At her heaviest, Anna Downie weighed 137kg.

She'd tried losing weight plenty of times and in a variety of ways prior to that point, in 2015.

With her yo-yo dieting over the years, she reckons she's lost a total of about 280kg. All to only put it back on again.

However, it was having a gastric sleeve operation - where 80 per cent of the stomach is removed - that turned her life around.

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Downie, 37, said she had always been active and played a variety of sports over the years, and also has a diploma in outdoor recreation and leadership, but it was starting up her own business several years ago where she went from being active, to sedentary.

"The only active job I had was being a commercial kitchen manager, lifting big pots and I was walking around the kitchen all day so I lost a little bit of weight doing that and then I started my business fulltime, which involves sitting down, and I put it all back on."

The sleeve has allowed her to lose the weight and keep it off, but not without a strict nutrition and training schedule.

Now she spends most of her spare either running in the hills around her Silverdale, Auckland, home or at the gym. She's already clocked up the Speights West Coaster and plans to tackle her first half marathon - the New Zealand Sotheby's International Realty Queenstown Half - on Saturday.

She's since dropped more than half of her body weight, 76kg, and is currently 61kg.

It hasn't been easy though. Given the size of her stomach she can't eat any more than a cup full of food at a time which means ensuring she has enough nutrients is a balancing act.

"I have spent the last four years being quite studious about what I put into my body in regards to macros, fats, proteins and carbs and figuring out what my body needs to run on.

"I was a size 24-26 and now a size 8-10. It's a lot of dress sizes, a lot of wardrobes, a lot of giving clothes away and swapping clothes too for people who are putting on weight."

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The surgery involved the food receptor hormones being cut out which meant she no longer knew when she was hungry.

So, she has to remember to eat.

"I can only eat about 3/4 cup to a cup of food, max. And I would never eat something like rice risotto or pasta, it's just too heavy. I eat very fresh food ... and all of the cheese. I just eat a lot of cheese."

She adopted the Keto diet - high fat and protein, low carb - to get the bulk of her weight down initially, before adopting her own method of dieting.

"It's all about being active for me and the right nutrition. It's an amazing feeling to be able to actually run and not have your body fighting against you or weighing you down. You feel so light."