EXCLUSIVE: How Rebel Wilson is being compared to Adele with dietitians saying she's already lost a staggering 18kg - as she vows to make 2020 her 'year of health'

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She recently vowed to make 2020 her 'year of health'.

And it seems all that hard work in the gym is paying off for Australian actress and comedian Rebel Wilson.

Dietitian and exercise physiologist Kate Save told Daily Mail Australia that she believes Rebel has lost up to 18kg (40lbs or 2.8 stone).

She's the new Adele! Rebel Wilson has already lost 18kg, according to a team of diet experts. Pictured left in December 2017, and right in May

Save, who is the director of Peninsula Physical Health and Nutrition and CEO of Be Fit Food, arrived at the estimation by studying before-and-after photos of Rebel. 

'My team aligned at 16 to 18kg given that her height is quite short [5ft 2in],' she told Daily Mail Australia. 

The man behind Karl Stefanovic's weight loss, Dr. Nick Fuller, told Daily Mail Australia this week that Rebel will have to focus on keeping off the weight once she loses it.

'Weight loss is not the challenging thing, preventing weight regain is,' Dr. Fuller said. 

'This is the challenge that Rebel will now face following her weight loss. Research shows that fewer than five per cent succeed on their weight loss journey long-term and this is due to strong biological forces that come into play when we lose weight.'

He said that the human body fights weight loss and that the metabolism slows down. 

'Our body fights the weight loss to ensure we return to our set point (our starting weight). Our metabolism decreases so we burn less energy at rest and our appetite hormones change in function telling us to eat more; consequently we end up back where we started.'

'The Interval Weight Loss program is designed to prevent the body fighting itself so a person can not only lose weight but keep it off.' 

Rebel, who hopes to slim down to 75kg, updated fans on her health and fitness journey during an interview on Sunrise on Monday. 

Go Rebel! Dietitian Kate Save who is the director of Peninsula Physical Health and Nutrition and CEO of Be Fit Food, arrived at the estimation by studying before-and-after photos of Rebel

Kindred spirits: Rebel's weight loss journey is similar to that of British singer Adele. The Hello hitmaker is pictured left in 2016, and right earlier this year 

'There's been a lot of working out, which was hard with the coronavirus restrictions,' the 40-year-old Pitch Perfect star said.

Rebel added that she was happy gyms were finally reopening in Australia because she was ready to step up her training and 'pump iron'.

'I had to go out to the park and lie on the dirt doing my sit-ups,' she laughed. 'It's a bit brutal, and it's getting cold in the mornings.'

Back to it: Rebel, who hopes to slim down to 75kg, updated fans on her health and fitness journey during an interview on Sunrise on Monday. 'There's been a lot of working out, which was hard with the coronavirus restrictions,' the Pitch Perfect star said

Going for gold: Rebel added that she was happy gyms were finally reopening in Australia because she was ready to step up her training and 'pump iron'

Last month, the Hollywood star vowed to slim down to 75kg (165lbs or 11.8 stone). 

'Even if you have to crawl towards your goals, keep going - it would be worth it,' Rebel wrote on Instagram.

'Try and give a little bit of effort each day. I know some days are frustrating as hell, you feel like giving up, you get annoyed at the lack of progress [but] good things are coming your way.'

Rebel's personal trainer, Jono Castano Acero, recently shared the star's workout routine with E! Australia.

During a typical week, she does a combination of HIIT workouts, mobility, weights and resistance, and has just one day per week of rest.

Keeping fit: Rebel's personal trainer, Jono Castano Acero (left), recently shared her workout routine with E! Australia. During a typical week, she does a combination of HIIT workouts, mobility, weights and resistance, and has just one day per week of rest

'Rebel and I are just focusing on enjoying training and getting the body moving,' he said, adding that she was 'pleasure' to work with because of her sense of humour.

In March, Jono told Daily Mail Australia his top tips to help people lose weight.  

The Sydney-based gym buff said that people should be working out for at least 45 minutes a day to shed the kilos.

Just do it: In March, former soccer player Jono said that people should be working out for at least 45 minutes a day to shed the kilos

Balance: While calorie intake is important, Jono says, dieters don't have to sacrifice cheat days

And while calorie intake is important, dieters don't have to sacrifice cheat days.

'At a minimum, I preach to always get in 45 minutes daily. Whether that be a workout, a walk or something else that's physical that you enjoy doing,' Jono said.

Jono, 28, believes that high-intensity interval training (HIIT) is the best form of exercise for weight loss.

'What I do for most of my clients looking to shed some pounds is incorporating a mix of different exercises into their session. Short bursts of intense cardio combined with resistance training and weights is my special formula,' he said. 

'It fires up the heart rate and calorie burn as well as builds lean muscle so your body continues to burn calories at a high rate throughout the day.'

Diet is just as important as exercise, and Jono recommends eating at a 'calorie deficit' if you want to lose weight.

'Think about it, if you train one hour a day, it's very important what you do in the other 23 hours,' he said.

'If you're looking to shed some pounds, it's important to eat at a calorie deficit meaning your total daily calorie expenditure needs to be higher than your calorie intake through what you eat.'

But cheat days are still okay, so long as you burn off the junk food with exercise.

'Find what works for you and stick to it,' he said. 

'Try not to cheat during the suggested time until you feel confident enough to have a treat and know that you will get right back on it after.'

Before and after: Rebel is pictured left in 2011, and right in January