January 11, 1984

Former President Reynaldo Bignone was arrested tonight in an investigation into the disappearance of two army conscripts, both Communists, in 1976. The arrest order was announced by Judge Carlos Oliveri after General Bignone left a court where he had testified for nearly five hours on the disappearances of two Communists. The general was already being held under what is known as ''preventive prison,'' the equivalent of indictment without bail, for investigation of criminal charges of abuse of authority, covering up an illegality and illegal detention.

A QUESTION often asked, especially by women, is, ''How much milk should I drink every day to get enough calcium?'' The answer, which often elicits a gasp, is you should consume as much calcium as there is in five glasses of milk. Few Americans, at least few adult Americans, come anywhere near that level of calcium intake. And the statistics on osteoporosis and broken bones show that we are paying a high price for our neglect of this vital nutrient. In just a few years osteoporosis, a disease once unheard of by most women, has become a common concern. And well it should be, among young as well as older women. For this debilitating bone disorder, which is virtually epidemic among American women past the age of 60, has its roots in young adulthood. Prevention should start as early as the teen years. Depending upon how long they live, up to 40 percent of American women develop osteoporosis, the first sign of which is usually a broken bone or collapsed vertebra. While men are not immune to this problem, for various reasons - especially the hormonal changes that accompany menopause - women are much more likely to develop it. After menopause, the average American woman loses an inch and a half in height each decade as a result of vertebral collapse. Far more serious, millions of fractures and tens of thousands of deaths occur annually because of osteoporosis.

ZITACUARO, Mexico IF Diana Kennedy feels blessed in her new home here, about 100 miles west of Mexico City, it is small wonder. It is a magnificent hideaway in the center of a mountain-ringed forest with tall poinsettias, bougainvilleas, fruit-bearing passionflowers and a thicket of wildflowers. The house is made of adobe bricks, a windmill produces electricity, the heat is solar and the water for cooking and drinking collected from rainfall. When I arrived there one recent morning Mrs. Kennedy was just stepping out of her pickup truck. ''I've just delivered the pig to the vet,'' she said. ''I had to deliver the milk to the market, and while I was there an old man said, 'Won't you buy my duck because I need the money?' So I did. My gardener just told me that one of the cows is pregnant, and I have more guests coming to dinner.''

Senator John Tower, chairman of the Armed Services Committee, who returned Monday night from a weeklong trip to the Middle East, said today that a withdrawal of American troops from Lebanon now would be ''disastrous.'' In addition, Mr. Tower, Republican of Texas, said at a news conference that moving the Marine contingent away from Beirut International Airport, where it has repeatedly been under fire, would not increase its security. In Lebanon, meanwhile, officials said that a plan to reduce hostilities by separating the warring factions was being delayed by the demands of groups allied with Syria. (Page 4.)

Washington and the Vatican established full diplomatic relations for the first time in 117 years. The action was described by spokesmen for the Reagan Administration as aimed at improving communications at a time when Pope John Paul II has become increasingly involved in international affairs. (Page A1, Column 6.) Many Protestant groups, some Jewish groups and some civil liberties organizations criticized the re-establishment of full diplomatic ties between the United States and the Vatican. (A4:4-6.)