ROCHESTER, Minn. -- As researchers struggle to better understand the machinery of COVID-19 hospitalization, they have turned up a most unlikely risk factor: male pattern baldness.

As reported in a recent letter to the Journal of American Academy of Dermatology, 79% of 122 men and 42% of 53 women suffering from COVID-19 in three Madrid hospitals were noted to have androgenic alopecia, otherwise known as male-pattern hair loss.

Like most of those faring worse from COVID-19, the subjects were all older. The median age of the men studied was 62.5, and that of women was 71. Balding accompanies older age, of course, but this was more balding than typical for the age ranges in question.

At 79%, the portion of men with hair loss in the Spanish COVID-19 hospital ward was potentially twice that of a matched group of males, for whom the normal prevalence of alopecia ranged from 31% to 53%.

At 42%, the portion of Spanish women hospitalized with COVID-19 who were balding had only slightly higher rates of hair loss than normal, given that the highest rate of so-called androgenic alopecia among women over 69 is 38%.

Newsletter signup for email alerts

The study was small and observational, though, meaning its conclusions were of limited use in describing cause-and-effect.

But it has been joined by a larger study of baldness and COVID-19, this one titled "Male Balding is a Major Risk Factor for Severe COVID-19." The authors, who are dermatology researchers West Virginia University, identified 1,941 hospitalized men in a health database known as the UK Biobank who had been tested for COVID-19. They then sorted them by positive or negative COVID-19 diagnosis, and looked up their level of baldness.

As part of an extensive health record, the UK Biobank participants had previously been tasked with describing their degree of baldness on a four-level severity scale, one ranging from virtually no baldness, to near total hair loss.

The researchers found that of 336 hospitalized men who tested positive for COVID-19 in the database, increasing baldness predicted COVID-19 positivity. The COVID-19 prevalence in hospitalized men was 15% for grade-one baldness, 16.8% for grade two, 18.1% for grade three and 20% for grade four. By contrast, increasing levels of baldness was evenly distributed among those who tested negative for the virus.

"While the exact mechanism remains unknown," the authors wrote, "severe androgenic alopecia seems to be associated with hospitalization for COVID-19." They added their belief that clinicians should incorporate baldness as a signal to identify those at greatest risk of doing poorly with COVID-19.

"I think it has to do with being male," says Mayo Clinic infectious disease specialist Dr. Greg Poland. "If you look at every country across the world, the numbers of those with infections are equally distributed among males and females, but deaths are slanted towards males."

Poland points to an emerging line of research spearheaded by a Johns Hopkins microbiologist. Dr. Sabra Klein has teased apart genetic and cellular machinery responsible for why men appear have less healthy immune systems in the face of novel viruses. While rates of COVID-19 are equally distributed between the sexes, more men have died from the illness, although the reason is unclear.

"I think it is a marker for being being male," Poland said, "or if you're a woman, having elevated male androgenic hormones. Obese women tend to have higher levels of male hormones. Many have a lot of hair loss. So I think that's the risk factor."

Still others have argued that baldness is sometimes a side effect of metabolic syndrome, a dietary disease of civilization which rests at the heart of the underlying obesity, inflammatory and cardiovascular conditions that put COVID-19 patients at greater risk of poor outcomes.

Noting that carbohydrates in the diet induce insulin resistance and an "endocrine cascade" leading to elevated production of androgens in both men and women, not to mention the decrease of a protein known as SHBG for the control of circulating androgen levels, the authors of a 2003 paper famously described male-pattern baldness, hypertension and coronary artery disease as all byproducts of a similar process.

"High glycemic-load carbohydrates, by inducing insulinemia ... represent a likely environmental agent that may in part underlie the promotion of male vertex balding," the authors Loren Cordain, Michael and Mary Eades wrote, a pattern "strongly associated with ... diseases, such as (coronary artery disease) and hypertension."

Be it through the dietary carbohydrates or gender, when it comes to the question of why some people do worse with COVID-19, it seems, hair loss has now become another piece of the puzzle.