(CNN) -- Don't doubt it: "Man flu" is real, or so says one Canadian researcher who was "tired of being accused of overreacting."
With many respiratory diseases, a man is more susceptible to complications than a woman, plus his immune system may be naturally weaker, according to research published Monday in the BMJ medical journal.
"Man flu" is a term used to chide men who are suspected of exaggerating their symptoms when sick from a cold or other minor illness.
"It's a frequently heard stereotype," said Dr. Kyle Sue, author of the study and an assistant professor of family medicine at Memorial University of Newfoundland in Canada.
When he was asked to give an "interesting" two-minute presentation to a group of people from various career backgrounds, he knew that proving that men were not exaggerating "could provide evidence for men around the world to defend themselves."
Sue began with a simple search for relevant studies to see whether men experience worse symptoms than women. He suspected this gender difference might even have an evolutionary basis.
What he found was a good deal of evidence that is "suggestive of an immunity gap," though it's "certainly not definitive," he said.
Other scientists argue there's too little evidence to say man flu exists.
Exploring the 'immunity gap'
Sue said, to begin with, women have a different response to vaccines that protect against the flu.
"There are a couple of studies that show women having more local and systemic reactions to the flu shot than men," he said. He added the evidence suggests that, overall, women may be "more responsive to vaccinations than men."
Other clues indicated that man flu is not an overreaction.
"Epidemiologic data from Hong Kong showed that adult men had a higher risk of hospital admission for flu," Sue said. An American study revealed that men died more often from flu compared with same-age women, regardless of underlying heart disease, cancer, chronic respiratory diseases and kidney diseases.
"However, neither study differentiated men and women based on other differences, like smoking and drinking rates (and) willingness to seek medical help," he said, and these unknowns might have influenced the results.
Still, Sue found support for the idea that men suffer more from viral respiratory illnesses than women because they have less-robust immune systems.
This "immunity gap" may be modulated by hormonal differences, in which the masculine hormone testosterone suppresses the immune system while the feminine hormone estradiol protects it.
"It is not commonly known that testosterone is immunosuppressive," Sue said, though "one study found that men with higher testosterone levels had less of an antibody response to vaccination."
If an immunity gap between the sexes is real, the evolutionary reasons why remain unclear, he noted. One theory is that testosterone boosts aggressive behavior and the development of secondary sexual characteristics and so allows men to win at competitions -- overriding the cost of the hormone's immune system suppressing effects.
Across species, the masculine strategy of "live hard, die young" means men are more likely to die from trauma than an infection, according to another theory.
One other evolutionary theory Sue noted is that worse symptoms would lead a man to conserve his energy by lying on the couch, which helps him avoid a predator (his boss), and voila: His chances of survival are immediately improved.
The importance of age
Though he intended the article simply as light fare for holiday readers, Sue's research is described as "just so" by Sabra L. Klein, associate professor at Johns Hopkins University Bloomberg School of Public Health.
Still, Klein, who was not involved in the new study, appreciates that Sue is helping to shed light on gender health differences, "which often are ignored."
"When we hear 'man flu,' we assume this means all males of all ages," Klein said. "This is not true."
Rates of hospitalization are consistently higher for very young (prior to puberty) and very old (over 65) males, she said. During the reproductive years, it is women who often suffer more severe disease, in part because flu is worse for pregnant women but also because women develop higher -- almost excessive -- inflammatory responses to flu.
"The point I want to make is that whether males or females suffer more really depends a lot on our age," said Klein, whose own research is referenced by Sue.
In countries where women have less access to health care or treatments, or where boys are more valued than girls, it may appear that boys and men are being hospitalized at higher rates. "These unfortunate facts create biases in our interpretation of data," Klein said.
"In my opinion, we do not yet have enough science to conclude that 'man flu' is real," she said.
Professor Helen Stokes-Lampard, chairwoman of the Royal College of General Practitioners in London, agrees. "Contrary to popular belief, and this article, the vast majority of robust scientific evidence suggests that flu is not sexist," said Stokes-Lampard, who was not involved in the research.
Still, some research suggests that men have more severe respiratory tract infection symptoms than women, she said.
"The best advice for anyone affected is to rest at home, drink plenty of fluids and to take over-the-counter painkillers," said Stokes-Lampard said. Most people will recover completely in a few days, no matter their gender, she explained.
However, if, after three weeks, your symptoms do not improve, your condition deteriorates, or you have trouble breathing, she suggests you see a health care professional.
Despite these contrary opinions, Sue believes that the available research points toward men suffering worse from colds and flu than women, but he called for "much better-quality research" to prove it.
One potential study, he said, could examine whether men with strong immune systems are less successful at mating compared with attractive, high-testosterone men with weaker immune systems.
"Can the blame for man flu be shifted to the people who select these men as sexual partners rather than the men themselves?" Sue asked. "I was surprised that there were far more female authors for the studies I cited than males."