Tommy Lasorda lost 35 pounds to save the Sisters of Mercy. It was a bet that Los Angeles Dodgers Orel Hershiser and Kirk Gibson made with their pot-bellied manager in 1989: Lose weight and we'll each donate $20,000 to your favorite charity.

The Sisters of Mercy in Nashville were being evicted from their convent with no place to go. "I said OK. I'm going to do it. I'm going to get that money for those nuns," Lasorda said last week from Dodgertown, the team's spring training camp in Vero Beach, Fla.

The rest is dietary history. Lasorda hooked up with the makers of Ultra Slim-Fast, lost the weight, the nuns are getting a new home and Lasorda has become the diet-shake company's premier spokesman, his pre- and post-gut a familiar image on television ads.

Well-known role models aside, it seems as if everybody knows somebody who has tried a weight-loss shake. In fact, products such as Ultra Slim-Fast and DynaTrim are so popular that they are the fastest-growing segment of the diet industry. You can't go into a drugstore or supermarket without seeing displays of the cans and boxes.

Just add milk, blend or shake, and you have a replacement for breakfast, lunch or a snack. Not surprisingly, America's latest fixation to overcome fat is fast, easy and promises swift results.

But do these diet shakes work, or are they just another quick fix for the nation's 65 million dieters? It's not difficult for most people to lose weight if they drastically reduce calories, but can these shakes help maintain weight loss -- the most crucial aspect of a weight-loss program? And most important, are they safe?

Certainly they are ubiquitous. Sales of meal replacements grew 28 percent in 1990, making it a $1.3 billion business, according to Chain Drug Review.

Ultra Slim-Fast and Slim-Fast, a lower-in-fiber product, are both made by Slim-Fast Foods, a division of Thompson Medical Co. Dominating the market by far, Slim-Fast last year attracted 23 million users by spending a whopping $100 million on advertising. Ultra Slim-Fast ads have also been made by Ed Koch, Christina Ferrare, Kathie Lee Gifford, six National Football League coaches, including the Redskins' Joe Gibbs, and others.

Made into a shake with milk or frozen yogurt, the product is available at TCBY yogurt stores, in vending machines, convenience stores, and was even spotted on a New York restaurant menu. Liquid Ultra Slim-Fast produced by regional dairies and packaged in milk cartons is now being rolled out nationally.

The number two player, DynaTrim, marketed by Lederle Laboratories, a division of American Cyanamid Co., can be prepared as a shake, mousse or frozen dessert. It has a projected ad budget of $35 million for 1991, according to Marketdata Enterprises, Inc., a New York research company.

There are also about seven or eight minor brands, which are sold mostly in health-food stores, according to the firm. They include Dick Gregory's Slim-Safe Bahamian Diet, Diet Fast, Twin Fast, Slim and Trim and Firm-a-Loss. In addition, observers predict that large pharmaceutical companies may be getting into the lucrative over-the-counter meal replacement market as well.

While consumers may find the products appealing because they are quick, easy and take the decision-making out of eating, many nutrition experts see those very attractions as disadvantages.

"It's really a no-brain approach to dieting," said Mindy Hermann, a New York consulting nutritionist and spokeswoman for the American Dietetic Association. "Nobody is teaching you how to eat once you've finished losing weight."

Gail Levey, another New York nutritionist, agrees. They're "an enormous deviation from the way people normally eat. Most of us do not live on liquids for two-thirds of our daily diet. It's not the way most of us eat or the way we want to eat," she said.

That sounds like Joyce Caggiano, a 28-year-old flight attendant from West Springfield who has observed that usage of diet shakes is rampant before March and September, United Airlines' weigh-in months for flight attendants. Using Ultra-Slim Fast for about three weeks, she followed the package directions to substitute a shake for a regular breakfast and lunch. But by dinner time, Caggiano said she was starving. "It wasn't a substantial enough replacement for food," she said, adding that once she had food in front of her, she couldn't control herself.

Still, there are others who have had the opposite experience. Robert Kelley, a men's suit salesman for Woodward & Lothrop, used to call Pizza Movers practically every night. The staff there got to know him so well that before he'd even placed his order, they'd say, " 'Oh, here's the one who wants the extra sausage and cheese,' " he said. He would also eat "meals" between meals. Kelley said he curbed his appetite by drinking an Ultra Slim-Fast shake for breakfast and lunch and having an Ultra Slim-Fast nutrition bar between meals. "I'd pretend it was a Reese's Peanut Butter Cup," he said of the nutrition bars.

But to Katherine Tallmadge, a District consulting nutritionist, drinking instead of eating a meal is "silly." People on liquid diets who don't learn about healthy food choices invariably go back to their old eating habits, Tallmadge said she has found. "It's so nutty to me that people fall for it," she added.

For the 220 calories in a serving of Ultra Slim-Fast or DynaTrim, Tallmadge calculated that someone could eat a half cup of brown rice, two ounces of shrimp, one cup of stir-fried pea pods, onions and carrots and a half cup raspberries. A breakfast equivalent would be a bowl of cereal with skim milk and fruit.

Barri Rafferty, spokeswoman for Slim-Fast Foods, said that while "we know what we should be eating for breakfast, this {Ultra Slim-Fast} provides you with a convenient way to eat healthfully." As for the criticism that diet powders don't improve long-term eating habits, Rafferty said that brochures inserted with the product provide examples and recipes of sensible low-fat dinners, and that the program encourages snacks to avoid overeating.

What's more, Rafferty pointed out that Slim-Fast Foods is continually increasing its number of food products. The company already sells Ultra Slim-Fast nutrition bars and puddings, is test marketing popcorn and cheese curls, and is currently rolling out Ultra Slim-Fast Entrees, a line of 12 frozen dinners made by ConAgra, makers of Healthy Choice.

At least anecdotely, it seems that those who have used the diet shakes and have been successful at losing weight and maintaining it (at least for a few months) are those who use them only as a tool, not a solution to their weight problem.

In fact, the now 187-pound Lasorda said he usually drinks an Ultra Slim-Fast shake once a day, perhaps as a snack, not a meal. For example, one day last week he had Cream of Wheat for breakfast, an Ultra Slim-Fast shake when he got to the clubhouse and soup for lunch. While he was losing weight and drinking two shakes a day, he was swimming regularly, which he still does. "Anybody who goes on a diet can lose weight. Most people put it back on if they don't exercise," he said.

Len Simon, a local urban affairs lobbyist who said he "never met a reception egg roll I didn't like," said that when "I would walk one way, my stomach would walk the other way." Realizing that he had to forestall his five-pound-a-year build-up, the self-described "mailbox" tried Ultra Slim-Fast and lost 35 pounds. He has kept the weight off since December.

"I don't want the Slim-Fast corporation to surround my house with a SWAT team and loudspeakers, but I don't think Slim-Fast itself makes you lose weight," Simon said. The first part of his weight-loss program was a self-examination period, a recognition that he was unhappy and needed to do something about it. He exercises regularly, learned how to downsize his dinner portions, and had a lot of family support. (His wife helped with the dinner portions and his kids would entertain him by chanting "Isn't it a riot, daddy's on a diet.")

As for safety, the effectiveness and claims of diet aids were highlighted last March, when Rep. Ron Wyden (D-Ore.) held hearings on Capitol Hill. Since then, the Federal Trade Commission has become more vigilant in its policing of diet products.

Graydion Forrer, staff counsel for Wyden's Small Business Subcommittee on Regulation, Business Opportunities and Energy, said that in the near future the FTC will be releasing results of between 14 and 16 investigations covering a whole range of diet aids, including meal-replacement powders, commercial diet programs, physician-sponsored products and others. Neither the FTC nor Forrer would divulge the specific brands under investigation.

As for whether over-the-counter meal-replacement powders are safe, nutritionist Hermann said that they have come a long way from the popular very-low-calorie liquid protein diets of the 1970s. Three servings of Ultra Slim-Fast and DynaTrim contain 100 percent of the U.S. Recommended Daily Allowance of vitamins and minerals, according to the package labels. They consist primarily of sugar, a protein blend and fiber blend, and both contain the artificial sweetener aspartame.

Most over-the-counter meal replacements derive more of their calories from carbohydrates and less from protein than the medically supervised liquid diets such as Optifast and Medi-Fast, according to Marketdata Enterprises. They are also used in conjunction with food, whereas most of the physician-supervised diets are liquid fasts.

In addition, the over-the-counter powders are considerably less costly than the products purchased from hospitals and physicians. John LaRosa, president and research director for Marketdata, said that the recession and talk-show host Oprah Winfrey's failure to keep her weight off after using Optifast have really helped the over-the-counter products.

But some health authorities caution that the over-the-counter meal replacements, because they are unsupervised, have more potential for abuse. Children and teenagers may misuse them, or those who mix the powders with water instead of milk may not be getting sufficient vitamins and minerals. A substantial portion of some nutrients comes from the milk, not the mixes.

In addition, C. Wayne Callaway, associate clinical professor of medicine at George Washington University, said that they can be a "big risk if they are used as a sole source of nutrition." The labels of both Ultra Slim-Fast and DynaTrim do caution dieters not to use them as a sole source of nutrition.

The other danger of a product such as Ultra Slim-Fast, Callaway quipped, is that "if you take it long enough, you end up looking like Ed Koch."