Can green spaces curb your cravings?

20 July 2019

Interview with 

Helen Keyes, ARU; Duncan Astle, Cambridge University

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Can green spaces curb your cravings? What's the genetics behind cannabis addiction? Katie Haylor dips into some of the latest neuroscience research with local experts - perceptual psychologist Helen Keyes from Anglia Ruskin University, and cognitive neuroscientist Duncan Astle from Cambridge University.

First up, Duncan looked at a paper about the genetics of cannabis use disorder...

Duncan - In essence it's a high dependency on the use of the substance. We already know that about 50 to 70 percent of the variance in whether you have cannabis use disorder is explained by your genes, but little is known really about the underlying biology of why it is that some people seem so prone to this highly habitual use of cannabis, and that's what the authors in this study have been looking at. And they've used a Danish sample of around two and a half thousand individuals who have cannabis use disorder and about 50,000 controls; and they have conducted what's called a GWAS or a genome-wide association study, which is where you put essentially all the genetic variants through an analysis and you're looking to see whether there are any specific genes which seem to really reliably predict who has cannabis use disorder and who doesn't.

Katie - How easy are these to do?

Duncan - They're really hard to do because you need a lot of people. So in fact two and a half thousand versus 50,000 - those sound like big numbers, but in the world of GWAS studies that's still pretty tiny. So in their analysis they found a specific gene which is the CHRNA2 gene, which is a catchy title, and that really reliably distinguishes people who have the cannabis use disorder from control subjects. But the really exciting thing is that what they then did was find some collaborators in Iceland. So in Iceland everybody who has a substance abuse problem mostly is treated within the same single clinic, which means they have a really big sample of about 5000 people who have cannabis use disorder. So they could then repeat their analysis with the Icelandic sample, and they found the same gene replicated. So that's really compelling data to suggest that this CHRNA2 gene is a really good predictor of why some people have cannabis use disorder and others don't.

Katie - And we know this is a causal link?

Duncan - So generally it's considered that if you find a single gene that it is causally related. But then it's interesting to start wondering, why is it causally related? So actually this gene has also been implicated in nicotine dependence, and so the authors suggest that it might have a more general role in addictive behaviours rather than in something specific to do with cannabis.

Katie - Oh I see. So you might be at increased risk of getting addicted to stuff; in this case we're talking about cannabis.

Duncan - Yes. And in many cases actually substance abuse problems seem to co-occur with other sorts of psychiatric difficulties: things like anxiety disorder, or psychosis, or depression. And there's an emerging view within the field that there's what we call a hierarchy of genetic risk, and that there might be some genetic factors which put you in an at-risk category of developing some kind of psychiatric difficulty, but not specific. And then presumably there are other environmental or further genetic factors that, further down the line, will further specify the kind of psychiatric risk that you have. Because these individuals with cannabis use disorder, they were able to then test whether they have a higher genetic susceptibility for all sorts of other psychiatric conditions, so things like risk of schizophrenia, depression, ADHD; and it turns out that they do. So there's this emerging view that there's sort of a general risk that might be genetic, but that becomes further specified by different kinds of environmental influences.

Katie - How would knowing that you have an increased risk help you if you were at increased risk of addictive behaviours, or getting addicted to cannabis?

Duncan  - Gosh that's a very good question. So the current model that we have for identifying and supporting people who have psychiatric difficulties is under massive strain. And in fact some people would say that it's failed altogether. And one of the reasons that it's under massive strain is because it's what we call a reactive system. So we wait until someone develops some symptoms and then we think, right, how can we help that person. And what lots of health care systems worldwide are trying to move towards is a proactive system, where we identify people who are at high risk - in advance of them developing their symptoms - and try and put in place preventative measures or strategies that might reduce the overall symptom burden in the population. And that's why these kinds of studies are really important: because we don't really know what are the main kind of risk factors for these different kinds of disorders. And that's why establishing what the genetic susceptibility might be is really, really useful. Because it means that in the future we're able to move to more of a preventative model rather than this kind of overwhelmed reactive model that we've been sticking with.

Helen looked at a paper asking whether access to green spaces can curb cravings...

Helen - Based on the past research that being in green spaces or seeing green spaces can reduce your negative affect, they wanted to say, well if there's a strong connection between negative affecting cravings, will exposure to green space help to reduce the frequency and strength of any cravings you're feeling? They surveyed 149 people across southern England. They measured their levels of depression, anxiety, and stress - so their negative affect. You would have to identify something that you found as an object of desire, so something you crave; and not surprisingly it was things like chocolate, and alcohol, and cigarettes, and coffee. And then they also measured people's local exposure to nature. So we could look at your postcode and see how much green space you have access to, how much green space you could see out your window, and they also looked at whether you had access to a garden or allotment on a daily basis. And they found having access to greener views from your residence and also having access to a garden or an allotment did significantly reduce the frequency and strength of any cravings you were having. So they found statistically that this was very much driven by a reduction in your negative affect. Any sort of access - even passive access to just viewing green spaces - reduced your negative affect, which in turn had the knock-on effect of reducing your cravings, which is what they expected to find. I mean I really would say that if you are struggling to get on top of your cravings, being outside or frequently viewing nature can have a nice effect for you.

Katie - So do we know what it is about looking at trees or green that has this effect?

Helen - I look at access to green space from a cognitive restoration perspective. So we know that if you're exposed to nature you will actually perform better on mental arithmetic, on vocabulary tests, a range of cognitive tasks. We think this may be something to do with the mental load associated with being in a built environment. So for example, how many things your eyes land on in a built environment is a lot higher than in a green environment for most people. Or it could be a more associative thing, so being in green spaces for a lot of us means we're not actually in work, we're away from perhaps the things that would be bothering us most and taking up a lot of emotional and cognitive space. We're not entirely sure what drives the effect but there's a lot of different hypotheses being tested. I would have liked them to also look at this cognitive restoration. It's tied into so many things around cravings like self-control, impulsivity... it would have been really neat to see whether the exposure to nature was driving the reduction in cravings through just the reduction in negative effect, or also through that increase in cognitive control, that restorative effect. I would also say about this study: it's very difficult, if you're going to talk about access to a green view from your home for example, or access to a garden or allotment, it's very difficult to disentangle that from socio-economic status. In general addictive behaviours, a higher proportion of addictive behaviours are associated with lower economic socio-economic status. And the authors didn't really address this. They did measure people's educational attainment level and they said that wasn't really connected with any of the findings, but they didn't delve any further into that. And I think it will be very difficult for us on the basis of this study to say there's a causal relationship between passively viewing green spaces and a reduction in cravings. However when we take this study in conjunction with all the other literature that has more carefully controlled for socio-economic status, we can fairly safely say that there is something happening when you're exposed to a natural environment, even possibly whether it's emotional or cognitive.

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