Friday, June 4, 2010

Growth hormone secretion drops with age, but not exactly in the way you would expect

Many people assume that growth hormone secretion drops with age in a somewhat linear fashion, as implied by this diagram. This assumption probably stems from attempts to model growth hormone variations with linear regression algorithms. This assumption is wrong.

Actual plots of growth hormone secretion patterns, with age on the horizontal axes, tell a different story. See, for example, the graphs below, from professionalmuscle.com. They match the graphs one sees in empirical academic papers. The graphs below (click to enlarge) are particularly good at highlighting some interesting patterns of variation.


On the left side, bar charts show secretion patterns grouped by age ranges during a 24 h period (at the top), during wake time (at the middle), and during sleep (at the bottom). On the right side is the actual data used to build the bar charts. As you can see from the graphs on the right side, the drop in growth hormone secretion follows a pattern that looks a lot more like an exponential decay than a linear pattern.

The drop is very steep from 15 to 40 years of age, after which it shows some fluctuations, going up and down. Interestingly, people in their 50s and 60s, at least in this dataset, have on average higher growth hormone levels than people in their 40s. Of course this may be due to sample bias, but the graphs suggest that there is a major drop in growth hormone secretion, on average, around age 45.

As you can see, there is a lot of individual variation in growth hormone levels. If you look carefully at the graph on the top-right corner, you will see a 50 year old who has a higher 24 h growth hormone secretion than many folks in 15-30 age range. This pattern of individual variation is common for the vast majority of traits anyway, and often the distribution of traits follows a normal, or bell-shaped, distribution. The bell-shaped distribution becomes clear when the traits are plotted based on frequency.

Growth hormone is secreted in pulses. In case you are wondering, growth hormone secretion in young women is higher than in young men. See the graphs below (click to enlarge), from this excellent article on growth hormone by Cummings and Merrian.


Yet, women do not put on a lot of muscle mass in response to weight training, regardless of the age at which they do weight training. This means that growth hormone, by itself, does not lead to significant gains in muscle mass. Androgenic hormones, like testosterone, play a key moderator role here. Muscle mass gain is the result of a number of things, including the combined action of various hormones. To complicate things further, not only do these hormones act together in an additive fashion, but they also influence each other.

Another reasonable conclusion from the data above on growth hormone secretion in young women and men is that growth hormone must indeed have major health-promoting effects, as most of the empirical data suggests. The reason is that, from an evolutionary standpoint, young (or pre-menopausal) women have always been the evolutionary bottleneck of any population of ancestral hominids. High survival rates among young women were a lot more important than high survival rates among men in general, in terms of the chances of survival of any population of ancestral hominids.

Higher survival rates among young ancestral women may have been enabled by higher levels of growth hormone, among other things. The onset of the metabolic syndrome, which is frequently in modern humans around age 45, may also be strongly influenced by falling growth hormone levels.

How can growth hormone secretion be increased after age 45? One obvious option is vigorous exercise, particularly resistance exercise.

8 comments:

Anonymous said...

A comment: I just wanted to let you know that I appreciate your blog and the work you do. You also leave insightful comments on other blogs in the general paleo-sphere (which is how I was directed to your blog in the first place).

Kindke said...

I dont like the bar graphs, Its silly to group 26 yr olds with 35 yr olds, obviously your growth hormone doesnt just instantly drop by half on your 26th birthday.

And it looks like they had a really small sample size for the 36-60 age group.

Overall its comforting to know that growth hormone, although while it does decline with age, that the drop is not so aggressive as one would expect.

Whats surprising is the very high variance in the 15-25 age group. I reckon this is good evidence that lifestyle and diet play a big role in overall growth hormone levels ( as we already mostly know ). I guess genetics too but I hate discussing something which is essentially unchangeable :)

Btw Ned great blog, look forward to your next post.

Ned Kock said...

Thanks Anonymous.

I agree with you re. the sample Kindke. The variability among the young folks reflects a number of factors, as you point out. One of those factors is the MUCH higher GH levels in young women compared with young men.

Jamie Scott said...

Good post Ned. One thing I picked up on about a decade ago when I completed my ex physiology degree was that a lot of the modelling that suggested various parameters declined with age (GH, Testosterone, etc), were based on cross-sectional samples. That is, they would take a man in his 20's, one in his 30's, one in his 40's, and so on, and measure the differences. From this type of sampling, it was suggested many of these hormones decline with age. Similar sampling was done with VO2max.

However, when you begin to map out how people's lifestyles change over time, one can often see that it is lifestyle & dietary factors that might be driving the biggest part of the decline.

A man in his 20's might still be relatively active, but the likes of alcohol intake increases, there are lots of late nights, diet might go out the window, etc.

Then in their 30's they are typically beginning to work long hours with work, starting a family (so disrupted sleep patterns), doing less physical activity.

In their 40's they are perhaps at a management level with career so have a significant amount of stress, alcohol intake may begin to rise again, the exercise of choice (if it is done) is chronic cardio, carb intake may be increasing as does waistline...

In their 50's & 60's they might be beginning to feel their own mortality somewhat and begin to make various lifestyle adaptations, including less stress, more exercise, better eating, etc - this mighth explain the increase in GH seen in this group.

When someone does a longitudinal study that follows a sample group over a 40-50 year period, getting the group to maintain primal-type living and shows a large decline over the ageing period, then I might believe this to be the case. At present, I'm not convinced that declines in hormone profiles & other physiological parameters are as big or as tied to age as what we have been historically lead to believe.

Ned Kock said...

Hi Jamie.

Very good points. And the data in this post is definitely cross-sectional. I would like to get my hands on extensive longitudinal data, but it is not easy to find.

Even the good longitudinal studies available, like the Framingham Heart Study, have only data on a very limited set of health markers. For example, for mean glucose, they have random glucose measures:

http://healthcorrelator.blogspot.com/2010/04/blood-glucose-control-before-age-55-may.html

The scenario you describe fits well, generally speaking, with the graphs on this plot though. A drop of GH in the 40s, and then some lifestyle adaptations bringing GH levels up in the 50s.

Byron said...

Great post and just in time.
2 weeks ago I started with l-tyrosine. Just for mood/stress issues. Side effect is an unexpected muscle growth. With more than one year of zero success by weight lifting a huge surprise. After more than 14 years LC and since one year VLC/keto I can´t believe that there was a amino deficiency. Maybe somekind of kickstart for a slow metabolism. Greetings.

Ned Kock said...

Hi Byron.

Tyrosine supplementation might have help reduce stress, which reduced cortisol levels. Elevated cortisol levels inhibit muscle growth and accelerate muscle catabolism.

In this sense, tyrosine supplementation may have had an effect similar to that of behavioral stress management.

viagra online said...

I think that the people didn't know about it because they do not have the chance of thinks in that many bar charts show secretion patterns grouped by age ranges during a 24 h period !!22dd