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鈥淩EDUCED sex drive? Decreased energy? Unwanted mood or body changes? Is it low testosterone?鈥�

TV adverts asking these questions have been bombarding American men for the past five years, often accompanied by images of handsome ageing men shooting basketball hoops or sharing romantic moments with attractive women.

The remedy, they suggest, is testosterone supplements; often billed as a quick ticket back to youth, fitness and virility. Elsewhere, the marketing is less pervasive, but worries about the physical deterioration that accompanies male ageing are similar.

But what if we鈥檝e got testosterone all wrong? Testosterone is widely perceived as the masculinity hormone; associated with virility, strength and competition, so it鈥檚 understandable that men might reach for it to restore a flagging mojo. However, testosterone may not be the express ticket back to youth many perceive it to be. Boosting levels may even be harmful. Neither does testosterone only facilitate stereotypically male behaviours like aggression and competitiveness; it may have a gentle side. 鈥淧op culture has promoted the idea that if enough [testosterone] is good, more is even better,鈥� says Bradley Anawalt at the University of Washington Medical Center in Seattle. 鈥淏ut it doesn鈥檛 make you a better man.鈥�

Between 2000 and 2011, prescriptions for testosterone supplements in the US more than quadrupled (see graph) and in 2011 an estimated . Although nowhere near US figures yet, sales of testosterone supplements in the UK, Australia and Germany are also on the increase.

Hany Omar, a 42-year-old mortgage broker in Washington DC, is among the growing ranks of testosterone aficionados. He had his hormone levels tested after he started to feel overly tired after regular gym workouts. At 380 nanograms per decilitre (ng/dl), his testosterone level was well within the normal range. There also seemed to be nothing wrong with his overall health, so he found a urologist willing to prescribe testosterone to anyone who tested below 400 ng/dl. Since last summer he has been injecting himself with testosterone every two weeks. 鈥淢y libido is stronger, I have more energy, I have great workouts,鈥� Omar says. 鈥淚 don鈥檛 want to call it the fountain of youth, but that鈥檚 what it feels like.鈥�

Man enough?

Testosterone is a product that caught the public鈥檚 fancy long before anyone could fully explain its merits, perils, or whether the balance between them justified its growing presence in men鈥檚 medicine cabinets. First synthesised in the 1930s, it was used for decades to treat conditions in which men are either born with extremely low testosterone, or lose their ability to produce it because of serious illness or injury. Testosterone drives the development of the testes, prostate gland and secondary sexual characteristics like body hair; in adult men it regulates sex drive, bone mass, muscle strength, and the production of sperm and red blood cells. 鈥淵ou need to have normal amounts of testosterone to feel good and maintain muscle mass and so forth,鈥� Anawalt says.

Of course, testosterone can also influence behaviour. Male swaggering and boasting are often dismissed as a displays of testosterone, as is aggression and risk-taking. This reputation possibly stems from a series of studies starting in the 1970s, which correlated extremely high testosterone with aggressive behaviour in criminals. More recently, high testosterone has been linked to stock market success among city traders.

However, recent research is painting a far more nuanced portrait of this hormone; suggesting that it prompts us to act in ways that might boost our social status 鈥� be that through aggression, or other, gentler means. 鈥淭his hormone is smart,鈥� says Jack van Honk of Utrecht University in the Netherlands. 鈥淚t finds other ways to reach dominance, without reverting to cheating or aggression.鈥�

Some of the evidence comes from a surprising source: women. All adult women produce testosterone, but in far smaller quantities than men, making it easier to assess how a temporary testosterone boost affects behaviour. 鈥淓ven though hormonal systems are not exactly the same in men and women, there are a lot of similarities in the way basic social behaviours are influenced by hormones, especially testosterone,鈥� van Honk says.

In one study, he and his colleagues asked women to view photos of strangers and rate them according to how trustworthy they looked. Women who received testosterone supplements were , but only if they were naturally very trusting 鈥� suggesting it might boost wariness among those who are at high risk of being misled by others, van Honk says.

In other situations, testosterone appears to boost generosity. In one study, women were given 鈧�20, and asked if they鈥檇 like to invest some of it in another player, the 鈥渢rustee鈥�. Any invested money would be trebled, but it would be down to the trustee to decide whether to give any money back. Investors given a dose of testosterone tended to make stingier investments, but trustees who were given with their payback 鈥� as if repaying the trust that had been placed in them. This supports the idea that testosterone adjusts behaviour to the situation at hand, says Maarten Boksem at Erasmus University in the Netherlands, who led the study. 鈥淏eing pro-social and actually taking care of the people we need is very important for maintaining high social status.鈥�

鈥淏eing pro-social and actually taking care of the people we need is very important for maintaining high social status鈥�

The conventional wisdom is that levels of testosterone peak at around the age of 20 and then slowly and steadily decline. Young men are often perceived to be less risk-averse and more aggressive. Combined with testosterone鈥檚 known role in physical development, this gave rise to the understandable conclusion that the reduced virility and vitality that occurs with age must be linked to testosterone deficiency. And when easy-to-apply topical testosterone treatments such as skin patches and gels began hitting the market in the early 2000s, testosterone took on a whole new persona as a cure-all for anything that might ail the ageing male, from erectile dysfunction to unwanted body fat. This despite a growing body of evidence suggesting that many of the major attributes assigned to testosterone might, in fact, be myths (see 鈥�Testosterone Truths鈥�).

Inevitable decline

Yet it is far from clear what constitutes a normal decline of testosterone as men age. One problem is that there is widespread disagreement about what level of testosterone counts as abnormally low. Surprisingly, there isn鈥檛 even a standard method of assessing it, meaning that sending the same blood sample to different labs can give different results. Testosterone levels can vary widely depending on several factors, including the time of day and activity levels. 鈥淭here鈥檚 also a problem that the 鈥榥ormal鈥� ranges for one laboratory may be decidedly different than they are for another laboratory,鈥� says Ronald Swerdloff, chief endocrinologist at Harbor UCLA Medical Center in California. Having recognised this, the US Centers for Disease Control are now trying to standardise testosterone testing.

Further clues about what testosterone is doing to our brains and bodies come from another unlikely source 鈥� a remote Amazon tribe called the Tsimane.

Curiously, baseline levels of the hormone are a third lower on average among Tsimane men than in their American counterparts. They鈥檙e certainly no less manly, at least in the conventional sense; they survive by hunting, fishing and farming. But recent research suggests that it鈥檚 not necessarily how much testosterone you start with, but how your body responds to it that counts. It also hints that a person鈥檚 baseline testosterone level 鈥� the focus of most clinical tests 鈥� may be a poor marker of what鈥檚 going on in their bodies.

In one study, Tsimane men provided saliva samples before and immediately after competing in a soccer match, which were tested for testosterone. Playing soccer caused testosterone levels to rise by about 30 per cent, just as they do in American men in similar scenarios. Further studies indicate that it also shoots up when Tsimane men are hunting or clearing horticultural plots 鈥� activities that raise their social status in the community, says Benjamin Trumble, an anthropologist at the University of Washington, Seattle, who led the research. The reason for these spikes is probably physical as well as behavioural; one of testosterone鈥檚 most important jobs is to supply energy to muscles.

The implication is that your ability to mobilise the hormone at times of need is potentially more important than how much testosterone you start with. 鈥淢en who can still chop trees in their 80s have the same relative increases in testosterone as men in their 20s and 30s,鈥� says Trumble, who has tested Tsimane men of various ages.

The idea of an inevitable plummet in testosterone with age is also called into question by studies of Tsimane men, whose levels seem to remain relatively constant throughout adult life. A recent study of 1500 Australian men suggested that they too might maintain reasonable levels of testosterone into old age 鈥� provided they maintain a healthy lifestyle. The five-year study showed an average 0.8 per cent decline in testosterone each year 鈥� but men who accounted for most of the decline. Removing them made the decline too small to be clinically significant.

Yet other studies also hint that avoiding weight gain and staying active could be crucial for enabling testosterone to do its job. When testosterone comes into contact with fat tissue, it changes into a form of oestrogen, thanks to an enzyme in fat cells called aromatase. Yet the area of the brain that regulates testosterone production doesn鈥檛 detect this change because the hormones have such similar structures. As a result, normal testosterone signalling in the body becomes confused. To make matters worse, oestrogen promotes the laying down of even more body fat, which causes further confusion.

It could be tempting, then, to think that taking a supplement would be a good idea for ageing men 鈥� particularly those developing a middle-age spread. Even if men can get by with lower levels of testosterone like the Tsimane do, surely very low levels of it are unhealthy? However, testosterone supplements might potentially do more harm than good. A recent study by Bu Yeap at the University of Western Australia and colleagues found that both high and low testosterone levels were associated with a shorter lifespan than middling levels of the hormone.

And in January, the US Food and Drug Administration announced that it was reviewing the safety of marketed versions of the hormone in the wake of suggesting an among men taking it. The European Medicines Agency announced a similar review in April.

Since testosterone can boost red blood cell numbers, one possible explanation is that it causes the blood to thicken, which might increase the risk of clots 鈥� effects the makers of gels and other topical products already state on their labels. However, much remains unknown, and the studies suggesting harmful effects were observational, not the double-blind, placebo-controlled studies generally relied upon to assess a drug鈥檚 risk.

Besides heart attacks, other studies have hinted that high testosterone levels might raise the risk of prostate cancer. Since cancer and heart problems are diseases connected with age, one suggestion is that the natural decline of hormones may be protecting us from diseases of ageing. 鈥淚f humans evolved certain systems to prevent or postpone disease, then the gradual reduction in testosterone level [with age] is something the body is doing to prevent damage,鈥� says John Hoberman, author of Testosterone Dreams. Proving this idea is difficult, however, because establishing the precise causes of age-related diseases is tricky.

鈥淭he natural decline of hormones as we age may be protecting us from disease鈥�

How do you keep your testosterone levels on an even keel rather than see them drop off a cliff? Artificially boosting testosterone may seem like a quick fix but we are still a long way from knowing whether such a strategy is either smart or safe. 鈥淭he medical community and patients are abusing testosterone therapy in ways that are completely outside of the evidence,鈥� says Daniel Shoskes, a urologist at the Cleveland Clinic in Ohio. 鈥淎 drug with such potential for both benefit and harm shouldn鈥檛 just be thrown around like this.鈥�

Why not adopt a healthy lifestyle 鈥� a study of 900 men found that obese men who lost an average of 8 kilograms through diet and exercise over a year had a corresponding increase in testosterone levels.

Given how patchy our understanding of testosterone is, a sensible first step for older men with a flagging mojo might be to reach for the running shoes, rather than testosterone supplements.

This article appeared in print under the headline 鈥淏igger than butch鈥�