The one thing Elsa Pataky wants you to stop doing

She may be married to a famously ripped Marvel superhero, but when it comes to fitness Elsa Pataky is as real as they come.

At 43, the Spanish actress — who relocated to Byron Bay with husband Chris Hemsworth and their three kids five years ago — is on a mission to share her fitness philosophy, which includes doughnuts (!!) and a refusal to use the scales.

"I never weigh myself," Pataky tells 9Honey Coach from Sydney's Park Hyatt.

"When I was younger, I used to. But I think it's a bad thing to do, because one day you might eat a little bit more or drink a little bit more and it shows on the scales and you start getting obsessed."

Pataky is promoting her new book, Strong, which explores her approach to diet and fitness. But don't expect any shortcuts.

Growing up in showbiz, Pataky has tried all the fad diets. "Sometimes they will help you lose weight really fast, but know that the weight you've lost will come back just as quickly."

Pataky says we should be making small diet changes with a long-term goal of a healthier and more active lifestyle.

"I don't believe in miracles," she says, adding that in the last few years she's had to work harder than ever.

"You really start seeing changes in your forties. With age, you start losing muscle and you have to work out even more, like I have to work much harder on my glutes because I want to keep them up."

That said, Pataky does believe in treating yourself admitting she has one "cheat day" a week.

"I love desserts and I love sugar," she says.

So what does an Elsa Pataky cheat day look like exactly? A break from her five-day training schedule and a green light to eat her favourite foods: "pizza and Krispy Kremes… one, two, sometimes three."

"It's not a life to be suffering all the time, you need to have those moments of pleasure like eating sugar and when I eat it I really want to enjoy it and be happy about it."

Strong by Elsa Pataky is published by Plum, RRP $34.99