disease, HS Brocklehurst Building, Hull Royal Infirmary, 220-236 Anlaby Road, Hull, HU3 2RW, United Kingdom. E-mail: . P.Albertazzi{at}Hull.ac.uk

To the editor:

Dewell et al. (1) in their article did not find any effects with 150 mg of soy-derived isolated isoflavones. Their results appear in contradiction with the bulk of data that have shown a beneficial effect of soy on lipid parameters, although these results were mostly obtained with soy preparations containing isoflavones rather than isoflavones alone in tablet form.

One of the possible explanations, as the authors correctly pointed out, is the fact that other components of soy, rather than the isoflavones alone, may be required for the effect of soy on cholesterol. There is also a second possibility. Once nutrients are isolated in tablet form, they become in all effects a pharmacological preparation. Tableting compounds may drastically influence their bioavailability, and hence, influence clinical effects.

A lot of resources are invested in testing bioavailability, stability, and absorbability of drugs, but supplements are not usually submitted to such rigorous testing. This is usually because of the lack of adequate resources. Thus, it is conceivable that if a compound is present but is not absorbed, it will not be clinically effective. Hence, the apparent contradictory effect is observed in clinical studies with supplements (2). Dewell et al. (1) correctly state the amount of isoflavones present in both the glycone and aglycone form but fail to give an indication whether any of these compounds were actually absorbed and in what levels.

Bioequivalence is the effect of different pharmaceutical formulations of identical compounds. An example may be two preparations of calcium chemically identical but pharmaceutically different. They may not be bioequivalent precisely because they may not have identical bioavailability. The source factors include associated ligand antiabsorbent, the pharmaceutical formulation that includes a broad range of physical and chemical factors involving the granulation of excipient; the coating, the hardness, and disintegration potential of the final formulation; and finally the size of the ingested load (3).

There are important factors to keep in mind when both performing clinical trials and advising patients on the use of these substances. Too often, results from food studies are used to support the marketing of isolated nutrients in tablet form. This is clearly unacceptable. Given the variability of food and the lack of standardization, results of clinical trials refer only to the compound tested and cannot be extrapolated even to preparations that are apparently similar, such as tablets containing isolated nutrients.

Above all, when using isolated nutrients such as isoflavones in tablet form, it is important to assess their bioavailability first, before attempting to assess clinical efficacy. A lot of these phytoestrogen preparations may in fact just be very expensive pellets that once ingested are, literally speaking, going down the drain! This lack of rigor will, if allowed to persist, ultimately undermine the credibility of the whole field of phytoestrogens among both scientists and consumers alike, thus dooming compounds that could potentially have extremely attractive health giving properties.

Received March 12, 2002.

References

  1. Dewell A, Hollenbeck CB, Bruce B 2002 The effects of soy-derived phytoestrogens on serum lipids and lipoproteins in moderately hypercholesterolemic postmenopausal women. J Clin Endocrinol Metab 87:118–121[Abstract/Free Full Text]
  2. Albertaizz P, Purdie DW The nature and utility of phytoestrogens: a critical review of the evidence? An unconventional approach to postmenopausal health. Maturitas, in press
  3. Heaney RP 2001 Factors influencing the measurement of bioavailability, taking calcium as a model. J Nutr 131:1344S–1348S