Health and lifestyle factors can have a dramatic impact on male fertility and, while some of this seems obvious (smoking is a deal-breaker), not everything is intuitive (there is such a thing as too much exercise). Infertility can feel arbitrary, but the truth is, for about 25% of men, modifiable lifestyle factors play a key role.
Smoking
When it comes to fertility, there are some things we can do in moderation: smoking is not one of them. Smoking has a hugely negative impact on sperm quality, especially concentration and morphology.
As you can see, every incremental increase in the number of cigarettes smoked and the duration of smoking habit (cigarette years = cigarettes smoked per day x number of years smoking), erodes semen quality.
And in case you think, “well, we can still get pregnant with poor semen parameters using ICSI,” in the case of smoking, you’d be wrong. ICSI Success rates for men who smoke are only half as good as they are for men who don’t smoke.
Alcohol
When it comes to alcohol and male fertility, there is a “threshold effect,” meaning consumption below a certain volume likely makes minimal difference. Alcohol can have an impact on the pituitary gland's ability to regulate and produce hormones, and so consumption above that threshold becomes problematic.
As you can see, after a certain level of alcohol consumption, sperm count and concentration begin to diminish. Most urologists agree that the upper-end of that threshold lies somewhere between 20 - 30 drinks per week, which in our book is pretty high.
Now, are the men who drink more somehow disadvantaged if they need to do IVF? Maybe not. Perversely, the best study we can find showed that men who drink in moderation (about a drink a day) might even have higher rates of IVF success than those who don't drink at all.
Coffee
There is surprisingly little literature on the impact of caffeine consumption on semen parameters or pregnancy rates. One small study revealed that IVF success rates were halved for men who consumed 265 milligrams of caffeine per day, or about two-and-a-half cups of coffee. Most urologists will tell you think they think coffee intake is probably like alcohol: in small quantities (e.g. a cup a day), it won’t have a meaningful impact.
Weight Gain
Excess weight impacts male hormone levels, specifically pushing rates of estrogen up, and rates of testosterone down. A man's weight directly impacts a couple’s level of fertility, even after correcting for rates of smoking and drinking. While the first study below shows an inverse correlation between weight gain and sperm count, the second study also discovered that the reverse is true: having a BMI that is too low may impact fertility, too.
A fundamental question is whether IVF, and use of ICSI, to some extent "bails out" overweight men and delivers comprable IVF success rates as those enjoyed by normal weight men. The answer is "we don't know". One large meta-analysis of over 30 studies concluded overweight men who participate in IVF have lower success rates than normal weight men. But there is an equally-compelling, well-designed single study out of Holland that not only shows IVF and ICSI success rates are comparable, but the difference in semen quality is not as wide many previous studies revealed.
Exercise
There are no solid studies looking at how fertility success rates correlate to exercise frequency, but there is reasonably good data showing that men who exercise frequently typically have higher sperm counts. What's more, erectile difficulties are tenfold higher in sedentary men.
At the same time, most urologists believe there is such a thing as too much exercise, and after 90 minutes of vigorous daily exertion, you may start doing more harm than good. The underlying premise is that prolonged exertion amplifies the heat and pressure in the testicles, which reduces sperm production. There is also a fair amount of data that cycling, even as little as two hours a week, has a meaningfully negative impact on sperm concentration.
In a small study comparing semen parameters of physically-active populations, triathletes had lower sperm counts, and generally worse morphology.
Drug Use
The studies focused on drug use clearly show an impact on semen parameters, but no studies have shown a direct relationship between drug use and pregnancy or live birth rates. As for marijuana, 12 of the last 15 studies have demonstrated some impact on hormone regulation, most specifically underproduction of LH from the pituitary. The same was generally true for prescription opioids, which included a link to hypogonadism (a reduction in testosterone production) and diminished sexual drive. As for cocaine, a drug that is used equally across age groups, the seminal findings come from a 1,300 person study out of Yale. While the trial was not randomized, it appears that men who took cocaine within the last year, or habitually over the previous five years, had diminished sperm counts and motility.