Testosterone-fuelled machismo masks an inner weakness caused by the male hormone, a study has shown.
Scientists have found men with high levels of circulating testosterone have less effective immune systems, as measured by their ability to resist flu.
The discovery could explain why men are more susceptible than women to a whole range of bacterial, viral, fungal and parasitic infections.
It may also be the reason why men’s immune systems respond less strongly to vaccinations against influenza, yellow fever, measles, hepatitis and many other infectious diseases.
“This is the first study to show an explicit correlation between testosterone levels, gene expression and immune responsiveness in humans,” said US lead scientist Professor Mark Davis, from Stanford University.
“It could be food for thought to all the testosterone-supplement takers out there.”
The researchers studied how the immune systems of 34 men and 53 women were stimulated by the flu vaccine.
As expected, the jab generated a bigger boost in protective antibodies in women.
Further analysis revealed a pattern of gene activity that in high testosterone men was associated with a weakened antibody response.
Like women, men with low testosterone were not affected the same way. For them, raised activity of the same genes bore no relation to how well they responded to the vaccine.
Ironically, one of the positive aspects of testosterone – its anti-inflammatory properties – may explain why it can weaken the immune system, said the scientists writing in the journal Proceedings of the National Academy of Sciences.
Inflammation, which can be extremely harmful to health, is merely an amplified immune response. Processes that trigger inflammation are also necessary for efficient body defences.
The scientists used state-of-the-art techniques to check the activity levels of some 22,000 genes in circulating immune cells.
Why evolution has seen to it that testosterone boosts male characteristics associated with strength, such as muscle power, risk-taking and aggression, yet weakens the immune system, remains a mystery.
One reason may be that dampening down the immune system makes men less susceptible to a potentially fatal over-reaction to infections, especially those from wounds, Prof Davis believes.
Men are more likely than women to suffer injuries from competitive encounters, as well as their traditional roles of hunting, defence and potentially dangerous physical work, he said.
“Ask yourself which sex is more likely to clash violently with and do grievous bodily harm to, others of their own sex,” Prof Davis added.
Compared with men, women had double the risk of dying from sepsis, a systemic out-of-control inflammatory reaction.