Alcohol absorption and metabolism differs in men and women
regardless of body weight. Women are more vulnerable than men to
alcohol-induced liver and heart damage.
Metabolism is the body's process of converting ingested substances
(anything we eat or drink) to other compounds.
Metabolism results in some substances becoming more toxic, and some
less toxic, than those originally ingested.
Metabolism involves a number of processes, one of which is referred
to as oxidation (combining with oxygen). Through oxidation, alcohol is
detoxified and removed from the blood, preventing the alcohol from
accumulating and destroying cells and organs. A minute amount of alcohol
escapes metabolism and is excreted unchanged in the breath and in urine.
Until all the alcohol consumed has been metabolized, it is distributed
throughout the body, affecting the brain and other tissues.
By understanding alcohol metabolism, we can learn how the body can
dispose of alcohol, and discern some of the factors that influence this
process. Studying alcohol metabolism also can help us to understand how
this process influences the metabolism of food, hormones, and
medications (drugs).
Blood Alcohol Concentration (BAC)
100 mg% is the "legal" level (BAC) of intoxication in most
States. 50 mg% is the level at which deterioration of driving skills
begins. (JAMA 255:522-527, 1986.) If the same number of drinks are
consumed over a longer period of time, a person's BAC will be lower.
The Metabolic Process
When alcohol is consumed, it passes from the stomach and intestines
into the blood, a process referred to as absorption. Alcohol is then
metabolized by enzymes, which are body chemicals that break down other
chemicals. In the liver, an enzyme called alcohol dehydrogenase (ADH)
mediates the conversion of alcohol to acetaldehyde. Acetaldehyde is
rapidly converted to acetate by other enzymes and is eventually
metabolized to carbon dioxide and water. Alcohol also is metabolized in
the liver by the enzyme cytochrome P450IIE1 (CYP2E1), which may be
increased after chronic drinking. Most of the alcohol consumed is
metabolized in the liver, but the small quantity that remains
unmetabolized permits alcohol concentration to be measured in breath and
urine.
The liver can metabolize only a certain amount of alcohol per hour,
regardless of the amount that has been consumed. The rate of alcohol
metabolism depends, in part, on the amount of metabolizing enzymes in
the liver, which varies among individuals and appears to have genetic
determinants. In general, after the consumption of one standard drink,
the amount of alcohol in the drinker's blood (blood alcohol
concentration, or BAC) peaks within 30 to 45 minutes. (A standard drink
is defined as 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of
80-proof distilled spirits, all of which contain the same amount of
alcohol.) The BAC curve, shown on the previous page, provides an
estimate of the time needed to absorb and metabolize different amounts
of alcohol. Alcohol is metabolized more slowly than it is absorbed.
Since the metabolism of alcohol is slow, consumption needs to be
controlled to prevent accumulation in the body and intoxication.
Factors Influencing Alcohol Absorption and Metabolism
Food. A number of factors influence the absorption
process, including the presence of food and the type of food in the
gastrointestinal tract when alcohol is consumed. The rate at which
alcohol is absorbed depends on how quickly the stomach empties its
contents into the intestine. The higher the dietary fat content, the
more time this emptying will require and the longer the process of
absorption will take. One study found that subjects who drank alcohol
after a meal that included fat, protein, and carbohydrates absorbed the
alcohol about three times more slowly than when they consumed alcohol on
an empty stomach.
Gender. Women absorb and metabolize alcohol differently
from men. They have higher BAC's after consuming the same amount of
alcohol as men and are more susceptible to alcoholic liver disease,
heart muscle damage, and brain damage. The difference in BAC's between
women and men has been attributed to women's smaller amount of body
water, likened to dropping the same amount of alcohol into a smaller
pail of water. An additional factor contributing to the difference in
BAC's may be that women have lower activity of the alcohol metabolizing
enzyme ADH in the stomach, causing a larger proportion of the ingested
alcohol to reach the blood. The combination of these factors may render
women more vulnerable than men to alcohol-induced liver and heart
damage.
Body Weight. Although alcohol has a relatively high
caloric value, 7.1 Calories per gram (as a point of reference, 1 gram of
carbohydrate contains 4.5 Calories, and 1 gram of fat contains 9
Calories), alcohol consumption does not necessarily result in increased
body weight. An analysis of data collected from the first National
Health and Nutrition Examination Survey (NHANES I) found that although
drinkers had significantly higher intakes of total calories than
nondrinkers, drinkers were not more obese than nondrinkers. In fact,
women drinkers had significantly lower body weight than nondrinkers. As
alcohol intake among men increased, their body weight decreased. An
analysis of data from the second National Health and Nutrition
Examination Survey (NHANES II) and other large national studies found
similar results for women, although the relationship between drinking
and body weight for men is inconsistent. Although moderate doses of
alcohol added to the diets of lean men and women do not seem to lead to
weight gain, some studies have reported weight gain when alcohol is
added to the diets of overweight persons.
When chronic heavy drinkers substitute alcohol for carbohydrates in
their diets, they lose weight and weigh less than their non-drinking
counterparts. Furthermore, when chronic heavy drinkers add alcohol to an
otherwise normal diet, they do not gain weight.
Sex Hormones. Alcohol metabolism alters the balance of
reproductive hormones in men and women. In men, alcohol metabolism
contributes to testicular injury and impairs testosterone synthesis and
sperm production. In a study of normal healthy men who received 220
grams of alcohol daily for 4 weeks, testosterone levels declined after
only 5 days and continued to fall throughout the study period. Prolonged
testosterone deficiency may contribute to feminization in males, for
example, breast enlargement. In addition, alcohol may interfere with
normal sperm structure and movement by inhibiting the metabolism of
vitamin A, which is essential for sperm development. In women, alcohol
metabolism may contribute to increased production of a form of estrogen
called estradiol (which contributes to increased bone density and
reduced risk of coronary artery disease) and to decreased estradiol
metabolism, resulting in elevated estradiol levels. One research review
indicates that estradiol levels increased in premenopausal women who
consumed slightly more than enough alcohol to reach the legal limit of
alcohol (BAC of 0.10 percent) acutely. A study of the effect of alcohol
on estradiol levels in postmenopausal women found that in women wearing
estradiol skin patches, acute alcohol consumption significantly elevated
estradiol levels over the short term.
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