Back to the idea of Nature versus Nurture

Happy New Year!
It has been too long since my last post but there were a few things that got in the way of my writing.  The AKC Agility Invitational and Christmas come to mind.  Prior to the trip out to Orlando for the Invitational, a few people emailed me the following link asking about my thoughts on the topic.  It's from Patricia McConnell's blog, The Other End of the Leash: http://www.theotherendoftheleash.com/your-dog-has-a-brain-in-his-gut

I always have enjoyed Patricia's writing and still do and was happy to see the topic surface on her blog.  After I read it, my initial reaction was "well, yes, of course emotions are linked to nutrition...how could they not be".  The reality is that nearly every biological system of the body is linked to the digestive tract in some way.  When it comes to the nervous system, one division of the nervous system in particular, the Enteric Nervous System (the topic of the referred to blog post) is responsible for control of the gastrointestinal tract of vertebrate species.  It is this system that automatically stimulates muscles and organs to start contracting (to move food through the gut) and secrete important digestive enzymes and hormones supporting metabolism.  In addition, dozens of neurotransmitters are active through this system.  Several of these neurotransmitters are indeed related to emotions and the one I'll refer to is serotonin (mostly because I feel people are familiar with it). 

Serotonin has been linked to behavior, including aggression, depression and hyperactivity, in many different species including rodents, dogs, humans and primates.  The research in this area is extensive and easy to find.  Biochemically, serotonin is derived from the metabolism of tryptophan, an essential amino acid found in proteins and chocolate (a reason why some of us find comfort in chocolate).  These types of complex interactions between and among nutrients and the nervous system  have given rise to the ideas of "cooling proteins" and "calming proteins".  Unfortunately, this is where research is lacking.  Although there is extensive research linking neurotransmitters to diet, there are many complex levels of these interactions.  The intuitive reaction that people have is to simply provide more dietary tryptophan by increasing protein or supplementing. However, it just isn't that simple.  In some animals, that simplistic idea may work but in others it won't. Why not?

Consumed dietary tryptophan must be converted to serotonin by an enzyme.  If there are reduced levels of this enzyme, then no amount of dietary supplementation of tryptophan can account for that.  In addition,  there are additional complexities in the fact that serotonin must be transported by  proteins (transporters) and the neural messaging is sent via proteins called receptors.   All of these proteins (enzymes, transporters, receptors) are all coded by very specific genes within  specific genomes.  Each of these genes I envision like volume dials.  Environmental factors (including diet) can turn them up (up-regulate) or turn them down (down-regulate)...hence the concept of Nature vs. Nurture?  My very favorite way of looking at this is the metaphor that genetics will load the gun, but environment pulls the trigger.  So in any animal that may have one or more of these genes down-regulated such that the result is less of the enzymes, transporters, or receptors, then serotonin's impact on the brain may be reduced.  These incredible complexities are why we have such conflicting data surrounding the issue of meat based diets on behavior.  Some studies show high protein diets are beneficial for animals with aggression, while others say high carbohydrate diets are better. 

So, what the heck does this all mean? Now that we know the genome of species including humans, dogs and cats, we can now begin to understand how particular nutrients are impacting specific genes of interest such as those involved in tryptophan and serotonin metabolism.  This area of research is known as nutrigenomics and is truly fascinating to me.  In other words, while we all have the same genome, our genetic expression varies.  The amount of tryptophan that my body needs may be vastly different from that of my husband.  We can begin to look more specifically at how individual nutrients are impacting each of these genes.  That's when we'll be able to determine what the real causes of behavior and temperament issues really are...is it a simple lack of dietary tryptophan (not likely) or is it a lack of an enzyme, a transporter, or a receptor.  If we can identify the issue correctly, then we might be able to figure out how to turn that dial back up or down.  It would make an enormous difference in how disease is diagnosed and treated. 

In Patricia McConnell's post, she indicated that probiotics might be useful for the ENS.  Sure! If you recall from my first blog about probiotics, I mentioned that I only use them when the systems are "off-balance".  Being off-balance may not manifest in loose stools and vomiting.  An unbalanced digestive system and enteric nervous system may indeed be observed through behavior problems, lethargy, appetite, etc... Can I say for sure that supplements or probiotics are going to help? No, I can't.  Because our animals (and us) are unique individuals, what works for one may not work for another.  Would I try it? Yes, I would.  Until we can unlock the mysteries of metabolism, there is some degree of anecdotal success that we must appreciate.  Recall, there are dozens of these neurotransmitters, metabolites, and hormones contributing to our overall biochemistry. I didn't even talk about stress hormones such as cortisol and how it impacts the ENS.  It is all of these complex unknowns  that make me appreciate the science of nutrition even more and why you will never hear me say there is "only one way" to feed you or your animals.  We simply don't know or understand them all yet.

Happy New Year!