For years, doctors have known that some heavy users of alcohol experienced dementia -- the loss of mental faculties.

But they thought it took many years and that most persons whose brains showed such deterioration were in their 50s and 60s.

Now there are disturbing reports that some irreversible mental deterioration may occur in the 30s. And the longer a person drinks heavily, the worse it gets.

The research results are controversial, but one research team that originally had said such dementia didn't occur has now reversed itself.

Last year, the team, from San Diego Veterans Administration Hospital, concluded, "Our findings suggest grounds for cautious optimism that even very heavy alcohol use is not related to neuropsychological impairment in the alcoholic who is in his or her late 30s."

However, on retesting a year later, the team -- headed by Dr. Igor Grant -- concedes, "Subtle changes may have occurred."

Meantime, other psychiatric teams rushed to confirm or deny the San Diego report, including a group at the national Institute on Alcohol and Alcohol Abuse in Rockville, Md.

Dr. Ichael . Eckardt, the senior author, noted Grant's original conclusion that alcoholism doesn't impair persons in their 30s "has serious implications, particularly in view of the enormity of alcohol as a social problem. If clinicians believe that no harm will result from drinking heavily to individuals who are less than 37 years of age, efforts to intervene in this large population might not be highly motivated."

The studies all employed objective psychological and performance tests.

For instance, Grants' group have a battery of memory tests, the Wechsler Adult Intelligence Scale (14 results, including three ratings of IQ), the Halstead-Reitan battery (mostly tactual performance tests), and about a dozen others.

Grant and his colleagues gave the same tests twice, a year apart, to the same three study groups: those who originally took the tests three weeks after alcoholic detoxification, those who took the test after 18 months of abstinence from alcohol, and a group of non-alcoholics.

The original test results, reported in the October 1979 issue of the American Journal of Psychiatry, showed few differences among the groups, though both groups of alcoholics scored more poorly than the non-alcoholics on such tests as name-writing time.

The San Diego results then were so different from previous studies showing deterioration for men in their 40s that they speculated, "the extra 3-8 years . . . of additional exposure to alcohol is critical to the emergence of specific neuropsychological deficits."

On retesting the three groups a year later, those in the original 18 month group and the non-alcoholics "improved significantly on several tests."

But Grant and his collegues, writing in the August 1980 issue of the same journal, found that the group originally freshly detoxified "did not improve significantly on any test, although they did increase their overall performance IQ."

Though 63 percent of the men in the detoxified groups, half the non-alcoholics (and even 20 percent of the long-term abstainers), had a drink during the year, only one man reported heavy drinking.

Yet the tests showed the group who had gone through detoxification just over a ysear before was beginning to be impaired, and some had deteriorated.

Futhermore, the fact their scores didn't improve may mean that alcohol had produced a "defect in incidental memory," which they said was an initial sign of "organic mental disorder."

In other words, when most normal people take a test, and take it again whether weeks or years later, there's a practice effect, and scores improve a little bit. (That's why educators recommend retaking critical tests like the Scholastic Aptitude Test.)

But for the detoxified group, that effect didn't appear.

They speculate some of these persons "have entered a process of change in neuropsychological abilities that is no longer correctable by abstinence."

Grant and his associates said the lack of expected improvement in follow-up testing was part of the evidence of early subtle changes in cerebral functioning "not yet measureable."

The National Institute team, headed by Eckardt, used many of the same tests, but compared people who were only 2-6 days from their last drink to those 14-31 days removed.

Using many of the same objective tests as Grant's group, they found results indicating brain dysfunction on 13 or 24 tests in the group only 2-6 days after drinking, and on 11 of 24 tests for the second group. "Increased consumption predicted decreased performance," they said.

They conclude, "Alcohol abuse in this age group is associated with clinically detectable . . . impairment."

So men in their 30s can be permanently damaged by drinking.