Fertility, Longevity, and Precocious Puberty

Healthy Seventh-day Adventist vegetarians may be longest living, but might that come at a cost? A semen analysis at a fertility clinic in Loma Linda, California, home to a large population of Adventists, raised questions about their sperm quality. Though still within the normal range, vegetarians had about a 25 percent lower sperm count. The few vegans tested also had a lower sperm concentration, but they made up for that with their 30 percent greater ejaculate volume. The vegans did have significantly fewer activated sperm, though, which is a sign of decreased fertility. The researchers suggested soy consumption as a possible mechanism, as Adventist vegetarians in California average about a half serving a day of plant-based meats, many of which contain soy. Soy phytoestrogens have been put to the test, though, and months of consuming up to the equivalent of nearly 20 servings of soy a day did not result in any adverse effects on sperm parameters.

There were only five vegans in the study; so, the sperm quality findings could just be a fluke, but if verified, this could reflect an evolutionary trade-off between fertility and lifespan first proposed nearly a century ago. Using a finely tuned laser, it’s possible to selectively destroy individual cells as the tiny roundworm C. elegans develops, and terminating the cells that give rise to sperm and eggs significantly extends lifespan. The same phenomenon can be demonstrated in fruit flies, potentially shifting the body’s priorities from reproduction to survival.

The fertility vs. longevity trade-off may be one of the reasons why spaying and neutering our pets can extend their lives. Based on a study of millions of dogs and cats, sterilized dogs live about 20 percent longer than “intact” dogs, and spayed cats live about 40 percent longer, and neutered cats a remarkable 60 percent longer.

What about men who have been castrated? Eunuchs seem to live 25 percent longer than non-castrated men. Up until the 1950s in the United States, the so-called “feebleminded” were involuntary sterilized by the state in a eugenics program upheld by none other than Oliver Wendell Holmes, who wrote for the majority on the Supreme Court: “[S]ociety can prevent those who are manifestly unfit from continuing their kind. The principle that sustains compulsory vaccination is broad enough to cover cutting the Fallopian tubes.” The heinous practice of compulsory sterilization did allow for a natural experiment, though, and one mental institution found that castrated men lived about 14 years longer than intact men in the same hospital.

A genealogy database of nearly 200,000 men and women across three centuries in 16 countries found that those who had fewer children seemed to live longer. Centenarians, for example, were found to have fewer children at later ages. This is not to suggest that having fewer children will make you live longer, but rather that constitutional factors that enhance human lifespan may come at the expense of reduced reproductive potential, another example of the antagonistic pleiotropy theory I talked about before. For example, selection for lifespan extension in model organisms can lead to longer-lived animals, but with reduced reproductive capacity. It makes intuitive sense when you consider the context of food scarcity.

In lean times, it makes sense to put off reproduction until the return of more favorable conditions to ensure long-term survival. In the infamous Minnesota Starvation Study that used conscientious objectors as guinea pigs during World War II, the study subjects rapidly lost their libidos after their calorie intake was cut in half. As one of them put it, “I have no more sexual feeling than a sick oyster.” The same calorie restriction that extends the lives of animals can also cause a reduction in the number of progeny. If you read my book How Not to Age, you’ll be familiar with the nutrient-sensing aging pathways—IGF-1, mTOR, and AMPK—that create kind of a seesaw between tissue acceleration and reproduction on one side, and tissue preservation and rejuvenation on the other. Thankfully, we can shift the weight to a more optimal balance with diet.

The later girls start their periods, the longer they tend to live. Each year later is associated with significantly lower risk of dying from heart disease and stroke, plateauing out with the lowest overall mortality among those who don’t start menstruating until age 15. Isn’t 15 a little late? It didn’t used to be. A century ago, the age of first menstruation averaged as late as nearly 17. Similarly, the age of breast development has dropped an average of about three months per decade over the last half century around the world––down to nine or 10 years old in the United States, necessitating a change in textbook definitions of “premature” puberty.

Earlier breast development (before age 10, versus 12 or 13) is associated with as much as 23 percent greater breast cancer risk later in life. And each year later a girl starts to menstruate is linked to significantly lower risk of cancers of the uterus, liver, skin, bladder, colon, lung, and breast. Thankfully, the age of puberty is something over which we have a degree of control.

Higher levels of IGF-1 are associated with earlier sexual maturity; so, it’s no surprise that girls eating more animal protein experience puberty significantly earlier––an effect not seen with protein from plants. A meta-analysis of 16 studies on diet and development found that one’s first period appears to start earlier by two months for each additional one gram per day of animal protein intake in childhood. So, for example, at seven years old, girls consuming more than 12 servings of meat a week had 75 percent greater odds of menstruating within the next five years or so compared to girls at that age eating less than four servings a week, and this relationship was seen for both red meat and poultry consumption. IGF-1 and other aging pathways may not fully explain these findings, however, as persistent pollutants that build up in meat, like DDT, have also been linked to precocious puberty.

In our last story, we look at how the early onset of puberty in girls may be due to endocrine-disrupting chemical pollutants in the meat supply.

“Early onset of puberty is considered a…risk factor…[for] a number of diseases in adulthood, including hormone-related cancers, [a shorter lifespan], metabolic syndrome, and cardiovascular disease.” The conventional thinking has been that the reason the age of puberty has been getting earlier and earlier is because our children have been getting fatter and fatter.

Well, our kids have been getting heavier, especially in the United States—we’re #1! But, while the age of a girl’s first period has been dropping in the U.S. and Asia, in Europe—despite their kids getting heavier, too—there hasn’t been a steady decline in puberty onset. So, maybe it’s less about how much kids are eating, and more about what they’re eating.

The most consistent link between diet and premature puberty has been animal protein consumption. For example, every gram of daily animal protein intake—that’s the weight of a paperclip—has been associated with about a 17 percent increase in the risk of girls starting their periods earlier than age 12. Why this link between animal protein and premature puberty? Well, we know meat increases the levels of the growth hormone IGF-1, and that alone is associated with early-onset puberty. But, maybe animal protein is just a proxy for the endocrine-disrupting chemicals that build up the food chain in animal products.

Recent reports have found “significant associations between exposure to environmental pollutants and sexual maturation.” This was done over in Europe. In the U.S., a similar relationship was found with the flame-retardant chemicals, for example, which are found mostly in poultry and fish, unless you’re eating cat food.

“Over the last three decades, human exposure [to these levels of industrial pollutants] in the U.S. has increased from virtually nonexistent to [almost everyone carrying them around now].” They appear to have multiple adverse effects, but of all the potential toxicities, endocrine disruption (meaning hormonal disruption) may be the main concern in children. Girls with the most circulating in their bloodstream appeared up to ten times more likely to start their periods early.

But, since these chemicals are found most concentrated in fish and chicken, maybe the level of these chemicals in their bloodstream is just kind of a proxy for their meat consumption. Whatever the reason, animal protein intake is associated with early-onset puberty, whereas plant protein has the opposite effect. Children with higher levels of vegetable proteins starting puberty seven months later than average, and children eating more animal protein may start puberty seven months earlier than average.

Soy seems most protective. “[G]irls with the highest levels of dietary isoflavone intake [the phytonutrients in soy foods] may experience their onset of breast development…approximately 7 or 8 months later than girls with the lowest levels of intake.”

What effect might these shifts have on disease rates? Well, delays in the timing of puberty in response to beneficial dietary habits (higher intakes of vegetable protein and soy, and lower intakes of animal protein) “may be of substantial public health relevance.” A later age of starting one’s period is related to a reduced risk of breast cancer, and a later first period is associated with lower total mortality (meaning a longer lifespan).”

Hence, “a delay in the timing of puberty by approximately 7 or 8 months,” which is “achievable with dietary modifications”—either more plants or fewer animals—”may translate into about a 6 percent reduction in breast cancer risk,” and up to a 3 percent decrease in total mortality. And, it’s not just a problem in girls; boys eating more meat in childhood appear to be more likely to grow up with the kind of abdominal fat deposits that increase risk for heart disease.