NEW FOREST REGION

Jaguar Enthusiasts' Club

Alcohol and Driving - How Much is Too Much?

This summer, as last, I was fortunate enough to be invited to the Salisbury Show by the New Forest Jaguar Enthusiasts Club, a perk of being acquainted with some of the members and, more importantly, the Chairman. The show happens over the first weekend in July and exhibitors range from the horticultural to the country craft and from the hunting dog trainers to the cider producers. Days are spent wandering the stalls and sitting by vintage Jaguars. Evenings are spent in the beer tent and nights in the adjacent camp site (in my tent, you understand - not just 'crashed out' in the field). Bliss.

As usual the Salisbury Plain IAM group had a stand, and this year they were accompanied by a Wiltshire Constabulary traffic officer who was not only allowing small children to clamber all over his Jaguar (we nearly invited him to join our stand), but was also running alcohol breath tests for the general public. As we all know, alcohol and machinery don't really mix, and that consuming alcohol impairs our ability to do things. So we shouldn't do things like drive or ride after taking alcohol. The reason for this is that at a cellular level, ethanol (or ethyl alcohol if you prefer) is an inhibitor of brain cell function. It binds to an inhibitory receptor found on cells in all areas of the central nervous system called the GABAA receptor. It enhances the action of the receptor, thus reducing any activity that the cell may have. Incidentally, this is also the mechanism by which sedatives such as temazepam and diazepam (valium) have their effect. So it's no surprise that alcohol reduces our reaction times and makes us feel sleepy - not good if you're driving. But why do we experience other effects, especially at lower doses of alcohol? Well, the brain is a complicated organ with lots of different centres controlling all sorts of things from the basics of respiration to the complexities of thought and movement. Somewhere in between there are control centres for alertness, aggression, lust, you name it! One of these centres is a controlling influence on all the others, a kind of cerebral Jimney Cricket on your shoulder telling you that you really shouldn't do this, that or the other. This centre works very well, which is why most of the time we don't wander around getting angry or flirtatious with everyone we meet. But even at low does of alcohol, this centre's function gets inhibited. Which is why after even one drink we become more gregarious and outgoing; our inhibitions drop a little. So it's generally agreed that a little alcohol can be a good thing at specific occasions, such as parties.

One of the other effects this lack of inhibition has is that it tends to make us think that we're doing better at something than we really are, and gives us a false confidence as a result. This can be really dangerous when driving, especially in combination with the slowed reflexes already mentioned. So why does the law allow us to have any alcohol in our systems when we drive? The answer lies in our culture. Europeans have, for a very long time, been alcohol drinkers. Brewing was a way of using and preserving some of the crops such as barley and grapes in a way that provided a pleasurable drink to enjoy. In the case of ale, it also provided a way of storing water because the alcohol inhibited bacterial proliferation. Incidentally, in the far east of Asia, they didn't discover brewing. They sterilised their drinking water by boiling it, and flavoured it using herbs such as tea. As a result, the gene pool in that part of the world contains an inactive variant for an enzyme called alcohol dehydrogenase, which degrades alcohols in our systems - the upshot of which is that some people from that part of the world have a very slow ethanol metabolism rate. They get very drunk very quickly. So we like our alcohol and the brewing industry is large and powerful. So we have a law which tells us that we can drink some alcohol and drive, but not too much. But how much is too much (legally speaking)? Finding a combination of a policeman offering breath tests and a nearby beer tent gave me the idea for an experiment.

I was going to be the 'guinea pig' and my friendly traffic officer was going to be the technician. He told me that for a breath test to be conducted we had to wait 20 minutes after my last sip - to allow for mouth alcohol to disperse so that it wouldn't bias the readings. I hadn't had a drink all day. I was fully sober. The experiment was on. I headed for the beer tent.

The legal blood alcohol limit for driving is 80mg/100ml blood. But what does this mean in real terms? To try to make some sense of how much is too much (legally), the government tells us that for an average human (whatever that is), it's 2 units of alcohol. And 2 units is the amount contained in either 2 shots of a (standard) spirit, or 2 (small) glasses of (weak) wine or 1 pint of (weak) beer. So armed with this information I bought myself a pint of 4.0% ABV (weak by my usual tastes) beer and sat down to drink it. A leisurely pint and a 20 minute wait for the breath test took about 45 minutes in all. Which gives me a little time to explain why the guidelines on how much is too much (legally) are almost useless. The reasons are two-fold; pharmacological and biochemical.

The pharmacological complexity is that of both dosage and volume of distribution. The dosage can be complicated. The 'unit' guidelines are vague about the strength of the drinks consumed, but in general they refer to relatively weak variants of the drinks listed. So not the 50% navy rum, the 14% red wine and the 9% barley wine that some of us are partial to on occasion. And have you tried to buy a 125ml glass of wine in a pub recently? The standard measure now seems to have grown to 250ml. And if you buy 2 of those, you get the rest of the bottle thrown in (not that you'll get much more wine out of it!). So it's easy to overdose on units. The volume of distribution problem is more simple. Ethanol is water soluble, which is good, because otherwise it would form an unpleasant oily slick on the top of whatever you are drinking. But what this means is that it distributes itself pretty evenly all around your body. So the more you weigh, the more drinks you can have before the dosage of alcohol in your body meets the 80mg/100ml of fluid of which blood is just a representative sample. So let me think about this……..the more beer I drink, the fatter I'll get so the more beer I can drink………. Er, no, sorry, I digress.

The biochemical influence is to do with how our bodies deal with the drink we've just consumed. Of course it enters our gastrointestinal tract, which does the job of digesting our food and absorbing both nutrients and sometimes toxins from it. Ethanol is readily absorbed from our gut, even from the mouth if it stays there long enough, but mostly it is absorbed from the stomach. Here the first variable comes in. If you've just had a large meal, then this will inhibit the rate at which the alcohol is absorbed. An 'old wives tale' suggests drinking milk before a heavy drinking session to reduce the alcohol absorbed. Milk contains a high level of proteins, which tend to coat the stomach wall and cling to the alcohol, thus reducing the rate at which alcohol is absorbed rather than preventing it altogether. Conversely, if you are drinking a fizzy beverage such as Champagne, the effervescence itself will enhance the rate at which alcohol is absorbed. So the speed at which we absorb alcohol depends not only on how fast we drink it, but also on what format it comes in and what we've eaten before. The rate of absorption is important because from the moment alcohol enters our body, we degrade it enzymatically (unless we're the specific genotype from the far-east mentioned before) and excrete it from both the kidneys and the lungs. So if it takes a while to absorb it all and we're constantly getting rid of it, then the amount in our body will follow some kind of curve, arcing upwards as absorption outweighs elimination and falling back again as we absorb less and eliminate more. Exactly where the peak alcohol concentration will occur in time, and how high that peak will be, will depend on the many factors of intake rate, absorption rate and elimination rate. The latter isn't simple either because the majority of the ethanol in our bodies is degraded enzymatically by the gut wall (hence never getting into out bodies in the first place) and by the liver. A feature of enzymes is that we can 'induce' them - make more of them if we need to. So if you're a regular drinker you'll have more alcohol dehydrogenase than a tea-totaller and will therefore be able to eliminate your body's alcohol quicker. As you can see, working out when and how high the peak alcohol content of your body will be after a drink or two, and therefore how much is too much for the law to allow you to drive, can be very difficult. I've always suspected that the guideline of 2 units is based on a very rapid dose given directly into the blood of an average sized human (10 stones?) with no time for elimination. Which brings me back to the experiment.

As mentioned before, ethanol is excreted from the lungs as well as by other routes, and therefore a sample of deep-down lung air will have the same concentration of alcohol in it as in the blood. This is the basis for the roadside breath test. The machine gives a read-out in units (arbitrary ones as far as I know), and the legal limit (equivalent to 80mg/100ml blood) is 35. What was my reading after 2 units of 4% beer on an empty stomach over half an hour? 16. Which means that for me, at 12 stones and moderate weekly alcohol consumption, the guideline of 2 units as the legal drink-drive limit would appear to be rather conservative. I'm assuming here that 20 minutes after my final sip of the beer it had all been absorbed. Drinking on an empty stomach, I suspect that this would have been the case.

But of course this doesn't mean that we now know that we can all happily have 2 pints (4 units) and drive home, because not only are we all different in terms of our absorption and elimination of alcohol, but of course any alcohol will impair our ability to drive safely. The legal limit is simply that, one over which we'll lose our licence; it's not one below which it is safe to drive.

As a follow-up, after an evening of drinking at the bar with the New Forest Jaguar Enthusiasts and a good night's sleep in the tent, I went back to the traffic officer the next morning for another breath test. We often hear of warnings that over-indulgence the previous evening may mean that we're still over the legal limit the next morning. While my evening's consumption couldn't be compared to a party night at Stringfellow's downing sequential margueritas followed by a 6am drive to work, it was worth seeing what my reading was. 0. Nil. Sober. I'd eliminated it all.

Of course these little experiments do not mean that it's safe to drink any amount of alcohol and drive, even the morning after. But it does shed some light on the legal standpoint to which we all have to adhere. If anyone thinks that their licence is safe after a couple of glasses of wine in the pub……………….

Paul Barrow.