Three measures of compliance in a soy intervention in premenopausal women

As part of a randomized dietary intervention study that examines the association of soyfood intake with breast cancer risk, we are evaluating the feasibility of adherence to a soy-based diet in premenopausal women (n = 213, aged 34-46 y). The study design includes an increase of dietary soy intake to two servings per day (50 mg isoflavones/d) for 2 y in the intervention group. Compliance with this intervention is measured by self-reported soy intake logs, repeated randomly timed 24-h recalls, and analysis of urinary isoflavone excretion. We used HPLC with photo diode array detection to determine isoflavone concentration in foods and urinary isoflavone excretion in overnight urine samples at baseline and 2, 6, and 12 mo. At baseline the two groups did not differ in dietary soy intake. Women in the intervention group chose from six different soyfoods provided at no cost. Intake logs show a strong preference for soymilk (38%) and tofu (31%) over soy nuts (16%), soy protein bars (19%), and soy protein powder (4%). Food logs showed that participants in the intervention group consume soyfoods, on average, twice per day and at least one serving on 68 of 70 d. According to the 24-h recalls, the estimated isoflavone intake is 9 times higher in the intervention group than in the control group. Increases in isoflavone intake shown by comparisons of 24-h recalls with baseline food frequency questionnaires are sevenfold within the intervention group and slight in the control group. Urinary isoflavone excretion results showed no difference between groups at baseline, no change in the control group for followup samples (n = 242), and a statistically significant fivefold increase for women in the intervention group (n = 243). These data indicate a very high level of compliance with the study regimen at this stage.