Why is Puberty Starting Earlier for Kids? | Dr. Andrew Huberman
00:00:00.000 | [silence]
00:00:02.400 | We know that one of the things
00:00:04.220 | that can trigger the onset of puberty,
00:00:05.860 | in particular in females,
00:00:07.900 | is that when enough body fat accumulates,
00:00:11.420 | that body fat releases a hormone called leptin.
00:00:14.860 | And that hormone leptin travels in the bloodstream
00:00:17.580 | across the blood-brain barrier
00:00:18.880 | and goes to the hypothalamus
00:00:20.420 | and can trigger the onset of puberty
00:00:22.020 | by activating the neurons
00:00:24.060 | that release gonadotropin-releasing hormone.
00:00:26.720 | So many people believe that one of the reasons
00:00:29.380 | that puberty is happening earlier and earlier in females
00:00:32.880 | is because of the accumulation of more body fat
00:00:36.340 | at younger ages than was observed 30 or 40
00:00:40.260 | and certainly 100 years ago.
00:00:42.020 | Now, I can already imagine a number of people are thinking,
00:00:44.740 | oh, this must relate to the obesity crisis.
00:00:46.980 | And indeed, there is a crisis of obesity.
00:00:48.860 | Obesity is something that is causing all sorts of problems
00:00:53.860 | with people's health at various levels, brain and body,
00:00:57.140 | and that is far more frequent today
00:00:58.780 | than it was even 20 years ago.
00:01:00.820 | So it is indeed a crisis
00:01:02.200 | because it has enormous detrimental effects
00:01:04.440 | for so many aspects of brain and body health and longevity.
00:01:09.420 | But this whole process of thinking about body fat
00:01:11.900 | signaling leptin to the hypothalamus
00:01:13.460 | and the onset of puberty
00:01:14.340 | doesn't necessarily have to do with the obesity crisis.
00:01:17.500 | It might relate, but it could also relate to,
00:01:20.060 | for instance, improved nutrition,
00:01:21.500 | which is allowing body fat stores to accumulate,
00:01:24.380 | maybe not to the level of obesity, but to accumulate earlier
00:01:28.140 | and at younger ages in females,
00:01:29.700 | which is then causing earlier puberty in females.
00:01:33.460 | To just highlight how that might be possible,
00:01:35.780 | I want to review some data
00:01:37.180 | that talk about the onset of menses, menstruation,
00:01:40.700 | that is puberty, in females according to country
00:01:44.220 | and according to age over the last 100 or more years.
00:01:47.980 | So what are the general trends
00:01:49.460 | in terms of the onset of puberty in females?
00:01:51.240 | Well, that's an easy one to answer.
00:01:54.020 | Over the last 100 years or so,
00:01:57.240 | the onset of puberty has been occurring much earlier
00:02:01.460 | with each passing decade.
00:02:02.940 | It's really an incredible set of statistics.
00:02:06.320 | I will provide a link to these data
00:02:07.780 | since I know a number of you are listening
00:02:09.100 | and not just watching on YouTube.
00:02:10.780 | This is from a study in which the onset of puberty
00:02:14.500 | has been analyzed from as early as the 1850s, right?
00:02:18.300 | In certain countries, there are data on that,
00:02:20.660 | out to the 1970s and in other countries,
00:02:22.500 | starting at about 1900, extending out to about 1990.
00:02:26.540 | These are ongoing collections of data,
00:02:29.100 | but just to give you a sense
00:02:30.500 | how the data are falling out
00:02:32.140 | in a couple of different countries,
00:02:33.500 | just to give you a flavor,
00:02:35.220 | but for those of you listening
00:02:37.260 | and for those of you watching,
00:02:39.340 | the essence of all of these findings
00:02:41.500 | is that puberty is happening much, much earlier
00:02:43.900 | with each passing decade.
00:02:45.180 | So for instance, in the United States,
00:02:48.640 | around 1900 or 1903, the average age of menarche,
00:02:53.140 | the onset of puberty in females was about 14 years old,
00:02:57.140 | whereas in 1990, the average age is 11.
00:03:01.020 | So that's a pretty significant,
00:03:02.660 | we can say acceleration of the onset of puberty.
00:03:06.660 | Now, of course, these are averages,
00:03:08.020 | so there will be exceptions.
00:03:09.460 | There's a distribution of data.
00:03:11.660 | Today still, there will be young females
00:03:14.100 | who will undergo puberty at age 11 or 10,
00:03:16.540 | or maybe even nine and others who will undergo puberty
00:03:18.900 | at age 13, 14, maybe even 16 or 17.
00:03:22.500 | However, if we look at, for instance,
00:03:24.980 | the data from Norway, which dates back quite far,
00:03:27.460 | they have excellent record keeping to 1850,
00:03:30.740 | what we see is that the average age
00:03:32.860 | of the onset of female puberty in 1850 in Norway
00:03:36.880 | was 17 years old,
00:03:38.780 | whereas in 1970, it's 13 years old.
00:03:42.780 | So this is a dramatic acceleration of the onset of puberty.
00:03:46.460 | And you see a similar trend in other countries as well.
00:03:48.620 | So if we were to look in, for instance, in the UK,
00:03:52.180 | they have a smaller data set,
00:03:54.060 | meaning it only extends back to about 1940,
00:03:56.820 | but the average age of the onset of puberty in the UK
00:04:00.300 | in 1940 was 13 and a half years old.
00:04:02.740 | Again, this is just for females.
00:04:04.720 | And in 1970, it was closer to 13
00:04:07.580 | with a trend towards declining even further.
00:04:09.940 | Unfortunately, they didn't continue to collect data
00:04:11.880 | out to 2022.
00:04:13.900 | And as a final point, if we were to look at,
00:04:16.120 | for instance, in Germany and Finland,
00:04:18.500 | the average onset of puberty in 1870
00:04:22.040 | was 16 and a half years old.
00:04:24.020 | By 1940, it was down to 13 and a half years old.
00:04:27.300 | So all of these data have borne out over and over again,
00:04:31.340 | regardless of location in the world,
00:04:33.020 | which is important because when you start to think
00:04:34.500 | about the obesity crisis, you can say,
00:04:35.900 | well, that's mainly in developed countries,
00:04:37.940 | believe it or not, or perhaps not surprisingly.
00:04:40.340 | And maybe it has to do with the obesity crisis.
00:04:42.580 | And yet I don't think we can conclude that at all.
00:04:45.380 | Something is happening, however.
00:04:47.380 | It could be increased body fat stores
00:04:50.220 | due to overeating and obesity.
00:04:51.700 | However, it could also be unrelated to obesity.
00:04:54.440 | It could be, for instance, improved nutrition
00:04:57.420 | and the availability of quality nutrition,
00:05:00.300 | which can signal the maturation
00:05:02.340 | of the brain and body mechanisms
00:05:03.840 | that trigger the onset of puberty,
00:05:06.080 | ovulatory cycle, and menstruation.
00:05:07.960 | So we want to be very careful about leaping to conclusions
00:05:10.400 | about what these trends mean,
00:05:11.640 | but the trends themselves are very, very apparent.
00:05:14.820 | And as a final point, I should also mention
00:05:16.740 | that there are a number of different behavioral
00:05:19.000 | and psychosocial, as they're called,
00:05:21.160 | interactions that can influence puberty as well.
00:05:23.720 | This has been most strikingly observed in animals.
00:05:26.720 | And so I don't want anyone to be alarmed
00:05:28.400 | or to leap to any great conclusions
00:05:30.240 | about the onset of timing of puberty in humans,
00:05:32.760 | but I'd be remiss if I didn't tell you
00:05:34.600 | about a certain result which shows
00:05:36.520 | that if a young female is exposed to the odor,
00:05:41.520 | not necessarily the pheromones, right?
00:05:43.200 | There's a distinction between odors that we perceive
00:05:45.800 | and pheromones, which are subconscious, right?
00:05:48.880 | That we don't actively perceive,
00:05:50.440 | but that can impact our biology.
00:05:52.440 | And pheromone effects in humans are very controversial.
00:05:54.600 | But we know, for instance, that if you take a female animal,
00:05:57.600 | and there's some evidence from humans
00:05:59.760 | that if you take a young pre-pubertal female
00:06:02.600 | and you expose her to the scent
00:06:06.400 | of a reproductively competent male for a series of days,
00:06:11.400 | but maybe even as short as a few hours,
00:06:16.020 | and she is also not regularly being exposed
00:06:19.740 | to the scent of her father,
00:06:21.780 | that she can undergo puberty earlier.
00:06:24.080 | That's right.
00:06:24.920 | There is something about the odor and/or pheromones,
00:06:28.740 | or perhaps something else that occurs
00:06:31.920 | when a young pre-pubertal female has a father
00:06:36.420 | that she's in regular contact with.
00:06:37.780 | He wouldn't necessarily have to live at home,
00:06:39.240 | but that is around a lot, that his smell, excuse me,
00:06:42.820 | is registered by her biological systems.
00:06:45.860 | That, I don't want to say protects,
00:06:48.020 | because it kind of skews the valence of the conversation,
00:06:51.180 | but that offsets or buffers the otherwise observed effect,
00:06:56.180 | which is that the scent of a reproductively competent male,
00:07:00.900 | if it's present often enough, or perhaps intensely enough,
00:07:04.140 | that it can trigger the onset of puberty in that female.
00:07:06.460 | In other words, the scent of a male that is not the father,
00:07:11.260 | and we think also that is not biologically related to her,
00:07:14.240 | can trigger earlier onset of puberty.
00:07:17.100 | And that effect can at least be partially buffered
00:07:21.580 | by her being in the presence of the scent
00:07:25.100 | from her biological father.
00:07:27.100 | Now, some of you are probably already leaping
00:07:29.020 | to conclusions about what this means.
00:07:30.540 | Should you not allow your daughter to be exposed
00:07:33.300 | to any males who are of reproductive age, et cetera?
00:07:35.940 | And that's certainly not what I'm saying.
00:07:37.860 | There's a huge number of considerations
00:07:39.300 | that go into that calculation for everybody
00:07:41.780 | and circumstances, et cetera.
00:07:43.700 | But the point is that the odors of individuals,
00:07:48.160 | both related, in particular, closely related,
00:07:51.400 | and non-related individuals,
00:07:54.600 | can shape the neural systems and the hormone systems
00:07:59.480 | that can trigger the onset of puberty
00:08:01.000 | or suppress the onset of puberty.
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