After being a long time lurker of this section of the forums on bb.com I've finally decided to make an account and start contributing.
First, a bit of background. I'm a medical student from a city in Scotland called Aberdeen (north east). I'm pursuing a physiology degree this year as part of an inter-uni intercalated program and thus I have some time to generate some articles/material that will probably appeal to the large majority of the advanced nutrition forum.
Most recently I wrote about the stress response and feeding behaviour
Here we go
Everyone gets stressed! In this article I attempt to distill why some people eat more and some people eat less than their normal during periods of stress. But before I begin this instalment of the geek chronicles, I think itís important to define what is meant by the word Ďstressí in the context of this article.
As there is no single test or biological parameter that can diagnose a stressful condition, nor one single stress response present in every stress related condition - stress, from a physiological point of view, can be defined in a number of ways (1). For the sake of simplicity, Iím going to take a definition from the literature and use that. If you want to explore deeper, have a read of the references.
ďA general distillation of the literature suggests that stress denotes a real or perceived perturbation to an organismís physiological homeostasis or psychological well-being. In its stress response the body uses a constellation of behavioural or physiological mechanisms to counter the perturbation and return to normalcy.Ē(1)
reference - (on website, can't link yet)
In simpler terms - stress is the result of any perceived or real challenge to homeostasis. Stress can psychological (emotional) in nature, or physiological (physical) in nature, or indeed a mixture of both.
For those confused - homeostasis is defined as the maintenance of a constant internal environment within the body (2).
Itís imperative to understand that homeostasis is essential for the survival of an organism and hence why the stress response from an evolutionary point of view, when acute and not chronic - is beneficial. The Ďfight or flightí response being an example of the stress response working as an evolutionary advantage (3).
Iíll give a brief overview of what happens below. (4)
A stressor is detected by one of the bodies many sensors
Examples being: the eyes, nose, proprioceptors, pain receptors and psychological input via emotional fluctuations and the limbic system
Stressors are processed and relayed through the brain
Coping mechanisms are activated in the body
These include the sympathetic nervous system and hypothalamic pituitary adrenal axis (HPAA) - as well as behaviour adaptations/reactions
The HPAA axis works to synthesise and release glucocorticoids (mainly cortisol) from the adrenal cortex of the adrenal glad
The sympathetic nervous system stimulates catecholamine release (Norepenephrine and Ephenephrine) - Catecholamines accelerate heart and lung function and inhibit digestive system and sexual function.
Cortisol mobilises the bodies energy stores and raises glucose levels in the blood as well as suppressing the immune system
This works synergistically with the catecholamines released by the sympathetic nervous system to prepare the body for the Ďfight or flight actioní
Itís fairly easy to see why this response is advantageous from an evolutionary point of view.
If we take a look at the stress definition above, any perceived threat or disruption to homeostasis results in the activation of this system. This includes emotional change or a change in environment - things that happen all to often these days in our fast paced social media fuelled lifestyles. Itís fair to say that all of us get stressed, and those who suffer will almost certainly find that their eating habits will change as a result. From anecdotal experience I know for sure that during exam times my usual diet structure and content goes completely out the window, and this has often led me to wonder, why is this?
The more intuitive of you will realise that people eating more when they are stressed, is somewhat paradoxical. Why would an increase in appetite and hunger for food have any place in a stress response where fight or flight is the main goal? In this article I try to explain why an overeating phenomenon is seen in people that are chronically stressed.
As described above, the HPAA plays an extremely important role in the stress response. Interestingly though, the HPAA also plays an essential role in feeding behaviour responses. (5). The neurones and cell bodies that regulate energy balance and food intake converge on an area of the hypothalamus called the paraventricular nucleus (5). The same area that contains the cells that release CRH (fig 1). There is evidence to show that there is overlap between the feeding and stress systems at the level of the HPAA (6). This isnít surprising really considering how closely they are anatomically related, coupled with the fact that regulation of energy and food intake itself is probably a very important part of the stress response from an evolutionary point of view.
The role that circulating glucocorticoids play in regulating in food intake is ambiguous, with some literature supporting it being orexigenic (7), (8), whilst other literature supports it being anorexigenic, (9), (10). No definitive conclusions can be made here, but what we do know is that chronic stress persistently elevates glucocorticoid levels. Furthermore, it has been observed that chronically elevated levels of glucocorticoids in the blood promote the accumulation of abdominal fat. (11)
Iíve taken this explanation straight form a journal (11).
ďExposure to chronic stress is also associated with the development of abdominal obesity in humans. Bjorntorp proposed that long-term stress which results in prolonged hyperactivation of the HPA axis, increases circulating glucocorticoids that bind to glucocorticoid receptors (GR) which are highly expressed in abdominal fat, thus activating lipoprotein lipase followed by inhibition of lipid mobilisation in the presence of insulin. This results in triglyceride accumulation and abdominal fat retentionĒ - (bijntorp et al., 2001), (5)
Itís not unreasonable for us to make the assumption that if someone is eating a caloric surplus, itís easier for them to put on weight when stressed than when not stressed, as conditions are more favourable for fat deposition/gain.
But do we actually eat more when we are stressed?
Note: When writing this section I referred heavily to the article Ďstress, eating and the reward systemí (Tanja C et al., 2007), (20) so itís only fair I give the authors credit. Give it a read if you want to delve deeper.
There is a large body of literature in rodents that points towards decreased energy intake when they are stressed (12,13). Some authors even use decreased food intake as a marker of stress in rats (14), which doesnít quite fit in with our anecdotal evidence. Fortunately for us, and this article, rats that are presented with highly palatable (enjoyable) food eat more when they are stressed. Stress itself causes an increase in palatable food specifically in rats. (15,16).
In humans, the literature reports positive and negative feeding responses to stressful stimuli (17,18,19). Some people eat less when they are stressed - but the majority favours positive feeding responses, with a particularly interesting study linking an increase in calorie dense food consumption and stress in women (20). I think itís important to stress that what determines the direction of eating is largely unknown (21). We do know that the HPAA plays a central role.
ďPeople may very well choose to trade off years of their life, or the possibility of disease or injury, in exchange for the current pleasure, excitement, or stress relief they get from foodĒ Jacob Sullum.
Now to put two and two together and attempt to explain why we eat more when stressed. in the western world.
Introducing Non Homeostatic eating.
Non homeostatic eating is interesting and is compelling evidence to support my anecdote that I eat more when Iím stressed. Now before I delve a bit deeper into this I think itís important to clear a few things up. Iím a luckily guy in that the stress I encounter is elective, I donít have to go to the gym, I donít have to go to University, I donít need to set myself deadlines and have high ambitions, but I do. For the most part - I put myself into these stressful situations electively, and can probably remove myself from these situations electively. Moreover, most of my stress come from a psychological source. Stressors that one can cope with are named in the literature as a Ďcontrollable stressorí, and itís hypothesised these activate the sympathetic nervous system more than the HPAA.
Some people arenít so lucky, some people are stressed because of extremely long hours at work, or get degraded on a daily basis by a tough boss. Is their stress response different to mine? Potentially yes - some authors hypothesise that more of the HPAA is activated for Ďthreat stressí, stress that one does not have resources or capability to cope with. (20),
So the theory behind non homeostatic eating holds more probably holds more true for someone in situation two, but it is likely applicable to most people. Lastly, a cornerstone of the non homeostatic eating theory is that people have access to highly palatable food, which Iíve assumed to be the case for the majority of the people reading this article.
Non homeostatic eating is stress induced eating of palatable food. Itís thought that this form of feeding behaviour actives the reward system in the brain (22), as evidenced by the release of dopamine when stressed, thus, promoting pleasure seeking behaviour. (23) This could partly explain why people turn to alcohol or drugs when they are stressed too, as itís this same reward system that makes us addicted to drugs. Itís been hypothesised that Palatable food, or Ďcomfort foodí, is thought to dampen the stress response (5), as evidenced by studies showing a lowering cortisol levels in humans and lower corticosterone levels in rats after the consumption of palatable food (22). Itís thought that this eating behaviour dampens the HPAA via negative feedback to the hypothalamus. In rats who have had an adrenalectomy, the HPAA is stabilised by the consumption of sucrose (24). We can assume that something similar may be going on in humans, but there isnít research out there for us to make a definitive conclusion. Non homeostatic eating definitely happens, we just donít fully understand it yet.
To conclude, I think itís fair to say that stress induced feeding behaviour is extremely complex and involves a number of neurohormonal pathways and cascades. Iíve barely scratched the surface in this article about the interaction between glucocorticoids and neuropeptides and their effects on various regions of the brain and have focussed mainly on the hedonic process of stressed induced eating. An area Iíve completely missed out would be pre and post natal stress and the resultant effects on food intake and bodyweight of offspring, this is mainly because itís an enormous field and I didnít feel it would have much appeal to the audience of this article. Iíve also not touched on some authors propositions about why they think psychological stress
References available on my website which I'm not allowed to link yet (not enough posts poverty crew)
www DOT jamesmcilroy.com/the-geek-chronicles/the-geek-chronicles-002-an-attempt-to-explain-stress-and-feeding-behaviour