Mia St. John might have become another Salma Hayek or Jennifer Lopez, a lovely Latina presence on the silver screen, had she not made a snap decision to score knockouts instead of merely being one.

Beset by a score of personal demons that she is only now willing to discuss, St. John – who recently had been released from a hospital, where she was being treated for obsessive-compulsive disorder – was in her early 20s and juggling acting, auditions and amateur Tae Kwon Do tournaments when she again arrived late for her acting class after it overlapped with her Tae Kwon Do training session.

In her new fitness book, The Knockout Workout, St. John writes of the challenge posed by her exasperated acting coach.

“Do you want to be a fighter or an actor?” the instructor of aspiring thespians asked.

“Without taking even a moment to rationalize, I let my heart speak for me. `I want to be a fighter,’ I said and walked out,” St. John reveals.

Now that her boxing career is winding down – she is 45-10-2, with 18 victories inside the distance, but with five losses in her seven most recent bouts – St. John, who turns 42 on Wednesday, is willing to bare even more than she did during that notorious 11-page Playboy  pictorial that certified her as the fight game’s most bodacious babe.

“As you get older, your reflexes slow down,” St. John said in a moment of quiet reflection during the Philadelphia stop of her book-signing tour last week. “But boxers still think they have it. You’re, like, `If I change this or that, I can still do it.’

“I’m at a point where I’m starting to really wonder if I can still do it, or if I’m just fooling myself. But if I did have to end it now, I can look back and say I fought the best. I eventually became a good boxer. I wasn’t the best, but I fought the best.”

But if St. John draws any satisfaction from having shared a ring with Christy Martin (against whom she was surprisingly competitive in losing a unanimous, 10-round decision in 2002), Jelena Mrdjenovich, Holly Holm, Jessica Rakoczy and Jaime Clampitt, it pales in comparison to having conquered her most formidable opponent: Her own inner doubts.

Even as she rose to prominence as the eye-candy lead-in to several of Oscar De La Hoya’s pay-per-view bouts in the mid-to-late 1990s, a heady period during which she was marketed by Top Rank founder Bob Arum with another sideshow attraction, Butterbean, as the pugilistic equivalent of Beauty and the Beast, St. John kept hidden the shame of a past marked by a drunkard father and the psychological scars inflicted upon her by white classmates who taunted her about her Mexican-American heritage, as well as her ongoing battle with alcoholism and bulimia.

“I didn’t come out with any of that until I reached my 40s,” St. John said. “I kept it very well hidden. No one knew except for the people in my most immediate circle. Bob didn’t know. Don King (her first promoter) didn’t know. I really wasn’t ready then to talk about it.”

All of which makes The Knockout Workout  more than just another tome aimed at getting America’s couch potatoes, particularly females, off the sofa and into a health-and-fitness program. Every celebrity with buns of steel and six-pack abs has authored such books, but St. John’s latest literary effort has the distinction of taking us on a tour through her hellish childhood, adolescence and young adulthood, while noting that the first step for women with self-esteem issues is to realize that they are empowered to change their lives.

“It’s something I’m always going to have to deal with,” she said of the bulimia and alcoholism that lurk in the shadows, a food- or drink-fueled binge away from again raising their ugly heads. “And that’s OK. We’re always growing, always evolving as human beings.

“What a lot of people, particularly women who can’t keep unwanted weight off, don’t realize is that it’s not about the food. It’s about how you feel on the inside. Unless you come to grips with that and learn how to love yourself for exactly who you are, you’ve never going to stop the cycle of overeating.”

Toward that end, before St. John, a divorced mother of two, lays out the physical aspects of her workout regimen, she advises women that they must first confront these causes that can lead to their consoling themselves with too much or the wrong kinds of food.

***Bad relationships.

***The stress of motherhood.

***A busy career and home life.

***A dysfunctional work environment.

Not that I’m the person to advise St. John on what to write or how to write it, but my take on The Knockout Workout is that its most compelling material is condensed within the first couple of chapters, when she goes public with a life that took too many wrong turns before it got back on course.

She tells that she had a “love-hate relationship with food and with her body,” brought on by the insecurity of living with an alcoholic father who was “an angry and oftentimes violent drunk.”

“We had glimpses of the man I knew my father could have been, if not for the demon that possessed him –which is how I chose to look at it,” St. John continues. “Unfortunately, those glimpses were few and far between, so my sister and I often hoped that he wouldn’t come home at all.”

To take the edge off the hurt she was feeling, St. John notes that “at 10 years old, I found solace in the same addiction that consumed my father: alcohol. I now knew why my father had drunk so much. Alcohol allowed me to escape.”

So, too, did an uncontrollable urge to scarf down everything edible she could put her hands on. Of course, bingeing had to be followed by purging. Induced vomiting is the release valve of bulimics.

“At age 13, I became obsessed with my weight,” she writes. “For every pound I lost, I felt as if I had deposited one more dollar in the bank. The skinnier I became, the better I felt about myself. Weight was the only thing I could control. By simply focusing on my weight and the caloric content of every known food, I could escape everything that was a mess in my life.

“I had so many reasons to self-destruct: my father’s unpredictable and explosive behavior, kids hurling racial insults at me and worse, calling me fat. I started to blame and resent my mother for being Mexican. I drank every day, all day, and not surprisingly was flunking my classes. Then, as if to torture myself further, I began to binge and purge. I ate whatever food I desired and then purged it by taking laxatives, throwing up, or even overexercising.”

So despondent was St. John that for a while, after her graduation from high school, she was temporarily homeless. It’s almost a miracle she pulled herself together long enough to get married (to soap-opera actor Kristoff St. John), bear him two children and to graduate from Cal State-Northridge with a bachelor’s degree in, of all things, psychology.

Marriage and motherhood, however, did not wash clean the stains of St. John’s haunted past. She divorced and once contemplated suicide. She was found huddled in a bathroom by her ex-husband, who took her to a hospital, helped get her cleaned up and urged her to enter treatment for the obsessive-compulsive disorder that constantly was tugging at her sleeve.

Finally sober and having gained a black belt in Tae Kwon Do, she sent a photo and resume to Don King in 1997. King signed her to a promotional contract, putting her on the path that eventually would lead to her current celebrity status.

But it was King’s archrival, Arum, who recognized that St. John was potentially a bigger draw than she had been as an unadorned female fighter with nice but hardly awe-inspiring boxing skills.

“Arum understood how important it was for me to be seen as a strong Mexican-American woman,” St. John writes of her second promoter, whose operation heavily tilts toward Hispanic audiences. “Arum knew who I was as a Latina, and he understood the importance of boxing as a Mexican sport.”

He also had eyes in his head, and those eyes immediately recognized that St. John was a hottie who would appeal to the same lascivious spectators that greet every between-rounds strut by a curvaceous ring girl with whistles and wolf howls. If sex sells – and it does; just ask any advertising executive on Madison Avenue – then why not package St. John as a jabbing, hooking object of lust?

Thus began St. John’s strange but well-paying journey as a regular presence on De La Hoya cards, the “Golden Boy,” of course, having his own base of lovestruck ladies who swooned over his matinee-idol good looks. Mia and Oscar were the Barbie and Ken of boxing, Chicano division, with Butterbean, the 340-pound heavyweight novelty act, tossed in for comic relief.

“He told me what he planned to do when he signed me,” St. John said of Arum’s vision of what she would bring to the table, so to speak. “Arum’s a very savvy businessman. When he took me from King, I was wearing all black. No makeup. When I went with Arum, all of a sudden I’m in pink and wearing makeup. And my name went from Mia St. John to Mia Rosales St. John, which included my name prior to getting married.”

St. John was judiciously matched, which is to say Top Rank sought out the same level of non-challenging opponents as it did for Butterbean, who was depicted as the sort of paunchy Everyman to whom beer-chugging slackers could relate.

One of Top Rank’s matchmakers, Ron Katz, was cut loose when he figured he had found another guaranteed loser in Mitchell Rose, who brought a 1-6-1 record into a Dec. 15, 1995, fight with the “King of the Four-Rounders” in Madison Square Garden. Too bad for Katz that Rose, a former New York Golden Gloves champion, got serious about the sport long enough to score a second-round stoppage of the well-bruised Butterbean.

“I only made a few fights for `The Bean,’ and I didn’t fare so well on one of them,” Katz said a few years later. “Let’s just say he is placed with very carefully picked opponents. I’m not saying the guys `The Bean’ fights don’t try. They’re just not very good, but then neither is `The Bean.’”

The same sort of creative matchmaking was employed in building St. John’s record.

Lucia Rijker, who was maybe the best woman fighter in the world when she was terminated by Top Rank and her gig as the company’s female face turned over to St. John, understood the economic reality of the switch.

“People sell what people buy,” she said. “If people don’t buy what people sell, it won’t be sold for long. When Bob sells Mia, it’s obvious someone is buying. They like the product, for some reason. But it has nothing to do with boxing. It’s entertainment.”

Arum wasn’t about to disagree. He had a good thing going with St. John and Butterbean, even if it was gimmicky, and you don’t change a winning formula.

“I learned long ago I could load up a card with good fighters that would bring me no additional business,” Arum said at the height of the St. John/Butterbean alliance. “That said, if my main event is a piece of crap, none of this would mean much. But the presence of Mia and Butterbean made a great event even more interesting.

“We’re not selling (St. John) as the world’s greatest female fighter. She’s an athletic, sexy-looking dame. We’re not deluding anybody here.”

Nor was St. John deluding herself. Full disclosure: Once, when interviewing her for an item in a notes column I was writing, my eyes drifted downward into the rather spectacular cleavage exposed by the halter top she was wearing. I looked up and noticed she had noticed that I had been noticing.

“Uh, sorry about that,” I said, embarrassed.

“It’s all right,” she replied, almost resignedly. “Everybody does it.”

And now?

“I don’t blame them,” she said of the Top Rank promotional machine that hyped her most visible attributes. “I knew how they were marketing me. I was what you’d call a willing victim. I gave in to it.”

When Playboy put St. John on its cover, “The Knockout” more or less gained parity with Martin, who could only claim to being the sole woman to make the cover of Sports Illustrated. Different strokes for different folks.

Eventually, of course, the Top Rank gravy train stopped chugging for both St. John and Butterbean. There were no hard feelings on St. John’s part; when something is over, it’s best to recognize that and move on.

“I always disliked it,” St. John said of her being regarded as sort of a tag-team partner with the excessively fleshy Butterbean. “I just wanted to fight. But I went along with it until it ran its course. I grew tired of it, Arum grew tired of it. I don’t know, maybe even Butterbean grew tired of it. You have to know when it’s time to step away from something.”

St. John recast herself as a serious boxer, taking on opponents who were more than capable of fighting back. She took her lumps without complaint, and in her most recent outing, a points loss to Brooke Dierdorff on April 4, she was head-butted several times, incurring a gash that required stitches. The ending was controversial in that the referee did not penalize or disqualify Dierdorff, but even if the decision had been overturned or changed to a no-contest, there is no undoing a cut that results in a scar. It’s doubtful that anyone, even St. John’s most persistent critics, would want that fabulous face to begin taking on a Jake La Motta configuration at this late date.

So maybe St. John never makes it to Canastota, N.Y., and the International Boxing Hall of Fame except as a tourist. Maybe she hasn’t won over all of those who doubted her grit, and she probably never will. But boxing gave her a platform to reach out and touch the masses with her true story and her charitable foundation.

She continues to cite De La Hoya as a beacon of what boxing is capable of doing, and not just because of what he accomplished with padded gloves on his fists. He built youth centers and charter schools, giving back to society in ways that transcend a stiff punch to the jaw. See, it’s not what you do to become rich and famous that matters; it’s what you do once you have attained those things that can transform a mere athlete or entertainer into a humanitarian.

“I’ve had a great career which allowed me to do so many things – my books, my DVDs,” St. John said. “I was able to start my foundation for Latinas in the United States and in Mexico.

“To me, what I did outside the ring meant so much more to me than what I did inside the ring. The boxing was a means to an end.”

Having come from where she was to where she is now, you just have to figure that St. John has to like what she sees whenever she looks in the mirror. On someone like her, even a scar can look gorgeous.