R. J. Reynolds Industries Inc.'s announcement on Sunday that it plans to acquire Nabisco Brands Inc. for $4.9 billion is the most dramatic example yet of the megamergers that are reshaping the food industry.

The proposed combination involves two well-known companies that themselves are the products of mergers. In 1981, Nabisco expanded its product line by acquiring Standard Brands Inc. for $1.9 billion, and in 1982 R. J. Reynolds established itself in the food business by acquiring Heublein Inc. for $1.6 billion.

Wall Street analysts said yesterday that the trend is expected to continue, as companies make acquisitions they hope will give them strong cash flow and an edge in the fight for shelf space in supermarkets throughout the world.

"A lot of companies want to acquire food companies because they can register 8 to 10 percent increases in operating earnings consistently, in good times and in bad," said Merrill Lynch analyst William Maguire. "They want food companies because of their strong balance sheets. A company like Reynolds may also think that tobacco may not provide it with the same profit margins in the future.

"Also, as the retailer gets smarter and smarter in determining which products to sell, you are not going to be in good shape in this industry if you are not big and do not have a lot of clout."

Acquiring Nabisco would be a major step in the quest of R. J. Reynolds, the nation's second-largest cigarette maker, to expand its consumer products division as the company lessens its dependence on tobacco products, which have an uncertain future. Since Reynolds acquired Heublein three years ago and picked up familiar brands including Smirnoff Vodka and Hawaiian Punch, there have been reports that the company was looking for a major food acquisition that would give it an international distribution system, which is one of Nabisco's strengths.

The merger also would provide Nabisco with additional dollars for marketing and promotion, because both companies are cash-rich. The giant company created by this merger would have annual sales of more than $18 billion and net income of more than $1.5 billion, and some Wall Street experts believe the company could have more than $500 million in excess cash flow annually to promote familiar Nabisco products such as Oreo cookies, Ritz crackers and Baby Ruth candy bars.

"This merger would generate a company with an awesome presence in the grocery store, as you combine two companies with very strong balance sheets and secure market shares in their products, for the most part," said Duff & Phelps analyst John Bierbusse. "For Nabisco management, this provides even stronger cash flow to support their brands. This merger is coming at a time when the industry is focusing on how well it handles its relationships with grocery retailers, and acquisitions highlight" that focus.

Mergers have been a popular way for food companies to expand. Increased distribution has been the most-frequently-offered reason for multibillion-dollar mergers such as the recent $2.7 billion combination of Beatrice Companies Inc. and Esmark Inc. Beatrice said that acquiring Esmark would enable it to take several popular regional brands to the national market.

Another underlying economic reason for the trend of food industry mergers is slow population growth, according to Martin Sikora, editor of Mergers and Acquisitons magazine. Sikora said slow population growth restricts the industry's growth. Several leading companies have decided that mergers are the most efficient way to grab a bigger share of the existing market, Sikora said.