Meet Stacey Hunt. She loves wine, hates protein powder and has zero willpower. How hard could sticking to a gruelling eight-week fitness challenge be?

The promise:

F45, lesser known as "Functional 45", claims to be "the most effective workout method for burning fat and building lean muscle". It involves circuit and high intensity interval training techniques packed into 45-minute resistance and cardio classes. By combining classes with a strict diet plan, the challenge promises to achieve a slimmer, healthier new you over the eight-week period.

An app connects you to a food plan and shopping lists and gives you additional tips and advice. As you progress through the challenge the diet changes so that in the first two weeks you're alkalising and detoxing your body, during the middle four weeks you build lean muscle, and in the final two weeks, you strip fat.

The app keeps you on track with your diet plan as you progress through each week. Photo / F45
The app keeps you on track with your diet plan as you progress through each week. Photo / F45

The history:

Founded in Australia by Robert Deutsch, the first F45 training studio opened in Sydney in 2012. Here in New Zealand, the first franchise launched in 2015. The aim was "to create a group-training facility that features the most dynamic and effective training styles to date".

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The science:

Functional training is the engagement in exercises that mimic or recreate everyday movement. High-intensity interval training, or HIIT, has been proven to be the fastest way to improve general fitness.

The reality:

The first week of the challenge was a shock to the system. You're in the detox phase so there's no alcohol, caffeine or sugar. Your diet consists mainly of lean white meat, eggs and heaps of greens. It took a good few days for my body to adjust and I was pretty miserable for a while. I had headaches from the coffee withdrawal, was grumpy and found some of the food boring and hard to eat. Protein powder features heavily over the eight weeks, and I can't stand it.

The middle four weeks focus on defining physique and building lean muscle. The nutrition plan moves to a high-protein focus, which gets a bit monotonous after a while.

Unfortunately for Stacey, protein powder features heavily throughout the challenge. Photo / Getty Images
Unfortunately for Stacey, protein powder features heavily throughout the challenge. Photo / Getty Images

The final two weeks are the hardest for most. Phase three features a high-fat, low-carb meal plan to help strip remaining body fat. It's a tough push to the finish line.

The challenge's exercise component is great. The classes are varied so there's no chance of getting bored. And there's really no way you can slack off - as soon as you set foot in the door you're guaranteed an intense 45-minute workout of burpees, push-ups, pull-ups, squats, lunges, box jumps ... the list is endless. You quickly learn what you love and what you hate (here's looking at you, sled work).

While you can feel yourself progressing as the weeks go on, the classes don't necessarily get easier. If it feels like they are, you increase your weights or amount of reps.

The main thing to know about this challenge is that it takes a lot of time and preparation. Making three meals and two snacks per day, each carefully weighed out, takes time.

Having a well laid-out plan made it a lot easier to stick to, until it came to social occasions. Though I ignored the no alcohol rule, I did significantly reduce my intake. It was hard, time-consuming and at times boring, but actually a lot more manageable than I thought going into it.

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The verdict:

The main thing to know about this challenge is that it takes a lot of time and preparation. Photo / Getty Images
The main thing to know about this challenge is that it takes a lot of time and preparation. Photo / Getty Images

There's a reason F45 has the reputation it does: it gets results. If you're willing to put in the work it will pay off.

My final body scans showed I was down 3.1kg and 2.2 per cent body fat. I'd lost 9.1cm from my abdomen and 4.6cm from my chest. I'd also dropped two points in my visceral fat level - a recording that indicates the amount of fat surrounding internal organs - so a good result for my overall health.

The team at F45 Grey Lynn are really supportive and encouraging and having a group of people around you going through the exact same thing helps. The motto of "team training, life-changing" is accurate. As much as I moaned my way through the eight weeks I would definitely do it again.