3.1 Comparison of the general characteristics between the case group and the control group
The average age of girls in the case group and the control group was 7.84 ± 1.05 and 7.71 ± 1.48 years (P>0.05), respectively (Table 1). Despite a higher weight and height of girls in the case group than in the control group, there was no statistical difference in BMI between the two groups (P=0.077). The rate of sleeping with light exposure, use of adult toiletries, and percentage of participants claiming heavy homework burden was significantly higher in the case group than that in the control group (P<0.05). In addition, the percentage of participants having more meat and fewer vegetables was higher in the case group than in the control group. There were no significant differences in other characteristics between the two groups (Table 1).
3.2 The dietary pattern of girls in the case and the control groups
In this study, the vegetable intake of girls in the case group (163.99±122.61 g/day) was significantly lower than that of the control group (217.07±147.54 g/day) (P<0.05). There were no significant differences in the intakes of red meat and poultry, fish and shrimp, milk, eggs, and soy products between the two groups as shown in Table 2.
Recommendations on "Student Meal Nutrition Guidelines" were issued by China's Health and Family Planning Commission in 2017 (18). The dietary intake of children in the case and the control groups were compared to the recommended intake for girls aged 5-12. In both groups, the intakes of poultry and red meat were higher than the recommended intake (P<0.05) while the intakes of vegetables and soy products were lower than the recommended dietary intake (P<0.05). In addition, in the case group only the consumption of eggs was lower than the recommended amount (P<0.05).
3.3 Frequency of dietary intake and precocious puberty risk
Conditional logistic regression models were used to explore the effects of frequency of dietary intake on girls with precocious puberty (Table 3).
In model 3, after multivariate adjustment, higher fruit intake frequency was associated with lower risk of precocious puberty. Specifically, the adjusted ORs (95% CI) for the risk of precocious puberty in fruit intake of 7-10 times/week group and 10 times/week group were 0.36 (0.14, 0.91) and 0.21 (0.05, 0.94), respectively compared to the <7times/week group (P for trend = 0.024). There was no association between the frequency of red meat, poultry, fish, shrimp, soy products, milk, vegetables, and egg intakes and risk of precocious puberty after adjusted for potential confounders.
3.4 Amount of dietary intake and precocious puberty risk
Conditional logistic regression models were used to explore the amount of dietary intake on precocious puberty (Table 4).
In model 3, after multivariate adjustment, a higher amount of red meat intake was associated with an increased risk of precocious puberty (P for trend = 0.012). Specifically, compared with consuming <25 g/day red meat, the adjusted ORs (CI) for the risk of precocious puberty was 1.95 (0.87, 4.39), and 2.74 (1.25, 6.02), respectively when consuming 25-50 g/day and 50 g/day red meat.
Also in model 3, a higher amount of vegetables and fruits intake was inversely associated with precocious puberty risk (vegetables: P for trend = 0.002; fruits: P for trend = 0.033). Compared with consuming <112.5 g/day vegetables, the adjusted ORs (CI) for the risk of precocious puberty were 0.31 (0.12, 0.77), and 0.25 (0.11, 0.59), respectively when consuming 112.5-240.0g/day and 240.0 g/day vegetables. Furthermore, when comparing the amount of fruit intake 200 g/day with <150 g/day, the adjusted OR (95% CI) was 0.53 (0.26, 1.09). Notably, the amount of fish, shrimp, soy products, milk, and eggs intake had no significant effect on the risk of precocious puberty, after multivariate adjustment.