by Aimu Agresti & Aili Riley
with reporting by Carlene Davis, Monique Meneses and James Vituscka

WHEN L.A.-BASED trainer Stevie Sant'Angelo got word that Jennifer Love Hewitt wanted to work with her this spring, she was skeptical. "I usually don't work with a lot of high profile people because they tend to be neurotic about food, and I don't need that," Sant'Angelo tells Us. But it took only a phone call to ease her mind. ''After three minutes of speaking with her, [I could tell] she's awesome inside and out," gushes Sant'Angelo, whose client, The Biggest Loser trainer Jillian Michaels, referred Hewitt. "Jennifer said the most important things. With her, it's not, 'I want to look good in that dress.' She wanted to start moving around because she thought it would make her feel better."

Mission accomplished

Since embarking on a new diet and fitness regime with Sant'Angelo in late May, the 5-foot-3 Ghost Whisperer star, 29, has shed a stunning 18 pounds, and set an impressive goal to boot. Fitting into her gown when she weds her fiancé, actor Ross McCall? Nope. It's running a marathon to coincide with her 30th birthday in February. "I am in a pretty good workout regimen that I like, so it inspired me to keep it up," Hewitt told Us last month. 'The energy level and the way I feel now is great."

That hasn't always been the case. In November, the actress was criticized after photos surfaced of her looking fuller-figured in a bikini as she frolicked in Hawaii with Scottish-born McCall, 32, while celebrating their engagement. The flak hit such a pitch, the star even blogged, "A size 2 is not fat!" And, after an unflattering bikini picture made it onto a national magazine cover, she sniped during one interview, "I worked 20 years in this business and that's how I got on the cover of People."

Still, Sant'Angelo insists that incident had nothing to do with her sudden shape up: "She told me, 'They said some rude, mean things, but that's not why I wanted to change. I'm still sexy, curvy.' She didn't buy what Hollywood was selling. She has a healthy attitude." A source close to the star, who puts in grueling 12-to-14-hour days on her hit CBS show, says she was inspired to get fit after an ankle sprain in March left her in a cast. "The show is very physically demanding. She wanted to get stronger so she wouldn't get hurt again," says the insider. "It was more about gaining strength than losing weight." Buoying that argument: She hasn't gone to extremes. "She wasn't interested in fads or dangerous, quick [fixes]," says Sant'Angelo. "She didn't do anything exotic; what she did was consistent. And she looks wonderful."

Diet Dos

Half the battle for this Hanes spokeswoman was learning to keep calories in check. "There is nothing as you get older that you can eat more of!" Hewitt told Us weeks before starting the program. "It just happens, you become more conscious. When you're a teenager, you feel invincible to all the things that can happen to your body that aren't good." Sant'Angelo customized a nutritional plan - which currently totals 1,250 to 1,500 calories per day - that is heavy on fruits, veggies and protein (Hewitt loves tuna and chicken). It even allows carbs before 3 P.M. "She doesn't need a full-blown, take-all-the-carbohydrates-and-starch-out diet. She's a healthy girl," Sant'Angelo says of the star, who, in no time, was too small for her Ghost Whisperer wardrobe. "Inside of two weeks, she had already dropped several pounds. It was easy: 1,500 calories can look immense if you know how to assemble them - a giant salad versus a latte at Starbucks." Sant'Angelo's menu also eschews the usual Hollywood staples of meal deliveries, power bars and energy shakes. "The less processed the better. When you teach someone to eat properly, it's about eyeballing portions," she says of her approach. "It's all about her going out to eat and being able to get food from work." It doesn't hurt that Hewitt, who doesn't drink alcohol, is a whiz in the kitchen and has gotten in the habit of whipping up healthy meals (a signature: her roast chicken with couscous) and brown-bagging her lunch. "She makes these turkey sandwiches she eats every day on the set. They're amazing and completely healthy: Jennifer's Special Turkey Sandwich!" reports the trainer. But the ultimate feat has been learning to curb that sweet tooth by loading up on veggies, fruit (she loves cherries!) and limiting herself to just a taste of the most sinful treats. "Once you've had a few bites of something, you've gotten that craving taken care of; she understands that," Sant'Angelo, who bans cheat days, says. "If your lifestyle is healthy, a bite of dessert isn't going to tip the boat."

Fit & Fabulous

Hewitt, who recently admitted to Us, "If I looked like Gisele Bundchen, that would make me stop exercising?" is nonetheless dedicated to her hour-long workouts three to four times a week at Sant'Angelo's Hollywood gym. "She said that genetically, the women in her family are predisposed to look like pears," Sant'Angelo says of the Texas native, who was once thought to be the inspiration behind ex John Mayer's hit "Your Body Is a Wonderland." (It's not true.) To combat a bottom-heavy physique, Sant'Angelo targets the legs, core and rear with plenty of cardio (jumping rope, running, spinning) and an exhausting routine of lunges, pushups, crunches, body planks and weight work (Hewitt is a champ at hoisting 18-pound kettlebells). On the days she's alone, the actress keeps her heart pumping with 40 minutes of swimming, running or roller skating. The results speak for themselves. "She loves the way her legs and booty are looking. Everything looks good. Her arms are getting defined, her tummy is flat," says a proud Sant'Angelo, adding she's not the only one who's noticed. Hewitt recently got a fawning phone call from a studio head: "They had noticed she'd done an amazing job and looked beautiful, healthy and radiant." Of course, her biggest cheerleader is closer to home. McCall often stops by the gym, with their pooch, Mona, in tow, to check on his girl: "He waits for her and talks with their friends in the gym," reports SantAngelo. "He's supportive and totally awesome."

Mind & Body

What needs no overhaul is Hewitt's attitude. The bubbly brunette routinely jokes her way through tough workouts, playing "Name That Tune" with Sant'Angelo as a distraction. She has a well-developed funny bone, once famously phoning Sant'Angelo's "complaints line," griping: "I'd like to complain about my booty. Every time I have toast, it appears on my backside the next day." Says Sant'Angelo, "Self-deprecating humor goes a long way." So does hard work: "She considers health a priority and peppers her week with fitness as opposed to it being last in line."

Keeping It Up

Hewitt's healthy new outlook is infectious. On the set of Ghost Whisperer (returning October 3), she's revolutionized the Craft Services table-which now overflows with fruits and veggies - and she has everyone on a health kick. "The ladies in hair and makeup are all eating the way she is. They're supporting her," says the trainer. They're also keeping her in line should she indulge in any forbidden fruits. (A major weakness? Hot dogs.) "They would rat her out in a second!" Sant'Angelo says. Like her physique, her confidence is also turning heads. "She's not self-conscious anymore. When she first came to work out, she was in baggy clothes. Now she's in tank tops and shorts," her health guru says. "It's changed the way she moves and walks, and her self-esteem."

It should all serve her well as she faces her next challenges: preparing for the Los Angeles marathon in March (she starts training in September) and planning for her impending wedding. But really, it's all about the happily ever after. "I want to be Ross' partner until I'm 80, so I need to live a long time," she says. ''You go to the gym more and you take care of yourself more just for the people around that you love so that you can have a longer life."