The effect of caffeine on fertility has been debated through the decades. While the connection between heat and reduced sperm count seems clear, there is no similarly clear relationship for caffeine. Excessive caffeine intake, as part of a collection of other unhealthy exposures, has been associated with adverse effects on sperm. There is no consistent evidence, however, linking moderate consumption of caffeine to reduced sperm quantity, quality or motility.
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When consumed in excessive amounts, it is possible that caffeine might adversely impact sperm production. A study in the April 2010 issue of "American Journal of Epidemiology" reported that the number and concentration of sperm were slightly reduced in a group of 2,554 young Danish men with a high intake of caffeinated cola -- more than 14 0.5-liter bottles per week. However, the high-caffeine men also tended to have more unhealthy habits, such as smoking, drinking alcohol and eating less nutritious food, so it is unclear whether the effects of sperm were due to caffeine or these habits. Other studies have found no relationship between sperm quantity and caffeine consumption.
Sperm motility influences its ability to actively swim and propel itself through the female reproductive tract. When sperm is directly exposed to caffeine in a laboratory, sperm motility is increased. However, this stimulatory effect has not been found in any studies involving caffeine consumption in men. In a report by the Practice Committee of the American Society for Reproductive Medicine and the Society for Reproductive Endocrinology and Infertility published in the September 2013 issue of "Fertility and Sterility," this committee of experts concluded that caffeine consumption in men has no effect on the concentration of motile sperm.