Low sperm count is an uncomfortable subject for some men to discuss, which may be why so many urban legends have sprung up about its causes.
While many grow up hearing that drinking Mountain Dew or shunning boxers can create barriers to fatherhood later on, studies haven't backed up those claims.
Meanwhile, a lesser-known condition that needs a doctor's diagnosis often goes overlooked.
"The most common treatable cause of male factor infertility is called varicocele," said Dr. Dan Williams, director of male reproductive medicine and surgery at the University of Wisconsin School of Medicine and Public Health.
Varicocele is a condition where varicose veins surround the testicle, typically the left one. Although the exact mechanism is unknown, Williams said the most widely accepted theory is that the varicocele increases the heat around the testicle.
"After we repair a varicocele, scrotal temperature decreases," he explained. Doctors don't know what causes varicoceles, but they are easily treatable and can only be diagnosed by a doctor.
"This is another reason it's very important for men to be evaluated by a urologist who's specially trained in male infertility," said Williams, noting that the common route of the man being evaluated by his primary care doctor or his partner's OB/GYN is often not enough.
"Just because they have sperm doesn't mean that there's not something harmful or treatable that we can diagnose that can help us to improve the sperm counts," he said.
In his own clinic at the University of Wisconsin, Williams notes, couples are screened together, with the woman seeing an OB/GYN and the man seeing a urologist.
"Couples are evaluated for male and female factor infertility issues simultaneously," he said. "Fertility really is a couples thing not just a male or a female thing."
But when it comes to figuring out on your own how to avoid infertility, what you hear can be misleading.
"The problem with a lot of these studies is there's something being reported in the medical news every single day, and often they're contradictory," said Dr. Abraham Morgentaler, associate clinical professor of urology at Harvard Medical School and Beth Israel Deaconess Medical Center and author of "Testosterone for Life." "In general, if there's something that makes a really big difference, the data are consistent and they'll show up often in a variety of studies under a variety of different conditions."
He notes that a lot of myths circulate around diet and fertility, but some of those have weak data and others show effects in animals that have not been replicated in humans. What he tells patients, Morgentaler said, is his fertility motto: "Live clean, stay cool. I think that's important."
"The point is that heat is a definite, no questions asked, has an impact on sperm production. And the live clean is that we know a number of recreational drugs appear to have some impact on fertility," he explained.
In the following pages, we see which alleged sperm-killers are culprits to avoid and which don't have evidence showing them to be all that harmful. Still, all couples with fertility troubles would do well to seek out a professional opinion.
"Most doctors now laugh that one off," said Morgentaler.
The reasons why boxers are said to be superior seem obvious. If the purpose of the scrotum is to keep the testicles away from the heat at the core of the body, briefs would seem to defeat that purpose.
"It is consistent with the concept that excess heat can be detrimental to testicular function," said Williams.
But the evidence doesn't seem to back that idea.
"There are no well-controlled studies that look into the choice of underwear, briefs versus boxers," said Williams. "There's no scientific data I know of ... head-to-head studies of boxers versus briefs showing a difference."
A study from the State University of New York at Stony Brook was published in the Journal of Urology in 1998 where 97 men with infertility were examined to see if their underwear choice made a difference.
Researchers found an average difference of less than half of a degree Fahrenheit in temperature of the scrotum between men who wore one underwear type or the other, with a margin of error larger than the difference.
"It is unlikely that underwear type has a significant effect on male fertility," wrote the authors. "Routinely advising infertility patients to wear boxer shorts cannot be supported by available scientific evidence."
If you are trying to avoid excess heat in the region, putting a computer that generates heat in your lap may not be the best idea.
"If there is excessive heat put out by the laptop, excess heat exposure from the laptop to the testicles may have a detrimental effect," said Williams.
"If there is prolonged use on the lap ... it's important to be aware of that," he said.
A 2005 study from Stony Brook also indicated that using a laptop on the lap can elevate the temperature of the scrotum several degrees.
But other concerns about modern electronics aren't as well backed up.
When it comes to radio waves generated by cell phones or wi-fi from laptops, it's unclear what if anything they do to sperm.
"This effect remains to be determined, but researchers are looking into that," said Williams. "Whether it's clinically significant remains to be determined."
"The take-home message on that is to avoid excess heat," said Williams. "Any prolonged exposure to excess heat can be detrimental to the production of sperm and sperm quality."
He noted that other heat sources, such as saunas, could present a similar problem.
"If men are regularly hot-tubbing or using the sauna and if they have impaired sperm quality or sperm counts, that would be one strategy to try to improve the sperm count or sperm quality."
While hot tubs can present a problem, Williams noted that studies have not shown how much exposure to them is too much.
Morgentaler noted that the problems hot tubs and saunas present are not surprising.
"The testicles live outside the body for good reason," he said. "Heat is bad for sperm production, so bad that…humans and many but not all mammals have the testicles outside the body in a pouch called the scrotum to keep the testicles cooler."
While noting that a fever can have a similar effect, Morgentaler said that a man who likes the occasional hot dip shouldn't be too alarmed that he is ruining his chances for fatherhood.
"An occasional hot bath or a single episode of a Jacuzzi is probably not enough to do anything," he said.
Avoid, If Only For Other Reasons
While obesity brings obvious health problems with it, a low sperm count may be one more to add to the list.
There are two possible problems obesity presents: a hormonal one and a physical one.
"There are two main potential effects that obesity can have on testicular function," said Williams.
"Obesity can affect the male hormones. Testosterone can get converted to estradiol in our peripheral adipose tissue and in men who are overweight there is increased conversion of testosterone to estradiol. That can effect sperm production or the quality of the sperm."
"The other effect is heat," said Williams. "When there's excess heat around the testes, then that can impair the testicular function." Excess adipose tissue, he said, acts as insulation and increases heat in the scrotum.
Sharon Moalem, a physiologist at the Mount Sinai School of Medicine and author of "How Sex Works," said the former explanation would be more likely.
"Where we live is so temperature controlled, it's not that big of a difference. It probably has more to do with hormonal [problems]," said Moalem.
"It may just be that obese individuals may have hormonal disruption."
Obesity should be avoided for many reasons, but Morgentaler noted that it should not be the primary concern from this list for obese men suffering from infertility. Most men who are obese, he said, have no trouble, although "there is some suggestive evidence that some men with obesity will have impaired sperm numbers."
While some of the effects steroids have may be overstated, their negative impact on fertility has not been.
"People who are on high-dose performance enhancing anabolic steroids sometimes can have damage to their hormonal system so that they may end up permanently sterile. That's one of the risks," said Morgentaler.
"One of the side effects of giving men even medical testosterone is that it will absolutely lower sperm numbers, even down to zero," he said.
While men are given fertility treatments to increase testosterone, those treatments are designed to increase the body's own production of testosterone.
"Whether it's injected or it's one of the topical gels, while it increases the body's levels, any exogenous testosterone will shut off the body's own production of testosterone, shutting off sperm production," explained Williams.
He noted that men who receive fertility treatments to get their body's to produce testosterone need to be patient, since it takes a few months for the effects to be realized.
"The treatment of male fertility takes time because it takes roughly 90 days for sperm to be made in the testicles and make the journey through the reproductive tract and end up in the semen," Williams said. "The patients and the couples need to understand that treatment for infertility will take time before you can see the benefits."
While smoking should be avoided for any prospective father for other reasons, it may keep a man from becoming a father in the first place.
"Not that we need any other excuse to get up on our soap box to discuss tobacco cessation," said Williams, but in addition to reducing lung cancer, quitting smoking can improve motility and sperm quality.
Moalem said the reasons for smoking's harm to sperm are probably related directly to its toxic effects on the body.
"If you have high turnover of cells, if there's environmental toxins, that may just interfere with your body's ability to make healthy cells," he said.
Given the image of a macho man with cigar in hot tub as the pinnacle of manliness, Moalem said, what we know about sperm production shows that it's in fact "as far away from virility as you can get."
Avoid, But For Other Reasons
As debates about drug legalization arise, the science isn't entirely clear on how these substances might affect our national fertility rates.
"The data around marijuana is not definitive but highly suggestive. It can lower the quality of sperm as well as probably the count," said Morgentaler.
"There's not a lot of good data out there about the effect of marijuana on sperm or sperm production," said Williams. "There are a few studies about heavy marijuana use possibly impacting men's hormone levels and the quality of the sperm, but these are smaller studies."
Moalem notes that some risks can arise from cocaine use affecting circulation to the scrotal region.
"You could have a stroke in your testicle," he said.
Some of the harm might actually arise from legal narcotics, as men on narcotic pain medications can have very low testosterone.
"If men are taking certain pain medications, certain pain medications contain narcotics, narcotics can affect men's hormone levels," said Williams.
"The data on vitamins and supplements for men and sperm numbers -- I don't think there's any definitive answer around them," said Morgentaler.
That isn't to say that men shouldn't focus on getting vitamins and minerals into their diets naturally.
"Having a health lifestyle and eating a well-rounded, balanced diet is important for a variety of reasons," said Williams.
He noted that a diet rich in antioxidants can help when sperm quality.
"When you start talking about food or nutritional supplements, there are a number of labs that show improved sperm quality in the presence of antioxidants," he said.
"I think the take home message is just a well-rounded, balanced diet. Excess of anything is not necessarily a good thing," said Williams.
Moalem pointed out that adding more spinach to a diet for its folate can have added benefits, even if it's unclear if or how it helps men's sperm.
"It's healthy anyways and it's not something people think about," he said. "And it's good for women who are trying to conceive as well."
While the benefits of folate aren't clear for men, it has been shown to benefit pregnant women, and presumably a couple trying to conceive is also eating the bulk of their meals together.
"A lot of people have heard that red meat is supposed to be bad for you. The good news is it's not something bad for your sperm," said Morgentaler.
Because of the number of groups that oppose meat eating for moral reasons, any study showing harm from meat eating is likely to be promoted.
Last week a story in the British press on possible hazards of meat eating drew enough attention that the National Health Service (NHS) felt compelled to let people know that the study was not as significant as the promotion made it out to be.
"The study was small and has limitations because of its design. This research should be regarded as preliminary evidence, and larger studies will be needed to explore whether diet does affect semen quality," the NHS said.
The NHS noted that the study found men who consumed more protein had sperm abnormalities, the study had not even looked at where the protein had come from.
"One of the challenges in designing studies that look at nutrient in male fertility is designing a well-monitored study of what men are really eating," said Williams.
He noted that the effects of foods and environments can vary between individuals.
"Those are factors that are very difficult to control when you're doing a research study about male infertility."
Undetermined -- But Worth Watching
"Unfortunately, there aren't any good well-controlled studies that investigate the direct effect emotional stress has on male factor fertility," said Williams.
He noted that the concern behind emotional stress is not on sperm production but on the effect infertility might have on the couple.
Because infertility can be so stressful, he said, he often refers couples to a counselor to help them through the process emotionally.
"Any strategies or coping mechanisms that help with stress management can certainly be beneficial as they go through the fertility process," said Williams.
"But whether or not the stress management directly improves the testicular function and production of sperm is unknown at this point."
Moalem notes that there might be some physiological reasons why emotional stress would cause fertility problems.
When the fight or flight response is triggered, he said, "you spend more energy…to get you out of a bad situation than to maintain the body.
"Our body's not geared toward reproduction as much."
For that reason, he said, when women are under a lot of stress, it can actually bring on an early period, perhaps the body's way of saying "It may not be the best time to have kids."
Moalem noted, however, that "I haven't seen any studies that do a good job for [the effect of stress on fertility]. It's very hard to study because it's so individual."
Obviously aging itself can't be avoided, but ignoring the biological clock when deciding when to have kids can be a problem for men as well as women.
"Certainly the impact of age and fertility is better established and more pronounced in women than in men, but there is growing evidence that with time the quality of men's sperm declines," said Williams. "There can be impaired sperm quality with advanced paternal age.
"There are some studies that demonstrate that when combined with advanced maternal age, that advanced paternal age can put offspring at risk for certain conditions like Down's syndrome, autism and schizophrenia, but there's active research going on in this field of advanced paternal age to better answer these questions."
Morgentaler, however, notes that advanced paternal age is still less of a concern than advanced maternal age.
"There is an age-related decline in sperm numbers," he said, but "Male fertility is very different from female fertility.
"That kind of abrupt change doesn't seem to happen for men," he said, adding that men in their seventies and eighties have fathered children naturally.
Radha Chitale contributed to this report.