"Just whatever I'd see on the table I'd go hard on," he said.
That all changed in 2015 when the promising young rugby player was selected to participate in a Cook for Life course run by food manufacturer Nestle.
"That was the best choice that I ever made."
The tutors at the programme taught Sa'u how to cook nutritious, affordable meals and developed an eating plan for him.
"I gave that [the plan] to my parents and that's when mum said, 'Oh this is a good programme.'
"My family were pretty big as well and they saw my progress and thought, 'I want to look like him.'"
Sa'u's mum, Moana, started cooking meals from the eating plan for the whole family.
Fatty pork and rice were banished, replaced with porridge and banana for breakfast, lots of fruit and chicken breast and green veges for dinner.
Sa'u met his Cook for Life tutors once a month for about nine or 10 months so they could monitor his progress.
He wrote a food diary about what he was eating and learned more about nutrition.
After a year and a half of sticking to the eating plan, Sa'u had shed more than 50kg to get down to 114kg.
Now, at 19, he said his life had changed. Because he had more energy he was more focused with his training, running more and exercising morning and night.
He has played for the ITM Cup team Tasman Makos and will soon head over the Ditch for a one-year contract with the Gordon Highlanders club in Sydney.
"My next goal is to make the international team for rugby, either for the All Blacks, Australia or Manu Samoa."
Sa'u said anyone who was thinking about changing their eating habits should "just think about your future".
"If you have a dream to chase you've got to do the hard yards and eat right."
His is just one of many success stories to come out of the Cook for Life programme.
Over the past six years Cook for Life has helped just under 3000 Auckland teenagers change their eating habits and lifestyles.
This year the company has partnered with Mangere Mountain Education Centre, a not-for-profit community organisation, to allow more Auckland teens to participate in the programme.
Nestle New Zealand chief executive Veronique Cremade said she hoped more than 750 schools would sign up to the workshops at the centre, which works with schools to educate kids about Maori heritage and agricultural science.
The centre's chief executive, Simon Kozak, said iwi leaders and tutors would teach teenagers how prepare healthy, affordable family meals, using traditional Maori crops grown in the centre's three communal gardens.
At the end of the workshop students tuck into the meals they have learned to prepare.
Cook for Life
Schools can sign up to the workshops by calling the centre on 09 634 7305 or emailing visitorcare@mmet.co.nz