We know that one of the things that can trigger the onset of puberty, in particular in females, is that when enough body fat accumulates, that body fat releases a hormone called leptin. And that hormone leptin travels in the bloodstream across the blood-brain barrier and goes to the hypothalamus and can trigger the onset of puberty by activating the neurons that release gonadotropin-releasing hormone.

So many people believe that one of the reasons that puberty is happening earlier and earlier in females is because of the accumulation of more body fat at younger ages than was observed 30 or 40 and certainly 100 years ago. Now, I can already imagine a number of people are thinking, oh, this must relate to the obesity crisis.

And indeed, there is a crisis of obesity. Obesity is something that is causing all sorts of problems with people's health at various levels, brain and body, and that is far more frequent today than it was even 20 years ago. So it is indeed a crisis because it has enormous detrimental effects for so many aspects of brain and body health and longevity.

But this whole process of thinking about body fat signaling leptin to the hypothalamus and the onset of puberty doesn't necessarily have to do with the obesity crisis. It might relate, but it could also relate to, for instance, improved nutrition, which is allowing body fat stores to accumulate, maybe not to the level of obesity, but to accumulate earlier and at younger ages in females, which is then causing earlier puberty in females.

To just highlight how that might be possible, I want to review some data that talk about the onset of menses, menstruation, that is puberty, in females according to country and according to age over the last 100 or more years. So what are the general trends in terms of the onset of puberty in females?

Well, that's an easy one to answer. Over the last 100 years or so, the onset of puberty has been occurring much earlier with each passing decade. It's really an incredible set of statistics. I will provide a link to these data since I know a number of you are listening and not just watching on YouTube.

This is from a study in which the onset of puberty has been analyzed from as early as the 1850s, right? In certain countries, there are data on that, out to the 1970s and in other countries, starting at about 1900, extending out to about 1990. These are ongoing collections of data, but just to give you a sense how the data are falling out in a couple of different countries, just to give you a flavor, but for those of you listening and for those of you watching, the essence of all of these findings is that puberty is happening much, much earlier with each passing decade.

So for instance, in the United States, around 1900 or 1903, the average age of menarche, the onset of puberty in females was about 14 years old, whereas in 1990, the average age is 11. So that's a pretty significant, we can say acceleration of the onset of puberty. Now, of course, these are averages, so there will be exceptions.

There's a distribution of data. Today still, there will be young females who will undergo puberty at age 11 or 10, or maybe even nine and others who will undergo puberty at age 13, 14, maybe even 16 or 17. However, if we look at, for instance, the data from Norway, which dates back quite far, they have excellent record keeping to 1850, what we see is that the average age of the onset of female puberty in 1850 in Norway was 17 years old, whereas in 1970, it's 13 years old.

So this is a dramatic acceleration of the onset of puberty. And you see a similar trend in other countries as well. So if we were to look in, for instance, in the UK, they have a smaller data set, meaning it only extends back to about 1940, but the average age of the onset of puberty in the UK in 1940 was 13 and a half years old.

Again, this is just for females. And in 1970, it was closer to 13 with a trend towards declining even further. Unfortunately, they didn't continue to collect data out to 2022. And as a final point, if we were to look at, for instance, in Germany and Finland, the average onset of puberty in 1870 was 16 and a half years old.

By 1940, it was down to 13 and a half years old. So all of these data have borne out over and over again, regardless of location in the world, which is important because when you start to think about the obesity crisis, you can say, well, that's mainly in developed countries, believe it or not, or perhaps not surprisingly.

And maybe it has to do with the obesity crisis. And yet I don't think we can conclude that at all. Something is happening, however. It could be increased body fat stores due to overeating and obesity. However, it could also be unrelated to obesity. It could be, for instance, improved nutrition and the availability of quality nutrition, which can signal the maturation of the brain and body mechanisms that trigger the onset of puberty, ovulatory cycle, and menstruation.

So we want to be very careful about leaping to conclusions about what these trends mean, but the trends themselves are very, very apparent. And as a final point, I should also mention that there are a number of different behavioral and psychosocial, as they're called, interactions that can influence puberty as well.

This has been most strikingly observed in animals. And so I don't want anyone to be alarmed or to leap to any great conclusions about the onset of timing of puberty in humans, but I'd be remiss if I didn't tell you about a certain result which shows that if a young female is exposed to the odor, not necessarily the pheromones, right?

There's a distinction between odors that we perceive and pheromones, which are subconscious, right? That we don't actively perceive, but that can impact our biology. And pheromone effects in humans are very controversial. But we know, for instance, that if you take a female animal, and there's some evidence from humans that if you take a young pre-pubertal female and you expose her to the scent of a reproductively competent male for a series of days, but maybe even as short as a few hours, and she is also not regularly being exposed to the scent of her father, that she can undergo puberty earlier.

That's right. There is something about the odor and/or pheromones, or perhaps something else that occurs when a young pre-pubertal female has a father that she's in regular contact with. He wouldn't necessarily have to live at home, but that is around a lot, that his smell, excuse me, is registered by her biological systems.

That, I don't want to say protects, because it kind of skews the valence of the conversation, but that offsets or buffers the otherwise observed effect, which is that the scent of a reproductively competent male, if it's present often enough, or perhaps intensely enough, that it can trigger the onset of puberty in that female.

In other words, the scent of a male that is not the father, and we think also that is not biologically related to her, can trigger earlier onset of puberty. And that effect can at least be partially buffered by her being in the presence of the scent from her biological father.

Now, some of you are probably already leaping to conclusions about what this means. Should you not allow your daughter to be exposed to any males who are of reproductive age, et cetera? And that's certainly not what I'm saying. There's a huge number of considerations that go into that calculation for everybody and circumstances, et cetera.

But the point is that the odors of individuals, both related, in particular, closely related, and non-related individuals, can shape the neural systems and the hormone systems that can trigger the onset of puberty or suppress the onset of puberty. (upbeat music) (upbeat music)