Some say that early puberty in girls may be caused by what we eat.
Is there evidence of that?
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TL;DR: This article has poorly constructed arguments. However, it does serve to provide references to support some association (not causation) between diet and early puberty.
First, let me summarise the key arguments in the linked article, just with regards to diet and precocious puberty, which is what the OP is asking about. (That is, I am ignoring claims about cancer, breastfeeding, mental health, etc.):
One of two mentioned factors in precocious puberty is consumption of cow's milk (no reference provided).
Cow's milk has a high fat content, and other bad things.
These are all linked to early puberty.
Hispanic child are less likely to consume cow's milk. (no reference provided). This argument peters out there. I see no actual claim that Hispanic children have later puberty as a result.
So, we can ignore statement 4. There is no reference for Statement 1 (or 4). I'm not going to argue Statement 2. The critical statement is statement number 3, and three references are provided. (Ref numbers #13, #14, and #15 in the article.) Let's look at those to see if they support the claim.
13: Stoll BA. Western diet, early puberty and breast cancer risk. Breast Cancer Res Treat (England) Jun 1998;49(3):187-93.
This paper is primarily about the increased breast cancer risk associated with early puberty. However, it does briefly summarise the purported causes of early puberty.
Improving nutrition is generally assumed to be responsible for the secular trend to ever-earlier onset of menarche in Western industrialised countries.
Nutritional factors are likely to be responsible and current option is that menarche is provoked by a critical accumulation of adipose tissue rather than through the effects of specific macronutrients . Studies in the USA and Canada have found that earlier onset of menarche is associated with a greater height and weight for age and that after allowing for these variables, there was no association with energy-adjusted intake of fat, protein or carbohydrate in the diet [23, 24]. Nevertheless one prospective study in Germany concluded that a higher energy-adjusted fat intake was associated with accelerated menarche and might be responsible for earlier body fatness .
14: Outwater JL, et al. Dairy products and breast cancer: the IGF-I, estrogen and bGH hypothesis. Med Hypotheses Jun 1997;48(6):453-61.
There is not one but three problems with this paper.
First, it looks at breast cancer, not early puberty. It is off-topic for our discussion, but on-topic for the general article.
Second, it doesn't conclude anything useful for the article
In conclusion, despite several intriguing hypotheses linking dairy product consumption and breast cancer, the available epidemiologic evidence does not support a strong association between the consumption of milk or other dairy products and breast cancer risk.
Third, it is published in Med Hypotheses, which is a great place to get ideas for future experiments, but not a great place to link to to show that the evidence supports your argument. [More info]
15: Kato I, et al. Factors related to late menopause and early menarche as risk factors for breast cancer. Jpn J Cancer Res (Japan) Feb 1998;79(2):165-72.
This paper was focussed on breast cancer, but relied on surveys of 17,000 Japanese women older than 40 to gauge some of the factors involved in both puberty and menopause.
Note the weakness is asking someone about what happened thirty years earlier, and a dubious assumption that eating habits varied in strength but not in character over the subjects lives. I've not reported the borderline associations (e.g. p>0.05).
Positively correlated factors for early menarche included:
Inversely correlated were:
Adipose fat levels are believed to be a trigger for menarche, although none of the cited articles show that directly, and there is some mixed evidence.
A survey (of moderately weak power) suggests correlations (not causation) between a number of dietary elements and early menarche.
The linked article makes some dubious claims.