Plasma levels of free daidzein and genistein are not affected by a high soy milk diet.

RM Harris PhD^a, T Pettitt PhD^b, DM Wood BSc^a, L Bottomley BSc^a, S Ayres MB ChB^c, D Rea MD^c, CJ Kirk PhD^a, RH Waring DSc^a

^aSchool of Biosciences, The University of Birmingham, Edgbaston, Birmingham, United Kingdom, ^bC.R.U.K, Institute for Cancer Studies, The University of Birmingham, Edgbaston, Birmingham, United Kingdom, ^cCity Hospital NHS Trust, Birmingham, United Kingdom

AIM: Soy isoflavonoids are widely described anecdotally as being beneficial in cases of breast cancer. Isoflavonoids are absorbed from the gut in the free-form and then conjugated with either sulphate or glucuronic acid in the body This pilot study evaluated the effects of a soy milk supplemented diet on plasma and urinary daidzein (D) and genistein (G) in women from the general population and those considered to be at risk of breast cancer. METHODS: “At risk” women were recruited from the breast cancer monitoring clinic at City Hospital while “normal” age-matched controls were volunteers from the local population with no family history of breast cancer. During the first week of the trial, the women were asked to avoid all soy-containing foods and in the second week to drink 500ml soy milk daily. Blood and urine samples were collected at the end of each week. Free isoflavonoids (untreated samples or those released by enzymatic hydrolysis) were extracted by 1:1 v/v chloroform:propan-2-ol, evaporated to dryness and redissolved in the hplc mobile phase (45% acetonitrile/54.75% water/0.25% glacial acetic acid). The samples were analysed on a Shimadzu QP8000Alpha quadropole mass spectrometer linked to a Shimadzu 10ADVP binary HPLC using triple ¹³C labelled daidzein and genistein as internal standards. RESULTS There was little evidence to suggest a difference between the normal (N) and high-risk (HR) women. There was no increase in free daidzein and genistein in plasma, but there were large increases in the levels of both sulphate (D sulphate (mean ± s.d.): N before soy 0.015 ± 0.03µM, after soy 0.25 ± 0.16µM; HR before soy 0.01 ± 0.03µM, after soy 0.16 ± 0.12µM. G sulphate: N before soy 0.02 ± 0.05µM, after soy 0.41 ± 0.36µM; HR before soy 0 ± 0.07µM, after soy 0.20 ± 0.39µM) and glucuronide (D glucuronide: N before 0 ± 0.02µM, after 0.13 ± 0.09µM; HR before 0.01 ± 0.02µM, after 0.09 ± 0.07µM. G: N before 0.03 ± 0.10µM, after 0.75 ± 0.50µM; HR before 0.02 ± 0.05µM, after 0.70 ± 0.77µM) conjugates. However, in urine, increases were seen in both free (D: N before 0.27 ± 0.32µM, after 1.2 ± 2.9µM; HR before 0.18 ± 0.26µM, after 2.0 ± 3.1µM. G: N before 0.05 ± 0.22µM, after 1.4 ± 4.8µM; HR before 0.03 ± 0.29µM, after 1.5 ± 2.2µM) and conjugated isoflavonoids (Sulphates, D: N before 0.26 ± 0.46µM, after 8.5 ± 20µM; HR before 0.25 ± 0.99µM, after 3.3 ± 4.3µM. G: N before 0.11 ± 0.18µM, after 1.6 ± 2.5µM; HR before 0.12 ± 0.27µM, after 6.6 ± 12µM. Glucuronides, D: N before 0.24 ± 0.48µM, after 18 ± 18µM; HR before 0.82 ± 1.4µM, after 12 ± 11µM. G: N before 0.05 ± 0.46µM, after 11 ± 13µM; HR before 0.27 ± 0.55µM, after 14 ± 19µM.). CONCLUSIONS: A moderately high short-term increase in dietary soy does not appear to affect circulating levels of free daidzein and genistein in either controls or women at high risk of developing breast cancer. If soy does have a beneficial effect on breast cancer, it may be acting via a conjugate or other metabolite.

Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in poster session 793 (Dietary Influences).