"I follow science and common sense," the model and registered dietician nutritionist tells PEOPLE. "Science says you should eat a variety of foods and eat more fruits and vegetables – I do that."
"I pretty much eat when I'm hungry and I don't eat when I'm not hungry," she adds. "That is my best philosophy for maintaining my weight."
And she's not one to hop on the trendy food train. "If there's foods I don't like, like kale, it doesn't mean that I'm not efficient in my diet, it just means I can eat broccoli and other green vegetables," says Musk, 67. "That's what people don't understand, is that as long as you're having a variety of foods in your diet, you don't have to have the food of the week that's everyone going crazy about."
The mother to Tosca, Kimbal and co-founder of Tesla Motors, Elon Musk, also relies on a diet rich in fiber.
"My kids always joke about how there was never any great food in my home," she says. "I remember my daughter at her 21st party, there was a big cake for her and she came over and her friends came along and she said, 'there's a cake in this house!' So they said, 'Why are you so surprised?' And she took them into my kitchen cupboards and opened them all up and she said, 'everything in here makes you poop!' And it's true."
Musk didn't always have such a handle on her health during her five decades of modeling. But two masters degrees in dietetics and nutritional science has helped her understand her body.
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"Many models are just naturally thin so we'll be on set and they'll eat the cookies and I'll say, maybe I can do that. Well, I can't!" she says. "I will gain two lbs. overnight very easily and it'll take me two weeks to lose it. You have to be responsible for your eating habits and it's really not easy and it's not fair but that's how it is."
The 5'8" size six model – who's booked campaigns for Clinque and Revlon – has also come to terms with her relationship with exercise: "I don't like exercise, I'm not good at it," she says, adding, she "drags" herself to workout about five days a week despite her distaste.
As for her advice for others, "the main thing is you have to eat well, stay out of the sun, and don't smoke. Those are all huge aging factors and you need to be active but stay positive," she says.