When I’m out with my friends at social events I’m often asked about how alcohol affects the brain. More specifically, the question on everyone’s mind is: when have I had one too many.
Well, this is not an easy question to answer because everyone is different, and because there are so many variables. What is too many for one 160 pound person may not be too many for another 160 pound person. However, because I’m supposed to be the expert, I have to have an answer that at least sounds authoritative, so here’s what I tell people.
I start with the basics. Before alcohol can get to the brain, it must be first absorbed, and then distributed throughout the body. Absorption and distribution are therefore the first things that must be considered.
- Absorption of alcohol – About 20 percent of the absorption takes place through the lining of the stomach. The remaining 80 percent takes place through the walls of the small intestine. Because the vast majority of absorption takes place after the alcohol leaves the stomach, the most important factor in the absorption of alcohol is simply the contents of the stomach. When there is food in the stomach, gastric emptying is delayed. The type of food also has an impact on absorption, as does the type of alcohol consumed. Generally speaking, full absorption takes place anywhere from 14 to 138 minutes after the last drink but some studies have shown that it may take far longer for full absorption to occur, perhaps as many as several hours. Trauma can also delay gastric emptying, as can certain drugs such as nicotine (yes, it takes longer to get drunk if you’re smoking cigarettes). The best way to keep from getting drunk is to always eat while drinking.
- Distribution of alcohol - After the alcohol leaves the small intestine, it must still get to the brain before it has any impact. Distribution rates do vary among individuals, and depend on factor such as heart rate and capacity. Generally though, distribution is rapid. It is essentially a function of water content, meaning the alcohol goes to where the water is. Body tissues that have more water will have more alcohol in them once the alcohol has been fully distributed.
The next important topic is the elimination of alcohol, and here’s where things get much more complicated and variable. Yes, it really does take more alcohol for some people to get drunk, but this is NOT because of tolerance. Instead it is because people that drink often usually have much higher clearance rates. Here are some additional details about elimination.
- Elimination of alcohol – The majority of elimination takes place in the liver. This is because the enzyme that breaks down alcohol is most concentrated in liver tissue. However, this enzyme is also present in many other tissues, including in the stomach. It is often said that alcohol is eliminated based on “zero order” kinetics, meaning the same amount will always be eliminated regardless of how much alcohol is present. Elimination is not dose-dependant. Women tend to eliminate alcohol at a rate slightly faster than their male counterparts, so yes, elimination rates are gender-specific. Most people eliminate alcohol at the rate of .01 to .02 per hour. So, if you’ve consumed enough alcohol to raise your blood alcohol content to .08, then each hour you will eliminate between .01 and .02 of that alcohol. However, elimination rates may vary from as low as about .008 to as high as about .036 per hour.
After reading this you should have a basic understanding of the complications of alcohol metabolism. For a much more exhaustive explanation of this topic, please consider purchasing the two volume text Defending Drinking Drivers, where you will find this topic and many others covered in great detail.