Alcohol tolerance vs. BAC

September 4, 2005 1:26 PM

What is the relationship between alcohol tolerance and blood-alcohol concentration? Is there any?

I'm sure the answer is really simple, but I have often wondered. Alcoholics and habitual drinkers tend to need quite a bit more to drink than moderate consumers of alcohol to visibly show the effects of alcohol. What is going on here? Does having a tolerance to alcohol mean that the alcohol is actually being metabolized more quickly, or does it really mean that the effects of the alcohol are not as deeply felt? Or to put it another way, two 200lb men (one a heavy drinker, the other a light drinker), bothing having consumed four beers in 90 minutes show the same BAC, even though one might seem staggered and the other merely buzzed?

posted by psmealey to science & nature (6 comments total)

Tolerance = ability to cope with intoxication.

There's negligable differences in blood alcohol content unless there's some compensatory mechanism such as increased production of the enzymes needed to metabolize alcohol, which I'm not sure actually happens.
posted by PurplePorpoise at 1:30 PM on September 4, 2005

I'm not sure if that happens either, but I do know that there are genetically determined differences in the amount of alcohol dehydrogenase (an enzyme involved in alcohol metabolism) in the stomach lining. Specifically, women have on average 30% less of it, and northern europeans have a lot more that south-east asians. The legacy of millennia of alcohol abuse, I imagine.
posted by atrazine at 5:48 PM on September 4, 2005

It is a combination of effects. The liver of the heavy drinker possibly burns off the alcohol more quickly. The rate of metabolisation is also very individual and depends both on genetics and circumstances. (As you know, women need less even if they weight the same for example, and you get drunk more quickly in an airplane).

But, mainly, the tolerance effect is due to the heavy drinker just being used to functioning under the influence and appearing "normal" whereas the light drinker is not. The braini adapts. There is a wealth of resources about this on the web if you search for alcoholism etc. I am sure quickly you will find sites that can explain this much better than any of us (well, apart from our doctor friends).
posted by keijo at 7:15 PM on September 4, 2005

IANAD, however this article on Alcohol Withdrawal is interesting. Specifically:

Ethanol (ETOH) is rapidly absorbed by the gastric and intestinal mucosa and enters the bloodstream. It is eliminated in small quantities by the lungs and kidneys but is primarily metabolized by the liver. With repeated exposure, alcohol dehydrogenase is induced, and, at higher concentrations, the microsomal P₄₅₀ enzyme system is also engaged in ETOH metabolism.

So if I'm reading that correctly, with repeated experiences the metabolism system brings more resources into play.

The part I find really interesting is in the next section, though:

Alcohol also potentiates the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). The activity of the neuronal chloride ion channel linked to the A-type GABA receptor is increased during acute exposure to high alcohol levels.¹⁸ This receptor is the site of action of benzodiazepines and may partly explain their benefit in alcohol withdrawal. Alcohol’s affect on GABA_A may contribute to alcohol’s anxiolytic, sedative, and motor impairment actions.³ Hyperexcitability and seizures may develop in alcohol withdrawal from a compensatory change in the number or function of GABA_A receptors following chronic alcohol exposure.³ Growth hormone and corticosterone are also implicated in the mediation of withdrawal syndromes.^19,20

If I'm reading that correctly, then bingo! The effects you describe are because alcohol activates GABA_A. In a severe alcoholic, their body adapts to high levels of GABA_A which is why the system hyperexcites during withdrawal. If then body adjusts to it then I assume the noticable effects are reduced. Any doctors want to weigh in on my interpretation of the material?
posted by sbutler at 9:07 PM on September 4, 2005

From what I have learned from forensic toxicologists, my understanding is as follows:

Your BAC is dependant on what you drink, how much you weigh, and your gender. This amount gets reduced by the proportion of what you drink entering your bloodstream (sometimes as little as half) and your elimination rate, which varies from 10 mg/100ml of blood to over 20 mg/100ml of blood per hour.

The more you drink and the more often you drink, generally speaking, the higher your elimination rate will become.

Tolerance to alcohol, as keijo wrote, is the ability to mask the outward signs of intoxication. There is some influence on the level of BAC which impairs - in the general legal context - the ability to drive. Some people, especially those who are intolerant, have their ability to drive impaired as low as 30 mg/100ml of blood. Everyone, including the extremely tolerant, has their ability to drive impaired at 100mg/100ml of blood.

Notwithstanding this impairment, which relates to judgment, visual acuity, ability to complete divided attention tasks, etc., a tolerant drinker may show no signs of intoxication at this level, while a more social drinker may start to have balance problems or slurring of speech (though these symptoms are more usually seen at 150mg/100ml of blood or higher).
posted by birdsquared at 9:59 PM on September 4, 2005

Both alcohol dehydrogenase and the MEOS can be induced (ramped up) with long-term alcohol use, causing the alcohol to be metabolized faster. However, it's probably the brain becoming accustomed to the presence of alcohol that's responsible for most of the effects noted above.

sbutler, sounds right to me.
posted by ikkyu2 at 10:08 AM on September 5, 2005