How much alcohol does it take to get intoxicated?
Many people figure a few beers at a ballgame or a couple of glasses of wine with dinner won't put them over the legal limit for driving. But how alcohol affects people is highly individual, with a number of factors in the mix.
Quick shots of liquor hit the bloodstream faster than slow sips of wine. Drinking on an empty stomach impairs reflexes more than consuming alcohol with food. And women and older drinkers generally hit legal intoxication levels sooner than men and younger people.
Carbonated beverages raise alcohol levels faster, because the gas irritates the stomach lining, causing alcohol to be absorbed faster. (Sweet or caffeinated alcoholic drinks aren't absorbed any faster, it just seems that way because people often consume more of them than they realize.)
Many Asians have a genetic variation that gives them a flush and a very rapid heartbeat from even a small amount of alcohol.
And factors like fatigue, stress, illness and depression can magnify alcohol's impact.
Eileen Wolter was driving home from an office Christmas party she had organized in Los Angeles in 1998. "I was definitely under a lot of stress," she says. She had had several mixed drinks, a few glasses of wine and very little food, but thought she was fine—until she took a turn too fast and hit a stop sign. She was driving with a flat tire and a broken wheel, causing even more car damage. A police car stopped to see if she was OK, and she flunked a breathalyzer test. "I blew a .09," says Ms. Wolter, who was arrested, fined $2,000 and sentenced to community service and alcohol education classes.
"I wasn't hurt—just humiliated and angry and scared. Dealing with all of it—and the fact that I could have hurt myself or someone else—made me realize what a stupid chance I'd taken," says Ms. Wolter, now a 40-year-old writer and mother of two who says she will never drink that way and drive again.
Drinkers who think they can tell when they've had enough are very often wrong. "Alcohol can affect your reflexes even if you feel fine," says Samir Zakhari, director of the division of metabolism and health effects at the National Institute of Alcohol Abuse and Alcoholism.
That's a key reason why many experts urge people who plan to drink any amount of alcohol not to drive, and vice versa.
In the U.S., it is illegal for adults to drive with a blood-alcohol concentration (BAC) at or above .08%, which represents the percentage of alcohol in the bloodstream. For drivers under 21, any alcohol in the blood is illegal.
The legal limit, once as high as .15% in some states, is now .08% in all 50 states. Some experts still consider it generous. Reaction time starts to slow at only half that amount, and much of the world sets stricter limits. It's .02% in China, .03% in most of India and .05% in much of Europe.
Many communities plan to join a nationwide crackdown on impaired driving between Aug. 19 and Sept. 11, including saturation patrols and sobriety checkpoints, especially during nighttime hours.
Such efforts have helped cut alcohol-related traffic deaths in the U.S. by almost 50% since 1980, though the number has plateaued in recent years at nearly 11,000 deaths annually, or one-third of all highway fatalities.
How does BAC work? One of the most important factors is how fast the alcohol is consumed, says Dr. Zakhari. It goes first to the stomach, then to the small intestine, where it is absorbed into the bloodstream and carried to the liver, where it is metabolized by liver enzymes.
"The liver can only break down the alcohol at the rate of about one drink per hour," says Dr. Zakhari, who likens it to how fast a ticket-taker can let concert-goers through a gate.
Consuming one drink slowly over an entire hour is unlikely to make a person inebriated, he says. But drinking more than that amount, or the same amount faster, will overwhelm the liver. The excess alcohol "goes into the bloodstream and every other organ in the body, including the brain," he says.
Once that happens, only time can unwind the effects, Dr. Zakhari says. BAC generally falls by .015% per hour for both men and women—but will rise again if you keep drinking. Drinking coffee, having a shower or splashing cold water on your face may make you feel more alert, but won't change your BAC.
Consuming food along with alcohol causes it to be absorbed more slowly, since a valve at the base of the stomach closes to allow for digestion before sending it along. Without this stop, the alcohol travels to the small intestine and into the liver faster.
What you eat along with the alcohol doesn't matter very much in terms of BAC. Fat, in, say, a marbled steak, slows the passage of food through the intestine, but only to a small extent. Likewise, drinking milk before consuming alcohol would have a negligible effect on blood levels.
Weight matters more than height, Dr. Zakhari says. A man who is 6-foot-4 and weighs 180 pounds will be as affected as a man who is 5-foot-4 and 180 pounds. But a man who is 6-foot-4 and 220 pounds will have a lower BAC after consuming the same amount. Women's bodies also tend to have less water than men's, which means the same amount of alcohol will yield an even higher BAC.
That was readily apparent at a recent dinner party, where guests were served alcohol and tested throughout the evening. A 110-pound woman, for example, reached a BAC of .079% with less than two drinks, while a 160-pound woman the same height had .05% on three.
Age matters, too. Older peoples' livers metabolize alcohol more slowly than younger people's. But excess alcohol can do more damage to young brains, since some portions are still developing, particularly those that govern impulse control and executive function.
Women's menstrual cycles are yet another factor: Alcohol metabolism increases about 10% right after ovulation.
People who drink heavily and regularly don't get as intoxicated as novice drinkers do on the same amount of alcohol, and tend to have a lower BAC because their livers eventually produce more of a particular enzyme (Cytochrome P450 IIE1) that breaks down alcohol more quickly.
But that may sometimes lull them into a dangerous complacency. Randy Strain, had just finished a stint with the Air Force, where off-duty drinking was common, in 2008 when he and his girlfriend were driving home on a rural road in Illinois after what he thinks were "about 20 beers." They were going about 80 miles per hour when they passed three police cars that were soon on his tail.
The ensuing 14 traffic violations and a DUI could have yielded a jail sentence, but a judge, noting his military service, only restricted his driver's license for six months, fined him $2,000 and ordered him to take 24 hours of classes.
Still, "it was a life-changing experience," says Mr. Strain, 27, of Oak Park, Ill. "Now I go out max one night a week and take a cab home. I will not even get into a car if I or someone else has been drinking."
