In this prospective study, dietary intake of animal protein was positively, and plant protein was negatively associated with the risk of occurrence of menarche after adjusting for all potential confounders. Considering protein-containing foods, higher intakes of poultry and low-fat dairy increased odds of menarche occurrence after adjusting for age, BMI, energy intake, early maternal menarche, but the associations were marginally significant after the inclusion of maternal education. The odds of early menarche decreased significantly with higher intakes of plant protein; neither animal protein nor protein-containing foods were significantly associated with odds of early menarche.
Similar to our findings, previous studies concerning the association between different protein intake sources and menarche consistently reported that girls attain menarche earlier by consuming more animal protein during childhood; however, the timing in which animal protein intake may influence menarche is controversial. In the US girls, animal protein during the entire childhood period from ages of 3–5 year, 6–8 years, and two years before peak growth velocity were suggested as a predictor of AAM , while in girls living in South-West England animal protein at age 3 and 7 but not 10 years were associated with earlier menarche . In contrast, in the German girls, animal protein intakes at age 5–6 years but not at ages 3–4 years were associated with early menarche . Plant protein intake at the age of 3–5 years was suggested as a predictor of menarche in the US girls, but no significant association was indicated between plant protein intake after the age of 5 years  while in the German girls, plant protein intakes at the age of 3–6 years were associated with later age at menarche . Our findings showed that dietary intakes of plant protein at ages between 6 to 14 years were related to menarche. Interestingly, the percent of animal and plant protein from total energy intakes did not differ across the age groups from early to late childhood, according to Berkey et al. and Gunther et al. studies [11, 12]. In our study, animal protein was about 7% of energy intake, which was lower than that in the US girls (about 9% energy intake) and in the German girls (about 8% of the energy intake) [11, 12]. Besides, the plant protein was 5.6% of energy intakes, which was higher compared to the US girls (3.78% of energy) and the German girls (4.3% of energy).
Regarding evidence on dairy intake and AAM, two studies conducted in the US girls [14, 16]; one of which suggested a higher risk of early menarche in girls with higher intakes of milk at the age of 9–12 years , while the other one showed the later attaining menarche in girls with higher frequency intakes of total milk and low-fat milk at ages between 9 to 14 years . In Chilean girls, also higher intakes of low-fat dairy, low-fat milk, and yogurt were associated with later AAM . We have previously reported a higher odds of early menarche in girls who consumed more milk at the age of 4–12 years . However, the other studies could not find any significant association between dairy and menarche [13, 17] .
In our study, the intake of fast food and red meat was not associated with AAM. Contrary to our findings, Jansen et al. reported that red meat intake frequency was inversely associated with AAM . Similarly, in the prospective investigation in the South-West England girls, meat intakes at both 7 years and 3 years were strongly positively associated with the occurrence of menarche . Consistent with our results, Carwile et al. could not find any significant association between girls' red-meat intake at the age of 9–14 years and AAM .
Other protein-containing foods such as poultry, fish, egg, and legumes have been less studied in relation to the AAM. In our study, poultry intake was inversely associated with AAM, although Jansen et al. could not show any relation between this food group and menarche among Colombian girls . Fish intake has controversial findings associated with the risk of menarche [13, 17]; we could not find any association between fish intake and AAM. Limited studies have been conducted to investigate eggs and legumes intakes in relation to AAM, which provided non-significant results [13, 17].
The previous studies were more interested in investigating the associations of dietary intakes of protein during early to mid-childhood rather than late-childhood, which is close to the age of menarche, perhaps due to the possible effects of puberty dietary intakes during the late-childhood periods . Girls enter puberty between the ages of 8 and 13 . The age of girls in our study ranged between 6 and 14 years; one-third of them were ≤ 8 years (mid-childhood), reflecting that most of them had entered puberty. Regardless of the possible effect of puberty on dietary intakes, habitual intakes of children may not be stable from early to late childhood. Therefore it makes it difficult to rely on a one-time point of dietary assessment for investigation on menarche. We also found considerable variability among studies in terms of dietary assessment tools, definitions of each food group, it's unit (gram, serving, gram/1000 kcal, etc.), and frequency (continuous, one serving/day, one serving/week, etc.). Besides, the substantial differences in the food products and the preferred food items consumption across the populations and food preparation methods render the evidence challenging to interpret collectively.
Prospective design and investigating different protein-containing foods are among the strengths of the study. However, our study does have its limitations; first, a small sample size reduces our ability to conduct subgroup analysis based on participants' baseline age. Second, because most of our participants entered puberty, the potential effects of each stage of puberty on dietary intakes could not be ruled out.