(Retriever, I think this post will also address some of your questions from your excellent comment on my last post!).
Previous studies of anorexia and birth month were too small to be of much use. You need a lot of cases to detect differences between patients and controls if season increases risk marginally. In this study, the researchers combined the results from several different UK studies, and then used some fancy statistical techniques such as "harmonic and spectral analysis" as opposed to the traditional chi squared test to wring as much out of the data as they could.
All told, 1293 cases of anorexia nervosa were compared to births in the general population from 1950-1980. The results:
Here is the pertinent graph (reproduced here with permission) and the source: linked at the British Journal of Psychiatry.
You will see there is a bit of a spike of anorexia cases for people born in the springtime, and a yearly nadir in October.
The discussion is short but sweet (and as the paper is free full text, I encourage you to take a look from the link in the first paragraph).
What these sorts of results show us is that early life and the neonatal period can plausibly be a time when suboptimal environments can eventually show up as disease later in life. Apparently there was a finding recently that an interaction between a dopamine receptor gene and season of birth influences body weight regulation in women with bulimia (1). But here is a problem with season-of-birth research - pregnancy is 9 months long, and obviously development occurs during all of that time, with vulnerable developmental windows of different sorts opening at different times all during the gestation. The environmental factors that could affect a seasonal change include maternal nutrition, sunlight, vitamin D, stress, temperature, and infectious disease (you can probably think of a few more).
Here is a quote from the study pertinent to Retriever's question and encapsulates my thoughts as well:
Interestingly, vitamin D levels have been shown to be associated with psychiatric disorders. Although the presence of low vitamin D levels in people with psychiatric illness may be the consequence of reverse causation, further support for a role of vitamin D comes from functional studies showing that it is also involved in neuroprotection and brain development.
So low vitamin D during development could plausibly cause problems - or maybe low vitamin D in mom is a sign of some sort of illness or problem, and that illness is the issue… if the first part is true, vitamin D supplementation will be protective. Otherwise, who really knows? Vitamin D has SO many roles in the body that pinning down the whys, wherefores, and causation chicken-egg issues can be super tricky.
It would be nice if some vitamin D levels were measured - in the Netherlands they keep samples of neonate blood and have measured vitamin D levels for schizophrenia research - I bet someone could do the same comparison with anorexia. It would still just be an association, but at least we could say vitamin D was definitely involved somehow if such a study were done.
Well, better get some shut eye as I have to be up early for the train into the city. Good night!
Interesting ... tho the thought occurred to me that November thru January is also holiday food time. I wonder if there's a link there as well?
ReplyDeleteInteresting. I wonder if other disfunctional eating habits such as emotional eating could have any correlation to birth month.
ReplyDeleteI remember a Rob Wolfe podcast talking about how summer is the time to grow in strength training and often the fitness world has it in reverse. Winter time is the time to slim down.
Vitamin D definitely plays a role in both for hormone development and bunch of other things I have no clue about.
I assumed the season of birth effect is a result of subclinical conditions being found in mothers (i.e. mothers of anorexic women are very thin due to a tendency to eat less/nervous appetite). Women who are at a marginal body weight are generally infertile, but fertility is enhanced by bright light so they may be temporarily fertile in summer. If a woman conceives in summer, she will give birth next spring.
ReplyDeleteI am thin and have had problems with hypothalamic amenorrhea (the sort anorexic women have) and bright light therapy has been very important to restore my menstrual cycles. Melatonin levels play a big role in mediating hypothalamic amenorrhea secondary to starvation/low body weight, as there is some redundancy between seasonal adaptation amenorrhea in animals and starvation amenorrhea. Higher melatonin features in both.
I wonder if they were to conduct studies where they controlled for history of subclinical eating disorders in mothers, if they would find that the season of birth effect would still exist. I suspect not. Prior to discovering that bright light mediates my fertility while at a low weight, the only time I would ovulate would be in the summer months.
One more thing. While I didn't say so in my previous post, I was implying that anorexia nervosa is highly genetic and heritable (the tendency to refuse food and develop this strange OCD like syndrome of compulsive starvation). While not all mothers of anorexics may be full off the ledge of anorexia, it's not unreasonable to assume that they tend to have trouble keeping weight on and eating properly and may be much thinner than average, possibly having subclinical conditions, since this disease tends to run in families like many other mental illnesses. Women at marginal fertility may be temporarily fertile in the summer due to bright light.
ReplyDeleteTrevor - Winter may be a time to slim down from an evolutionary environmental perspective, but the fact that most of our evolution we tended to lose weight during the winter is precisely the reason it is in practice very hard to lose weight INTENTIONALLY during winter. Our whole neuroendocrine system changes during the dark and cold period so as to promote fat maintenance (and gain if possible).
ReplyDeleteHibernation is not a binary state; seasonal adaptation exists in all animals who evolved in environments with seasonal shifts, including human beings. Beginning in fall, our bodies change, dopamine and the sympathetic nervous system and melatonin levels reorient our metabolism/weight, energy, sleep habits , mood, fertility, blood sugar control. The hibernation period of winter means that dwindling light and temp (less SNS/dopamine, more melatonin) results in worsening blood sugar control, increasing appetite and body fat, decreased energy/movement/activity, increased sleeping, decreasing fertility (and in women who are already marginally fertile they may develop hypothalamic amenorrhea in the fall and winter, just like a hibernating animal).
The reason this occurs is because we have evolved to detect signs of impending winter, and preemptively adapt so as to minimize the risk of death. Less moving, more eating, more blood sugar, less pregnancy, more sleeping all means you will live through winter to see it out to spring.
Even seasonal affective disorder (and mild bipolar?) can be considered a sort of advantage. If you go into a torpor in the fall and come out in spring with a bang, you've sorta got an advantage on people who aren't as good at detecting- and responding -to both adversity and opportunity.
But ya... it is totally NOT easy to lose weight in the winter. Just because during evolution people tended to thin out due to lack of food availability does not mean that our bodies find it easier to lose weight when it is cold and dark given a constant modern agricultural food supply in america 2011. If food supply is equal it is waaaay easier to lose weight in summer.
I'm one of those peeps who is hyperresponsive to seasons (possibly because of my mood problems, or possibly my mood problems are a result). I *always* lose heaps of weight beginning in mid to late spring, and this continues until early fall. When the temp comes up and the light increases, my whole *body* changes, I get tons of energy, my mood increases, I eat less, I am much more active. It feels like I have so much more energy. My blood sugar control is far better. Sometimes I get hypomanic sx where I feel fantabulousastic and the energy is just pouring out of me and I"m off the walls buzzing. This generally only occurs in late spring or summer (although I have triggered the symptoms by using light therapy during the winter, it only *naturally* occurs in the spring and summer).
Starting about october the fattening begins again (as well as the decline of energy, mood, activity, etc). This is also the time my depressions are worst and more frequent. I can get depression any time, but the fall and winter is always a bigger risk.
It would make sense to me that if the majority of the pregnancy occurred during spring and summer, the mother would have more vitamin D in her body to be going to the baby that is born in October than if the pregnancy was fall/winter.
ReplyDelete