shows some demographic and dietary characteristics first for the subjects who provided two serum samples (Group1). As can be seen there was a fairly even gender split, and the average age was 65 years. Nearly 60% were vegetarian though about a quarter of these eat some fish. Soy protein intake was high, on average nearly at oriental levels, although 12 subjects were essentially non-users with average intakes <1 gram per day in the six 24 hour recalls. Serum levels of isoflavones reflected this relatively high average intake. Twelve subjects from the 22 assessed, reported eating soy-containing foods in the day before the blood draw, six at breakfast, seven at lunch, and five at dinner.
Selected characteristics of the 28 study subjects (proportions or means[SD]).
Descriptive data from Group 2, who provided urine samples, is generally very similar, except that there are fewer males (p=0.13) and somewhat more non-vegetarians (p=0.43). As neither soy intake or isoflavone excretion varied significantly by gender this difference is unlikely to bias the comparison between groups.
The adjusted intra-class correlation (95% confidence interval) for logarithm of serum isoflavones (genistein plus daidzein) is 0.201 ( 0.0 – 0.452 ); for log(serum daidzein) it was 0.112 (0–0.520); for log(serum genistein) it was 0.282 (0–0.68). Regression results where log(morning serum isoflavone levels) are predicted by soy intake the previous day (meal by meal) are shown in . One would expect b3 to be largest, as it reflects the effect of soy intake at supper the evening before the blood draw, and is thus closest in time. This is the case, and the smaller anticipated effects of soy intake at breakfast and lunch could not be demonstrated in this small sample. Intake the previous day explains about 25% of the variance in log(serum isoflavone) values.
Linear Regressions of Log(morning serum isoflavone levels) on soy intake† at breakfast, lunch and supper the previous day.
Our estimate of the correlation between log(urinary genistein) and the mean soy protein intake (corrected for with-in person dietary reporting errors), Corr(U,μr), is 0.50. The same statistic for daidzein is 0.46. Thus reports a sensitivity analysis for different proposed values of Corr(μu,μr) (that equal or exceed 0.50). The estimated ICC for urinary isoflavones lies between 0.56 and 0.83. Given the vagaries of absorption and metabolism of isoflavones, it seems improbable that Corr(μu, μr) exceeds 0.90, and this means that our best estimate from this data is that ICC of a urinary estimate exceeds 0.56. It should be pointed out that as the recalls and the urine were not separated by more than 6 months that the ICC for urinary values is over a shorter time period than that evaluated above for the serum.
Estimated intra-class correlation (ICCu) of urinary soy isoflavones (daidzein + genistein): A sensitivity analysis*