Fitness guru Kayla Itsines has inspired millions of girls in her now-famous Bikini Body Guide community.

But none more so than New Zealand-born psychotherapist Jessica O'Brien, 26, who has ultimately gained weight to be healthier and look slimmer.

On Wednesday O'Brien uploaded a picture to Instagram showing the major changes gaining 4kg made on her physique, according to Daily Mail.

A post shared by Jessica (@got.it.girl) on

The picture on the left was taken before she started Itsine's "BBG" fitness regime, at 57kgs and the mirror selfie on the right is 48 weeks into the lifestyle change at 61kgs.

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"I never had the confidence to wear a crop top before BBG. Sure, not every day is a good one workout wise, but it has given me such a great sense of purpose and that's where the accomplishment comes from," O'Brien, who now lives in Sydney, wrote in a post on her Instagram page.

She told Daily Mail Australia: "I used to do a lot of cardio, mostly running, and sometimes yoga. I guess I was fit for a runner, but not fit in the general sense.

"I figured that out as soon as I started Kayla's workouts," she said.

It wasn't until she saw pictures of inspirational women on their own fitness journeys that she decided to give BBG a chance.

A post shared by Jessica (@got.it.girl) on

"It involves three resistance exercises a week - I like to call these 'dead by Kayla days' - and three LISS (Low Intensity Steady State) days a week. I normally go for a walk on my LISS days.

"I started to notice my body changing after taking transformation pictures," she said.

But although her body was thinning, her weight was increasing.

Now that O'Brien understands this to be a good thing she fiercely uses the hashtag #screwthescales in protest of worrying about a fluctuating number.

"It can be so disappointing for women on their fitness journey to look at the scales and see nothing changing. So don't take that number on face value."

As for posting her weight and images on Instagram, that didn't always come so easily for the 26-year-old.

"I'm naturally quite shy so I wasn't sure if I wanted to put myself out there like that.

"But the online fitness community is so loving - often sending private messages to check in with everyone's progress - I knew I wanted to be apart of that," she said.

The BBG fan revealed some of her tips for gaining weight, looking thinner and feeling lighter.

"Figure out where your proper fat burning zone is and stick to that heart rate. I found if I stayed in that zone while running my waist thinned dramatically and my abs were more obvious.

"Also make sure you're sticking to a schedule. I'm exercising six times a week so I need to make sure I prioritise my time in order to fit everything in.

"Make sure you're getting enough protein. When I started BBG I was on a vegetarian type diet and wasn't supplying my body with the energy it needed. I find my abs grow fastest when I'm getting high enough levels of protein.

"Oh, and join a community! Fitness groups are a great way to stay on track and feel motivated," she said.

The only thing O'Brien warns against is counting macros.

"I was doing it for a little while and I know some people use apps to track your calories but if you have an obsessive personality these can be dangerous.

"I was definitely noticing myself falling into that obsessive category."