What Determines Male Fertility?

What affects a man’s ability to successfully reproduce?
21 March 2017

Interview with 

Professor Allan Pacey, University of Sheffield


It seems that modern living might make us less fertile as a species, but what about males in particular? Sperm counts on the whole seem to be in decline but what is it that determines the number of sperm produced by a man, and what role does aging play? Chris Smith spoke to Allan Pacey, a Professor of Andrology at the University of Sheffield...

Allan - There’s some controversy about whether or not sperm counts have declined or not. In 1992, there was a famous paper published in the B.M.J by Elizabeth Carlson suggesting that sperm counts from the 1930s to the 1990s had seen a year on year decline. It gets a little bit fuzzy as we go further forward from that because some studies say that there hasn’t been a change, some studies say that there has. But it’s certainly clear that couples are having fewer children; more men are seeking infertility diagnoses and treatment, but I think we’ve got to remember that some of that may be social as much as it might be biological.

Chris - So if we assume for a minute that sperm counts might be on the move, what factors might account for them dropping?

Allan - Well there’s two stages in a man’s life where sperm counts might be affected. The first might be a surprise and that is processes that happen before he’s even born. As a foetus, the developmental processes that develop his reproductive system and, ultimately, will influence the size to which his testicles will grow will have an effect on his sperm counts as an adult, but they’re processes that occur before he’s born.

Then when a man is an adult, we have to take into account all of the things that may impact on his life at that moment in time that may affect his sperm production. So, for example, there are certain drugs and medications that we know will decrease sperm production and may do so permanently. But, similarly, there are temporary effects such as temperature and we know, for example, that men who wear pants that are too tight are more at risk of poorer sperm counts.

So, when we’re studying male fertility, we almost have to take a two step look at processes that might have occurred before he was born and those that are occurring now.

Chris - So, there’s a developmental predisposition to your maximum fertility, and superimposed on that is how you live your life. What’s the reason for that developmental predisposition towards testicular size and fertility? Why should that effect be there?

Allan - I think it’s just part of the developmental programme, and external things that may affect that might be aspects of maternal diet, aspects of maternal hormonal environment in the womb. If you’re to build a factory, how big do you build the factory will depend on how many cars, for example, you may make when the factory opens, and the developmental processes define how big the factory gets. Whereas, you don’t know how efficient the factory will be until the production line starts rolling, and that only occurs as puberty.

Chris - When the production line does start rolling, tell me a bit more about the kinds of factors that do influence sperm count? You eluded to temperature, that’s one of them, and people wearing tight pants, but there are others?

Allan - There are others. We know that temperature has an effect, as I’ve said, and there are many studies both from epidemiology, but also experimental studies in animal models that show that if you elevate the temperature of the testicles you decrease sperm production.

But then there are subtle influence of aspects of diet. We know that men who are eating more processed, refined foods generally have a poorer sperm production that men who are eating five portions of fruit and vegetables a day and are eating more mediterranean diets, for example. There are also temporary issues to do with alcohol to some extent, I think you really have to be in the realms of going into binge drinking before we see an alcohol effect.

But then there’s also an impact of things like smoking and the compounds within cigarette smoke. Smoking doesn’t alter the efficiency of the sperm production process, but what it does is it allows the introduction of damage at the genetic level so the sperm that are produced are, ultimately, less functional because their DNA is a bit more chopped up and a bit more mashed up. So I think, when we think about sperm production, we not only have to think about the efficiency in the speed of it, but we also have to think about the quality of the sperm that come out at the other end.

Chris - I was going to ask that. Because one aspect of this is how many sperm can you physically make and that’s going to be important, but then equally, if all those sperm that you make, you make millions, and they’re all defective, you’re not really any better off than someone who makes none?

Allan - Indeed. And there is a theory that men whose sperm production process is too quick also, generally, makes poorer quality sperm simply because - to take the analogy, there isn’t enough time to put the wheels on.

Chris - So timing really is everything? What about ageing though because, increasingly, people are deferring  having a family till they’re older. There are lots of people now who are on their second family. Are there effects there?

Allan - Yeah. There are subtle effects in ageing. But it does become a little more interesting actually because there are slight changes in how many sperm an older man produces, and how well they swim, but they're really quite subtle. But at the genetic level when you look at the health of the children born to an old man, compared to the health of children born to a younger man, you see that there are increased risks of having a child with a genetic disease. The kind of things that we see are increased risks of schizophrenia, down syndrome. Things like achondroplasia, which is a form of shortening of the bones which leads of a form of dwarfism. So that’s probably indicative of the fact that as a man gets older then the quality of the genes in the sperm that are produced is not as good.


I wonder if I often use adjuvants like viagra (sildenafil) will it affect my sperm and reproductive abilities?

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