WHEN THE Food and Drug Administration approved aspartame for use in soft drinks on Friday, the rules of the game changed. Instead of the soft drink industry welcoming the low-calorie sweetener, it may not be anxious to use it. In fact, according to Dr. Howard Roberts, of the National Soft Drink Association, "there is a possibility that we would formally object" to its approval.

Aspartame has already been approved for use as a tabletop sugar substitute, and for use in dry foods and dry drink mixes such as lemonade. G.D. Searle & Co., which markets it under the trade name NutraSweet and in packets as Equal, sought FDA approval for its use in soft drinks in early June, and the agency initially postponed its approval because of some new questions raised. Roberts said the soft drink association is still concerned with those issues.

For one thing, Dr. Richard Wurtman, a scientist at the Massachusetts Institute of Technology, raised health issues concerning its application to diet sodas. For another, attention has been focused on how long it keeps its sweetness on the grocery shelf. Moreover, the industry has reservations about the product because it is much more expensive and not as available as the tried and very popular saccharin.

Dr. Sanford Miller, director of the FDA's Bureau of Foods, said the agency examined all of Wurtman's data and concluded it "simply didn't support his hypothesis," and that the soft drink companies were "all of a sudden concerned, out of the clear blue sky," about aspartame's sweetening stability. Searle says that the information it submitted to FDA adequately answered concerns about aspartame's safety and shelf life.

When exposed to high temperatures over prolonged periods, the two amino acids in aspartame break down, creating a byproduct and destroying aspartame's sweetening power and taste (a taste that many observers tout as superior to the metallic aftertaste of saccharin). The temperatures at which this breakdown occurs, the length of time involved and the effect on safety as well as taste were documented by Searle in its recent petition for aspartame approval. The National Soft Drink Association felt that the existing information was incomplete.

In a letter submitted to the FDA on June 24, the NSDA said it was designing a study to determine aspartame's shelf stability in a U.S. market. "It has been suggested that because soft drinks experience different time/temperature conditions in the U.S. than in Canada which had already approved aspartame for use in soft drinks , aspartame in U.S. soft drinks could involve quite different stability characteristics," the letter stated. "The more persistent warmer temperatures in some parts of the U.S. will result in more extensive aspartame breakdown." The association requested that the FDA "consider these issues." Roberts said the agency "approved it without responding to us at all."

According to the FDA, the agency did not respond because it felt the issues were adequately addressed by Searle. Miller of the FDA said that aspartame deterioration is easily monitored by appropriate controls on shipping and storage. "There's no reason to assume that the warehousing facilities in Canada are any worse or any better than here." Besides, said Miller, if the soft drink deteriorates, "the stuff would taste lousy and if it goes far enough it smells bad. If they the soft drink companies really are concerned about this deterioration, they don't have to use it at all. There is nothing in the decision that tells them to buy it."

In fact, some of the companies may not do so. "Business is very good without aspartame. They the diet soft drink companies need it like a hole in the head," said Emanuel Goldman, a beverage industry analyst for Montgomery Securities, earlier this week.

While some big companies--Coke, Pepsi and 7-Up--wouldn't say whether they would go after aspartame, others were willing to offer opinions. "We're perfectly satisfied with saccharin," said James Ball, spokesman for Dr. Pepper.

Growth has been fat for the diet soft drink industry--which now accounts for almost 20 percent of the entire soda business. New sugar-free products (Diet Coke, Sugar Free Like) are catering to the demand from weight watchers (not to mention the caffeine-free drinks that appeal to the health conscious) and contributing to the steady increase of diet sodas' share of the market, currently $3.7 billion. And all those bottles and cans need lots of artificial sweetener; out of the 8 million pounds of saccharin used in the United States last year, about 5.5 million pounds of it went into diet soft drinks.

Besides the proven success of the diet drinks without aspartame, there's also the high cost of the new sweetener to be considered by the companies. While saccharin's wholesale price is $3 a pound, aspartame's present price tag is around $90 a pound--a cost that the company, G.D. Searle & Co., attributes to a "difficult and lengthy manufacturing process." In addition, 50 percent more aspartame is needed to equal the sweetening power of saccharin (aspartame is about 200 times sweeter than sugar; saccharin is 300 times sweeter).

Needing more of the sweetener is an added disadvantage for the companies. There's not enough to go around as it is--at least for the time being.

"There's no question that we could not provide every soft drink company with enough," said Richard McGraw, spokesman for Searle. The company has plans to step up production through a $160 million plant due to be completed in 1985, but until then, McGraw said, the company will have to ration it. That's what Searle has been doing up to now with aspartame, which sweetens about a dozen products in the United States (such as Quaker Oats Halfsies, Country Time lemonade, Swiss Miss pre-sweetened hot chocolate, Lipton's pre-sweetened tea, Wyler's instant lemonade), plus soft drinks in Canada (saccharin has been banned there), and foods in 22 countries around the world.

To reduce the price of manufacturing to the companies, improve the taste to the consumer and strengthen aspartame's stability, some in the industry are talking about using aspartame in conjunction with saccharin, said Keith Keeney of the Calorie Control Council, a trade association which represents firms that produce dietary foods and beverages.

Mixing and matching sweeteners may raise concerns among consumer advocates. According to James Turner, a consumer-interest attorney, once alternatives to saccharin are found, then the reasons for keeping saccharin on the market disappear.

On the other hand, if soft drinks are sweetened only with aspartame, that "would increase our sweetener costs about eight times," said Mike Weinstein of A & W. Those costs, he added, would be passed on to the consumer.

Another possible disincentive to using aspartame is that the use of saccharin apparently has not hurt sales of diet soft drinks despite the mandatory cancer-warning labels they carry. "If you stopped 20 people on the street, 19 would not know or would have forgotten about the warning. Everybody's interested in the body beautiful," said Roberts of the NSDA. In addition, the costs of relabeling "would cause a pause" in the industry, he said.

But Dr. Michael Jacobsen, of the Center for Science in the Public Interest, emphasized that consumers shouldn't forget the mandatory warning on saccharin-containing foods which says: "Use of this product may be hazardous to your health. This product contains saccharin which has been determined to cause cancer in laboratory animals."

The FDA itself is taking a new look at saccharin, in light of data submitted by the Calorie Control Council. While the studies were submitted as data to verify the safety of saccharin, Miller pointed out that the studies--"much better than what we have"--reveal that saccharin produced bladder tumors at lower levels of consumption than they had found before, though still very high levels. In addition, Miller said there are other sweeteners coming on the market that the FDA is considering, such as Acesulfame K, which has already been approved by the British. "I would like to see several of these on the market, so there is a choice, and the exposure to any one of them becomes much smaller," Miller said.

Dr. Richard Wurtman, the professor of endocrinology at MIT who corresponded with the FDA prior to its approval of aspartame in soft drinks, had raised questions about the potentially widespread consumption of the sweetener with carbohydrates, when his experiments with both humans and rats suggested that drinking aspartame-sweetened Kool-Aid with a carbohydrate-containing snack might have induced a change in brain chemicals.

Wurtman, who said he testifed in favor of (and continues to support) the limited use of aspartame in those products first approved, such as Kool-Aid, calculated that the FDA's newly approved acceptable daily intake of the sweetener (for soft drinks as well as other approved uses) is comparable to a 154-pound person consuming 88 packets of Equal per day.

Wurtman said he hoped the FDA and Searle would monitor future reports of neurological or behavioral symptoms observed in taking large doses of aspartame along with carbohydrates and said he plans to continue doing studies himself. consumption than they had found before, though still very high levels. In addition, Miller said there are other sweeteners coming on the market that the FDA is considering, such as Acesulfame K, which has already been approved by the British. "I would like to see several of these on the market, so there is a choice, and the exposure to any one of them becomes much smaller," Miller said.

Dr. Richard Wurtman, the professor of endocrinology at MIT who corresponded with the FDA prior to its approval of aspartame in soft drinks, had raised questions about the potentially widespread consumption of the sweetener with carbohydrates, when his experiments with both humans and rats suggested that drinking aspartame-sweetened Kool-Aid with a carbohydrate-containing snack might have induced a change in brain chemicals.

Wurtman, who said he testifed in favor of (and continues to support) the limited use of aspartame in those products first approved, such as Kool-Aid, calculated that the FDA's newly approved acceptable daily intake of the sweetener (for soft drinks as well as other approved uses) is comparable to a 154-pound person consuming 88 packets of Equal per day.

Wurtman said he hoped the FDA and Searle would monitor future reports of neurological or behavioral symptoms observed in taking large doses of aspartame along with carbohydrates and said he plans to continue doing studies himself.