Having spent almost two decades of studying what makes us gain weight, as well as how to shed those pounds at the gym, there are few more qualified than Jillian to help the stars get red-carpet ready -- and she's clearly not afraid of a few crunches herself, judging by her abs.
But what about the rest of us who can't stretch to a Tinseltown-style trainer?
As someone who admits to finding it hard sometimes to haul herself to the gym, Jillian has an un-starry attitude to exercise.
What's more, her eating philosophy acknowledges that weight loss is a mental and physical process, with a focus on teaching people to let go of unreasonable goals and embrace realistic ones.
Jillian's online programme -- which talks about self, science and sweat -- allows for the fact that we are all unique, so you can personalise your exercise routine to something you can perform at home.
There is also online support and recipes, and her brand of no- nonsense motivation.
Here's the science bit. Much of Jillian's work, as espoused in 'Master Your Metabolism', is based on detailed science and endocrinology.
"Welcome to the future -- this is the era of genome mapping, stem-cell research and nutrigenomics (the study of how food communicates with our genes).
"Yes, calorie counting and exercise are very important but they are not the whole story," she writes.
"Underneath the dieting and workout programmes are the little messengers that carry information from your body to your brain and vice versa. These 'little messengers' are your hormones."
According to Jillian, hormones tell your body everything it needs to know about food: if you're hungry or full; whether to store the food you've eaten or burn it as fuel; how to move and consume energy stores.
"Your body can't work the way it's supposed to if any of your hormones are out of tune," she says.
"But instead of trying to isolate one hormone at a time -- which is totally impossible -- this book is about how you can naturally optimise all of your hormones. And how you can do it without taking dangerous or expensive drugs."
What can I eat?
All that may sound complicated but it's simple: ditch the junk.
Jillian avoids confusing formulas which involve low-fat, low-carb, high-protein science.
Instead, there's a simple equation -- the amount of calories you take in should be less than the calories you put out.
Her plan is personalised according to your body type, your metabolism and your goals. The diet then calculates how many calories you need each day to lose weight, and offers ideas on meals to suit your metabolism.
Remove, restore and rebalance -- that's the mantra. Remove the anti-nutrients by clearing out all the processed foods and synthetic chemicals in the kitchen which screw up your metabolism.
Restore the hormone power foods, and rebalance your timing and portions.
Says Jillian: "The first part of the plan is to remove all of the chemicals in your environment -- your cleaning supplies, your beauty supplies and your food supply.
"The second part is to restore power nutrients to your diet. The third part is to make some easy changes to rebalance your life, everything from getting enough sleep to working out correctly, to what time of the day you should work out, to how much you should be eating."
Jillian is big on eating clean, fresh foods free from preservatives, trans-fats, artificial flavours, colours and sweeteners.
Her top tip? "Go organic. That would be the one thing -- it's not even exercise. Our food is corn derivatives, food derivatives and chemicals. It is making us fat and sick."
Nothing is off the menu (though it's best to avoid those trans-fats), but Jillian wants you to establish healthy eating habits, and recognise the emotional triggers which can cause you to overeat.
On the menu
Here's an example of Jillian's meal delivery package, a service she is offering to her LA clients:
Breakfast -- mixed grains and berries; lunch -- prawn and mango salad; dinner -- wild halibut with ratatouille; dessert -- wholewheat fruit tartlet.
And because she has a sweet tooth, Jillian doesn't let her dieters suffer.
"I try to get my dark chocolate in somewhere throughout the day," she admits.
"I have to have it. I work it into my calorie allowance and I will always have 200 calories of chocolate... always!"
While Jillian may be popular with the stars, not everyone can have an A-list body, and she is honest about that.
The diet acknowledges that not everyone has the build to become an action hero or a supermodel; you have to work with what you've got.
Accordingly, there is an emphasis on having realistic goals which involve recognising your true body shape, then setting up healthy eating and exercise habits, and steering clear of those emotional triggers which can set you off on the wrong path.
"When you're skinny, you're not necessarily a healthy person," Jillian says. "And when you're obese, you're not disease-free; you're struggling with fatty liver disease, diabetes, stroke, cancer, sleep apnea, acid reflux, and the list goes on."
What about exercise?
There's no easy way to say this. Exercise is the major component of Jillian's strategy for weight loss. It is the best way to burn fat, and it's the way to get calories 'out' to keep the programme's equation balanced.
The lady herself is honest about how tough she can find a workout: "If I don't work with a trainer, I'm like, ' Okay, I ran my 30 minutes, I'm going home'.
"People often ask me, 'Why do you work with a trainer?' And I say, 'Because he's a monster. He pushes me in ways I wouldn't push myself'."
Seriously? "If I didn't work out and watch what I ate, I'd easily be 200lbs," she admits. "I'm not the girl who loves to go to the gym. And I'm not the girl who loves broccoli, either.
"But I am the girl who feels potent, capable and confident when she feels strong, and I don't want to give that up. To me, it's about building a better body."
Jillian's programme offers a comprehensive weekly exercise plan, suited to your level of fitness. This fitness planner explains the muscles involved and how to properly warm up and perform each exercise.
It also provides exercise tips, variations and progressions, complemented by a fitness diary that
helps you work through issues and keep track of your achievements.
Why should I try this?
Jillian's no-nonsense approach has earned her an army of fans. Her other books include 'Making the Cut: The 30-day Diet and Fitness Plan for the Strongest, Sexiest You', intended to help anyone gearing up for a big event, and her first book, called 'Winning By Losing', focused on the psychological and behavioural aspects of losing weight.
And there's a reason why she's a mentor to those who've reached a huge low point -- and high weight -- on 'The Biggest Loser'.
"I use fitness to empower people. It makes people feel strong and confident and potent, and that strength transcends into other parts of life," Jillian says.
"And now I understand it's the same with your diet and other aspects of your lifestyle. Once you make the decision to take control of what goes into your body, you're able to harness that power."
Be warned though: Jillian certainly doesn't pack any punches, even with her trainer.
"'Making the Cut' is not some namby-pamby 'lifestyle' book that's going to waffle on about moderation for 'better health' and leave you with the warm fuzzies," she says. "It's about seeing how far you can go, getting a little crazy, and maybe, along the way, making that ex of yours want you back."
Sounds like quite a promise, doesn't it?
"You will feel strong. You will be strong. It's about getting in the best shape of your life -- so channel that inner badass, and let's get ripped!"