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.