Alcohol, cigarette smoking and obesity had no appreciable effect on sperm appearance. Previous research on the same group of participants reported a similar lack of effect for these variables on the number of swimming sperm, a measure known as 'motility' (see BioNews 661) .
'This previous study also found that there were relatively few risk factors that men could change in order to improve their fertility', said Dr Andrew Povey, a co-author of both papers. 'We therefore have to conclude again that there is little evidence that delaying fertility treatment to make adjustments to a man's lifestyle is unlikely to improve their chances of a conception'.
The research, by scientists at the Universities of Sheffield and Manchester, recruited fertility patients who had all been trying to conceive with their partners for at least a year. All participants completed detailed questionnaires on their lifestyle habits.
The effect of cannabis the scientists observed was more pronounced in younger men. Dr Allan Pacey, the study's first author, told the Independent that this was most likely a dose effect. 'Younger men may be more inclined to smoke cannabis more frequently and in higher doses than older men', he said.
To explain the effect of cannabis, the authors refer to research performed in mice which showed that cells involved in making sperm allow cannabis to bind. When the drug does so, it affects the cells' DNA which, in turn, could cause abnormal sperm shape. Only sperm with 'normal' morphology (shape and size) are thought to be able to navigate the reproductive tract and fertilise an egg cell.
The study also found that samples obtained during the summer contained more sperm of abnormal morphology. This could not be explained by temperature rises; there was no similar effect observed in participants wearing tighter underwear, which would also result in higher temperatures.
The researchers claim theirs is the largest study yet to investigate the effect of lifestyle choices on sperm morphology.
Of the 2,076 study participants, 376 were affected by abnormal sperm morphology, as defined by the World Health Organisation's (WHO) criteria. The authors say that since their analysis, the WHO guidelines have changed; if they re-analysed their data according to the new criteria, they would likely find fewer cases.
The study was published in the journal Human Reproduction.