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Results

Supplementation and age at menarche. Ages at exposure, potential years of exposure to supplements, age at interview and sample sizes of those who had reached menarche by 1992 are presented in Table 1.

TABLE 1 Exposure to supplementation (1969-1977) by year of birth for cases that had reached menarche by 1992 ¹

¹ Thirteen cases had not reached menarche by 1992; n = 806

The mean age at menarche for the entire sample is 13.71 ± 1.29 y (mean ± SD), and does not differ by supplement type; the values are 13.75 ± 1.22 y for Atole and 13.74 ± 1.36 y for Fresco villages (Table 2).

TABLE 2 Mean age at menarche by supplement types

¹ Thirteen cases had not reached menarche and are not included; n = 799.

² Difference = Atole - Fresco.T tests comparing differences at each birth year were all nonsignificant at P<0.05.

Age at menarche does, however, vary by year of birth (Fig 1). There is a decline in mean age at menarche over time for both groups; the overall rate of decline is ~0.69 y over the 15-y period as calculated by a regression of age at menarche on year of birth.

Determinants of age at menarche. Table 3 shows the results of two separate regression models: with age at menarche as the dependent variable and supplement type, year of birth, a second degree polynomial of year of birth, SES score and village size as covariates in both models. Model 2 includes these variables and interactive terms. In Model 1, supplement type is not a significant predictor of menarche but all other covariates are. Older subjects reached menarche later and higher SES was associated with earlier menarche. Village size (large = 1, small = 0) was associated positively with age at menarche.

TABLE 3 Determinants of age at menarche: multiple linear regression analyses

¹ 1 for Atole and 0 for Fresco supplements, and abbreviated as SUPPL.
² Coded as 1962 = 1, 1963 = 2, etc., and abbreviated as YRBIRTH.

³ Second degree polynomial term for year of birth.

⁴ Composite score based on characteristics of home and household possessions in 1975. Scores are a standardized variable with a mean of zero. SES = socioeconomic status.

⁵ 1 for large and 0 for small villages.

⁶ Interaction of year of birth and supplement type.

⁷ Interaction of SES and supplement type.
⁸ The degrees of freedom are fewer than the original number of subjects because of missing data for some of the independent variables.

In Model 2, significant two-way interactions between year of birth and supplement type and between SES score and supplement type are included. Other two-way interactions between year of birth and SES score and between village size and supplement type also were tested but were not found to be statistically significant. The interaction between year of birth and supplement type in Model 2 reflects the fact that for cases who were exposed to the supplements at older ages, menarche occurred earlier in Atole villages compared with Fresco villages. In cases exposed at younger ages, menarche was later in Atole compared with Fresco villages (see Fig. 1 and Table 2).

The positive interaction between SES score and supplement type suggests that, at high levels of SES, exposure to Atole was associated with later menarche; at lower levels of SES, on the other hand, exposure to Atole was associated with earlier menarche.

Results using survival analysis to incorporate cases who had not reached menarche (n = 13) were nearly identical to those in Table 3 and therefore are not presented.

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