A Massachusetts Institute of Technology scientist warned yesterday that aspartame, the chemical about to sweeten many of Americans' soft drinks, may also harm their brains if they drink too much of it.

Dr. Richard Wurtman tempered the warning by saying that if bottlers limit the amount of aspartame per drink, as he believes they may, there is probably "little, if any, chance of harm."

There are still many unknowns, and it is "highly important," he added, that doctors, the Food and Drug Administration and beverage-makers all watch aspartame users for ill effects like dizziness or changes in sleep, appetite or mood. The FDA, in approving aspartame, said it was aware of Wurtman's concerns.

Wurtman made yesterday's warning in a letter to the New England Journal of Medicine, an important medical journal that screens letters closely for possible importance.

It is not his first word. He objected in July when the FDA approved aspartame for soft drinks. He cited three studies with groups of 30 to 36 rats in which giving fasting animals either aspartame or glucose (blood sugar) caused changes in blood plasma or brain chemicals.

The upshot, he told the medical journal, is that drinking aspartame-containing beverages while consuming any carbohydrates--"a sandwich or cookies or potato chips or pretzels or many things"--may double the aspartame effect on the brain in vulnerable persons.

Among the possibly vulnerable, he said, are "more than 4 million Americans" who are unrecognized carriers of the gene for phenylketonuria, a serious cause of mental retardations in newborns. Also, "millions" of persons with conditions like high blood pressure, Parkinson's disease, hyperkinesia (muscular over-activity) and even insomnia. Also, persons taking drugs like MAO inhibitors (commonly given persons with depression) and levodopa (for Parkinsonism).

The aspartame dose used in the animal studies, he wrote, was "consistent with the amount an 8-year-old child might consume during a hot afternoon if the sweetener was added to soft drinks at the level currently used in Canada (about 500 milligrams per liter)" and the child drank three soft drinks. If the child also ate a sandwich, he said, the effect could be doubled, and if the child were a phenylketonuria carrier, for example, the effect could be doubled again.

This is still true, he said in an interview yesterday. But he has since been told, he said, that Coca-Cola will use only 100 to 120 milligrams of aspartame per liter, "and if other bottlers do this too," these "moderate levels are likely to be safe."

Wurtman has said the same thing in a letter to Dr. Arthur Hull Hayes, FDA commissioner, telling Hayes he will not file a formal objection to aspartame's new use, but calling further studies of aspartame "of the utmost importance."