Picking at one of the six small meals she eats a day - grilled cheese sandwich, potato salad, and a cup of walnuts, in this case - Shirley MacLaine sat in her Watergate Hotel suite Saturday explaining why somewhere on her way to turning 40 a few years ago her life suddenly became the pits.

It was, she says, about five years ago - about the time she had worked a year for McGovern and he lost . . . about the time she had all but "no'd" herself out of the movie business by turning down scripts . . . about the time she lost her sense of humor, got dissatified, gained 25 pounds . . . about the time she had a mid-30s crisis.

It was, she says, worse than her teens. The moment in her life when she had to decide whether to "go on living positively" or, well, keep on eating.

So she stopped eating. And started jogging, exercising, dancing - and now, at 43, has danced her way back into a successful nightclub act whose performances she sandwiched in between finishing her third book and the movie "The Turning Point" - a marvelous MacLaine/Anne Bancroft tour de force whose world premiere has brough her to Washington. A movie she likes a lot. A movie she'd pay to see if she weren't in it.

"You know," she begins, gulping down her words with a swig of skimmed milk, "you never know what you're going to get with a movie. You start out thinking it's wonderful and it can turn to s--- before your eyes. Ours didn't. It caresses you, it's sweeping to look at, it makes you feel good, it doesn't have gratuituous sex.

"Actually, it ended up having more depth to it than I thought it would. For instance, I think the relationship between the mother and her daughter is one of the most important things in the film. Last night I was talking to my own daughter Sachi on the phone about [the movie] and she got so excited she almost hopped a plane and came here today."

Sachi Parker, one of the most interesting sides of Shirley MacLaine's life, is the daughter from her 25-year marriage to producer Steve parker, with whom she has not lived for years but also has not divorced and probably never will. She is an Asian scholar, currently doing postgrad work at the University of Hawaii, who reads and writes both Chinese and Japanese fluently and thinks show business is a smashing bore. MacLaine says that mother and daughter are very close even though Sachi's rearing was one to set Dr. Spock reeling.

"Sachi was an accident, actually, and it's probably a good thing, otherwise I don't know if I would have had her," says MacLaine with a characteristic candor. "At that time I didn't know myself well enough to know if I wanted a child or not."

As for Sachi's upbringing, well, says MacLaine, it was probably harder on her than it was on Sachi, who spent her first seven years with MacLaine in America and the next five with her father who opted to live in Japan. Sachi then made up her own mind to attend schools in Europe before settling halfway between, in Honolulu.

"No Sachi didn't live with me. But she spent Easters, Christmases and summers with me. Yeah, there were times I felt I should be more of a conventional mother and then I'd talk to her about it and she said, "No. I think what you're doing is right." Does she resent me? No, I don't think so. At least she's never expressed it. In fact she's always complimented me on it - always says she's glad I allowed her to reach her identity on her own sooner than most of her girl friends."

For MacLaine, identity is a big number. The only number. The thing she is always onto - racing for it, jogging for it - attacking whatever urge she must in order to fulfill the sum parts of her whole.

And when she talks about it - and she does a lot - when she tells you that the "most pleasurable journey you take is through yourself . . . that the only sustaining love involvement is with yourself . . . " it comes out okay, isn't gooey, doesn't sound like a chapter out of "Your Erroneous Zones," because MacLaine is careful to qualify herself . . . admit that for her at least it is the partial battle cry of the "compulsive overachiever" - the decathlon competitior competing only with herself.

"When you look back on your life and try to figure out where you've been and where you are going," she will tell you, "when you look at your work, your love affairs, your marriages, your children, your pain, your happiness - when you examine all that closely, what you really find out is that the only person you really go to bed with is yourself. The only person you really dress is yourself. The only person you really eat with is yourself. So in the end life must be what you do with yourself.

"And all the wonderful surroundings, like people you live with, your friends, your co-workers, are all extensions but they're not you. The only thing you have is working to the consummation of your own identity. And that's what I've been trying to do all my life. People always want to know where my drive comes from. Well, all I can say is my drive is what I have to do. See, what always finally happens to me is that the not trying is harder than the anticipation of failure if I do try. That's why I started writing. It was harder not to write and as you know there's nothing harder than writing."

However, life with MacLaine, says MacLaine, "can become a group therapy session." There is, after all, that directness, that damnable directness of hers that she finds refreshing but can scratch, says, 95 per cent of the male roster at least.

That does not include, however, writer pete Hamill, who despite his recent escorting of Jackie Onassis is still, says MacLaine, the main man in her life. Until a year ago the two had lived together for five years). "Pete's and my relationship is probably better and closer than it's ever been," was MacLaine's remark - and only remark - on that subject.

But, then, she added, between her books, poliitics, movies and night clubs, well, she doesn't have that much time for "deep personal involvements" anyway. "Life is feast to me," is how she puts it, "and sometimes I'm hungry and sometimes I'm not. I mean I do know women who have four lovers at once and I'm amazed. If I had all that going on I wouldn't be able to figure out when to jog and when to make love."

Of course, she added, lightig up yet another Vantage and laughing, "I suppose you've noticed that settling down doesn't exactly run in our family anyway."

Speaking of which MacLaine says she is in fact very close to her brother, Warren Beatty, despite some popular opinion about their sibling rivalry and all that.

"I just saw Warren two weeks ago," said MacLaine. "Miss Lillian came out to stay with me in Malibu and when I asked her who she wanted to meet she said, 'I just want to meet your brother.'" So, she continued, smiling, "my brother graciously consented to come over, leaving, of course, three girls at the doorstep. And after Miss Lillian went to bed, we stayed up until 8 in the morning just talking.

"See, the reason people say we don't get along I guess is because we don't see each other that much. But I live in New York and he lives in California. I mean he loves Hollywood. he lives there. Can you believe it? And in a hotel yet. He bought a house, had it renovated and has never been in it. He tells me it's a palace but he still lives in two rooms at the Beverly Wilshire. If those walls could talk, huh"

And does sister think brother will ever marry.

"I doubt it," She deadpans. "Warren likes to do his own shirts."