Today in the U.S., about 16 percent of girls enter puberty by the age of 7, and about 30 percent by the age of 8. A recent study determined that the number of girls entering puberty (defined by breast development) at these early ages has increased markedly between 1997 and 2010.
Trends in Age at Menarche
The average age at menarche in Western countries began declining during the early part of the 20th century due to increased consumption of animal products and increasing calorie intake; the decline slowed in the 1960s, and now in the U.S. there has been a more recent surge in early puberty starting in the mid-1990s.
In Europe, in 1830, the average age at menarche was 17. Similarly in the 1980s in rural China, the average age at menarche was 17.3 In the U.S. in 1900, the average was 14.2. By the 1920s, average age at menarche in the U.S. had fallen to 13.3 and by 2002, it had reached 12.34. Similar trends are occurring in other Western nations. For example, age at menarche in Ireland has declined from 13.52 in 1986 to 12.53 in 2006. In Italy, a recent study showed that girls’ age at menarche was on average 3 months earlier than their mothers’.
Taking all this data together, we can estimate that the normal, healthy age at menarche under conditions of excellent nutrition without caloric excess would probably fall somewhere between 15 and 18. But today in the U.S., about half of girls begin developing breasts before age 10, and the average age at menarche is less than 12 ½ and still declining.
Why Is this happening?
The neurological and hormonal systems that regulate pubertal timing are complex, but research has identified a number of environmental factors that may be contributing to the decline in age at puberty:
Increasing rates of childhood overweight and obesity
Several studies have found associations between higher childhood BMI and earlier puberty in girls. Excess body fat alters the levels of the hormones insulin, leptin, and estrogen, and these factors are believed to be responsible for the acceleration of pubertal timing by obesity. Also, physical inactivity may decrease melatonin levels, which can also affect signals in the brain that trigger pubertal development.
Increased animal protein intake
Higher total protein, animal protein, and meat intake in children age 3-7 have been associated with earlier menarche in multiple studies. In contrast, higher vegetable protein intake at age 5-6 is associated with later menarche.
High protein intake elevates IGF-1 levels and promotes growth, which could accelerate the onset of puberty – IGF-1 contributes to pubertal development on its own and in part by its involvement in estradiol signaling.[4,16] Meat and dairy consumption in children may also reflect ingestion of environmental endocrine-disrupting chemicals (EDCs) that have accumulated in animal tissues (see EDCs below).
Other dietary factors:
High dairy consumption is associated with earlier than average menarche. Soft drink consumption is associated with early menarche.
Children with lower nutrient diets (based on analysis of macronutrients, vitamins, minerals, and certain whole foods) tend to enter puberty earlier.Overall our modern diet rich in processed foods, dairy, processed meats and fast food is disruptive to normal development and aging. Early puberty is an early sign of premature aging.
Exposure to endocrine-disrupting chemicals (EDCs)
EDCs are hormonally active synthetic chemicals that either mimic, inhibit, or alter the action of natural hormones. These chemicals are ubiquitous in our environment, and are considered by scientists to be a significant public health concern. Although EDCs are thought to pose a threat to adults as well, children’s bodies are more sensitive to exposure to exogenous hormones.
Chemicals are not currently tested for their endocrine disruption potential before they are approved for use and enter our environment, and there are endocrine disruptors in a vast array of products we come into contact with every day, including organochlorine pesticides, plastics, fuels, and other industrial chemicals.