Why are testosterone levels falling?

Forget war, forget climate change, recessions, pandemics. Today we’ll talk about a real crisis. The decrease of testosterone levels. Just look at all those headlines. There must be something going on. But what? Are testosterone levels really falling? If so, why? And how much of a crisis is it? That’s what we’ll talk about today.

The worry that men are becoming too feminine isn’t new. It’s been in the newspapers since there’ve been newspapers. The blame has, among other things, been put on juice boxes, sleek electronics, face creams, lilac pajamas and embroidered bathrobes, marxism, and living in high rise buildings. New is however the trend to self-medicate with testosterone pills. In the US, annual prescription sales for testosterone supplements have increased from 18 million dollars in 1988 to 70 million in 2000 to more than 2 billion in 2013.

The probably most prominent advocate for boosting your testosterone levels is Tucker Carlson, an American TV host. He’s seriously worried about the supposed decline of manliness and, among other things, suggests that men tan their balls to increase their testosterone levels. This is what his vision of the future man looks like.

So I made a PhD in physics and somehow ended up on YouTube talking about people tanning their balls. How do I explain this to my mom?

Apparently the idea started with a paper from 1939 by researchers from the Psychiatric Unit of the Boston State Hospital. The irradiated five patients with a mercury lamp in different body parts and found that the largest increase of testosterone levels happened when the target was the scrotum.

I don’t know want to know what else happened at that place. But even leaving aside the somewhat questionable circumstances, 5 patients in a psychiatric unit are not a representative sample for half the world population. There’s no evidence that irradiating your family jewels will do your testosterone levels any good. And the US Food and Drug Administration cautions against the use of testosterone unless there’s an underlying medical condition. They say that the benefits and safety of testosterone supplements have not been established.

But let’s step back from the testosterone craze for a moment and talk about the scientific basic. Testosterone and estrogen are the most important human sex hormones. We all have both, but men have higher testosterone levels, while women have higher estrogen levels. Testosterone in men is, among other things, responsible for changes during puberty, muscle mass and strength, hair growth, and sperm production.

Men with low testosterone levels may suffer from fatigue, depression, and sexual dysfunction. But too high testosterone levels also aren’t good. One of the consequences is muscle growth, which is why the stuff’s used for doping. But the side-effects are mood swings, aggressive and risk-taking behavior, skin problems, hair loss, and elevated cholesterol levels. Also, if the testosterone level is too high, the body tries to produce less of it, which leads to a reduced sperm count and shrinking of the testicles. And since the body converts part of the testosterone to a form of estrogen, high testosterone levels can, among other things, lead to breast development. So, more isn’t always better.

All of which brings up the question how much testosterone is normal? This question is surprisingly difficult to answer.

To begin with, testosterone levels change during the day, especially in young men, where they peak in the early morning. That’s why testosterone levels are measured in the morning, and another reason why early-morning classes should be illegal.

But that’s not the only reason testosterone levels vary. According to a 2020 study testosterone levels change with the seasons and are higher in summer. They are also known to change with partnership status. According to a Harvard University study published in 2002, married men have lower levels of testosterone than single men, and the more time they spend with family, the lower the testosterone level. Other studies have shown that men’s testosterone levels drop when holding an infant, or even a baby doll, and that the level goes up again after divorce.

That’s all very interesting, but these are all quite small effects. What we want to know is what’s a normal average level?

In 2014, a group of researchers did a meta-analysis to find out. They collected the data of 13 previously published studies and found that testosterone blood levels in men peak at about 19 years of age with a mean value of 15 point 4 nanomoles per liter. They then fall slightly to about 13 point 0 by age 40. Be careful, this plot has a log scale on the vertical axis. The authors found no evidence for a further drop in mean testosterone with age, although the variation increases as men get older.

Medical guidelines in the United States currently say everything above 11 point 1 nanomoles per liter is normal, below 6 point 9 is too low, and in between there’s a grey area where they send you from one doctor to another until one can decide what to do with you. In Europe they think men should have a little more testosterone and the guidelines are 12 and 8 nanomoles per liter, respectively.

Now that we know what we’re talking about, what’s the deal with the falling testosterone levels? First off – the headlines are right. Testosterone levels really are falling. This has been backed up by several independent studies. And it isn’t even news.

One of the best-known papers about this was published by researchers from the US in 2007. They tracked three groups of randomly selected men from Boston, during the 1980’s, 1990’s and 2000’s.

They found that the later-born men had lower testosterone levels at the same age as the earlier born ones. You can see this in this graph, where the solid lines are the mean values, and the dotted lines are the 95 percent confidence levels.

Don’t get confused about the numbers on the vertical axis. This graph uses different units than the ones we had before, but even so you can clearly see that indeed testosterone levels are falling. For example, a 70-year old man in the first study group from the 1980s had a higher total testosterone level than the youngest man in the second group. They found that the average levels declined by about 1 percent per year, so men born 15 years later would have 15 percent lower testosterone levels at the same age.

In case you think something odd is going on in Bostonin particular, similar studies have found the same elsewhere in the US and also in Europe. For example, a Finnish study from 2013 found that older generations of men had higher testosterone levels at any given age range compared to younger generations. It’s not a small difference.

For example, for men aged 60-69 years born in between 1913 and 1922, testosterone levels were 21 point 9 nanomoles per liter, but for those born between 1942 and 51, it was only 13 point 8. Finnish men born during the 1910’s had more testosterone in their sixties than men born in the 1970’s when they were in their twenties.

It's not just testosterone, and it’s not just men. Strength levels seem to be decreasing in general. A 2016 study measured the grip strength of about 250 healthy full-time studentsaged 20 to 34 at Universities in North Carolina. They compared the results to measurements from 1985 and found that grip strength had significantly increased both for men and for women. It seems firm handshakes really are going out of style.

But in all fairness, this wasn’t a particularly large study. But here’s another example from a meta-analysis of 50 studies that included a total of 25 million children, age 9 to 17, from 28 countries. It was published by a group of Australian researchers in 2013. They reported that children today take 90 seconds longer to run a mile than kids did 30 years ago, and that seems to be a global trend.

Okay, so we’re all getting weaker and slower and spend our days watching YouTube. But why? The brief answer is that no one really knows, there’s just speculation, so let’s have a look at the speculations. First of all, the changes happen too quickly to be genetic adaptations.

But one suspect factor is food. According to a recent meta-analysis done by researchers from the UK eating too much protein can significantly decrease testosterone levels. They found that diets with more than 35 percent protein decreased testosterone levels by 37 percent. 35 percent protein is a lot. The average person in the developed world eats less than half of that, so it doesn’t explain the observed trend. But if you only eat meat, it quite possibly has consequences.

A related issue is the increasing number of obese people, which we just talked about some weeks ago. We know that in men, a high body mass index is correlated with lower testosterone levels. And according to a 2014 paper by researchers from Australia, the causation goes both ways. That is low testosterone can cause weight gain, and weight gain lowers testosterone levels, which can create a self‐perpetuating cycle of metabolic complications. However, the studies that documented the fall in testosterone levels found it even for men with normal body mass index, so this doesn’t explain it either.

Another factor might be smoking, or rather, the lack thereof. That’s because some chemicals contained in tobacco prevent testosterone from converting to other hormones, which can increase the mean testosterone level. A meta analysis from 2016 found that men who smoked had higher mean testosterone levels than non-smokers with the difference being about 1 point 5 nanomoles per liter. In women the difference wasn’t statistically significant. So, the overall decline in smoking might have had an impact on the overall decline in testosterone levels. But again, that alone doesn’t explain it.

What are we to make of this? It seems plausible to me that several factors are at play. As we have seen, testosterone levels change with living circumstances. The world is a more comfortable place today than 50 years ago, so maybe testosterone just isn’t needed as much as it used to. And then the changes in diet and smoking add on top of that. Is that enough to explain it? I don’t know. As scientists like to say “more work is needed”.

Is it something to worry about? Well, that depends on what you want the world to be. Carl Sagan once referred to testosterone as a poison that causes conflict. He said “Why is the half of humanity with a special sensitivity to the preciousness of life, the half untainted by testosterone poisoning, almost wholly unrepresented in defense establishments and peace negotiations worldwide?” However, he then continued, “Testosterone also causes the kind of aggression needed to defend against predators, and without it, we’d all be dead. [...] Testosterone is there for a reason. It’s not an evolutionary mistake.”

Personally I see the decrease of testosterone levels more as a reaction to our changing environment than reason for concern. The world changes and we change with it. We study tree rings to find out which years were good years and which years were bad years for the trees. And maybe in ten thousand years from now, scientists will study testosterone levels to find out which times were good times and which times were bad times for us.