From 1980-2010, the number of obese American children aged 6-11 more than doubled, from 7% to 18%.

While childhood obesity can presage a host of problems, greatly increasing the risk of cancer, heart disease and diabetes, less discussed is the effect it will eventually have on a person’s sex life.

In the new book “XL Love” (Rodale), health-policy journalist Sarah Varney informs us that these negative effects begin surprisingly early for many, such as how overweight girls begin puberty younger than their healthy-weight counterparts.

“Some 15% of American girls now begin puberty by first or second grade,” Varney writes. “Over the last quarter-century, the age at which American girls begin menstruating has decreased by 2.5 months.”

Carlton Gorton, a doctor in Belzoni, Miss., an area with higher-than-average rates of obesity, told Varney that “he has discovered girls as young as 5 and 6 years old who are developing pubic hair. These alarming signs of puberty are usually related to his patients’ ample body fat.”

Some of these young patients, whom the doctor describes as “off the growth chart,” even come to him for monthly hormone shots that “hold them off from going through puberty.”

Early puberty, meanwhile, leads to a host of dangerous issues.

“Among girls who go though early puberty,” Varney writes, “there is an increased incidence of depression; alcohol, tobacco and substance abuse; riskier sexual adventures; teen pregnancy; and even suicide attempts.”

Girls who get their periods earlier are “more likely to be sexually active sooner than girls the same age and are more likely to become pregnant and contract sexually transmitted diseases.”

Varney also references a study that “found that women with early periods had an elevated risk for unwanted sexual touching and an even greater risk for forced sexual activity.”

In the teen years, excess weight can also be an obstacle for a girl’s romantic development.

One 2005 study found that “a teenage girl’s odds for a romantic relationship…dropped 6 to 7 percent for every 1-point increase in her body mass index.”

With dating becoming that much more challenging, overweight girls often wind up in one of two categories, neither positive: Some become late bloomers, leaving them behind in knowing how to forge real relationships as they approach adulthood; or, seeking approval, they have sex earlier, leading to even lower self-esteem and continually high risks of pregnancy and sexually transmitted diseases.

A teenage girl’s odds for a romantic relationship…dropped 6 to 7 percent for every 1-point increase in her body mass index. - Study from 2005

Margaret Villers, an assistant professor of obstetrics and gynecology at the Medical University of South Carolina in Charleston, examined more than 20,000 responses in a study of teenage girls.

Villers and her team found that “when obese girls did have sex,” they were “three times as likely as healthy-weight girls to have had sex by the age of 13; 30% more likely to have sex with more than three boys by the end of high school; [and] less likely to use condoms.”

Varney writes that based on feedback from therapists, “Overweight teenage girls can be reluctant to refuse any advances out of fear that they’ll have few chances in the future for romantic and sexual attention.”

Furthermore, a 2011 study found that “extremely obese high-school girls” were “less likely to have sexual intercourse overall…But when they did have a sexual encounter, 42% reported using drugs or alcohol — four times the rate of healthy-weight girls.”

This is troubling for a host of reasons, not the least of which is that “since girls who were drunk or high often ‘did more’ sexually than they originally intended, [they] were more likely to have sex without birth control.”

Jennie Noll, a professor at Penn State who participated in that study, “worries that heavy girls who become sexually active aren’t developing mature sexual identity that will serve them well on their journey to adulthood; they’re just stuck in a cycle. [Some guy is] going to call her beautiful, [and then she thinks], ‘It felt good when that kid came on to me at the party. I slept with him.’ But it doesn’t do anything to move [her] on in a developmental fashion.”

“The picture that is emerging of heavy teen girls and sex,” Varney writes, “is one of extremes: girls who enter adulthood romantically and sexually inexperienced, and those who begin sex very early, by the age of 13, and pursue sexual encounters more recklessly.”

“I don’t think either of those cases,” Noll says in the book, “is going to have healthy relationships unless they’ve found Prince Charming.”

In addition to the psychological and maturation problems these factors set in place, there is also, according to Varney, the fact that 92% of girls who are obese as teens will remain so as adults, leading to problems such as feelings of sexual inadequacy and difficulty reaching orgasm due to decreased blood flow to the clitoris.

For every 50 pounds overweight you are, you lose an inch of penis. - Dr. Edward Karpman

A 2006 study even found that “one out of three obese women said that because of her weight, she ‘usually or always did not enjoy sexual activity, had little sexual desire, experienced difficulty with sexual performance and avoided sexual encounters.’”

The news is little better for heavy men. While overweight boys might not have the depth of challenges overweight girls do, once into adulthood — where 80% of those boys will remain overweight — the extra weight creates similar feelings of inadequacy, along with unique and troubling problems of their own.

“For every 50 pounds overweight you are, you lose an inch of penis,” Dr. Edward Karpman, a California urologist, says in the book.

“A man’s penis is actually fixed to his abdominal wall, holding it in place,” writes Varney, based on Karpman’s explanation. “The more a man’s fattening belly grows outward, ‘the more it eats their penis,’ leaving them with, according to the doctor, ‘this little nubbin of a penis.’”

Extremely obese men face an extreme version of that, called “buried penis syndrome.”

As Varney explains, “abdominal fat and skin drape out and over a man’s pubic area, causing a host of problems.”

There are plenty of reasons for America to lose weight. “XL Love,” with its portrait of sexual dysfunction and loss of intimacy, shows us one of the saddest.