Now, we know exactly how she achieves her famous physique - at last!
Aussie-born Mary reportedly follows the Scandinavian Diet, not Keto as first thought.
"A closer look at the Scandinavian diet shows people living in Denmark, Sweden and Norway, have a much lower intake of carbohydrates and higher intake of good fats and proteins," news.com.au reports.
Unlike many diets that warn off bread, the Scandi Diet promotes to consumption of black bread or rye bread instead of the typical white, which is low-GI and packed with fibre-rich grains and seeds.
Not surprisingly, Mary's diet is also rich in omega-3 - bring on the salmon!
Nutritionist Susie Burrell told the website that Scandi Diet-devotees are also big fans of the humble potato!
"While potatoes often cop a beating here in Australia, they are a firm staple in the diet of Danes as are all root vegetables including parsnips, sweet potato and turnips which thrive in cold temperatures and form a base to the warming casseroles, stews and soups that frequent the menu of these cooler climate countries," she explained.
And, "they are much higher in fibre and nutrients with fewer carbs than a big plate of pasta or a rice based meal."
Other day-to-day food staples for Mary include goat's milk and cheese, and fruit, seeds and nuts, which "add a dose of extra protein and good fat to keep the overall profile of this diet relatively low in carbs, helping those who follow it to keep leaner and healthier long term."
"The approach to dairy is slightly different in Scandinavia with dairy often consumed via cultured yoghurts and drinks including quark, which we are now learning are exceptionally good for keeping our gut healthy," Susie added.