Re: Why has the age of menarche decreased?

Steven:

The onset of puberty is dependent upon the hypothalamus-pituitary-gonadal 
axis shifting its function to secrete hormones in the appropriate pattern.  
Each level must be competent to 1) do its own job & 2) respond to 
signalling from the other glands/organs.  For example, at the onset of 
puberty, the total amount of gonadotropin-releasing hormone secretion 
from the hypothalamus (an area in the brain) increases. This signals the 
pituitary gland (located just inferior to the hypothalamus) to secrete more 
luteinizing and follicle-stimulating hormone. These hormones signal the 
ovaries to increase their function so that steroids can be produced and 
eggs are prepared for ovulation.  The ovarian steroids must complete the 
cycle and communicate how things are going at the ovary to the hypothalamus 
and pituitary.  This cyclic relationship must be balanced in order for 
normal function to continue. 

The largest single reason for decreased age of menarche is improved 
nutrition.  As the level of nutrition increases, the bodies ability to 
store energy as fat increases.  It has been shown in ewe lambs that there 
is a target body weigth must be reached in order for estrous cycles to 
begin. If the lamb is held at lower weight, there will be a delay in onset.  
It can be inferred that this phenomenon may occur in humans as 
well.  

Improvements in disease prevention and control also contributes to the 
overall health of the human population, therefore, contributing to enhanced 
reproductive function.

The human population has become much more mobile in the past two centuries. 
This provides the opportunity for gene pool mixing and increasing the 
opportunity for increasing the variability within the gene pool. Thus, 
providing for the opportunity for enhanced biology.

I hope this helps.

David Mallory, PhD
Associate Professor
Department of Biological Sciences
Marshall University
Huntington, WV