While many people recognize Maye Musk as the oldest model to become the face of CoverGirl or as the mother of Tesla co-founder and CEO Elon Musk, she also had a successful career as a registered dietitian for 45 years and still keeps up her credentials.

Though some of her son Elon's Silicon Valley contemporaries, like Twitter and Square CEO Jack Dorsey, are fans of internet-famous diets like intermittent fasting, gluten-free diets and the ketogenic diet, Maye Musk is not sold: "I never did the fad diets, I have to stick to science," Musk tells CNBC Make It.

Maye Musk, 71, is the a dietician and at 69 became the CoverGirl model.

Photo credit Mark Seliger

Intermittent fasting, an eating pattern that entails alternating between periods of fasting and eating, has become mega-popular among high-profile figures from Dorsey, who says it makes him more productive, to actress Jennifer Aniston. But while studies on mice suggest that intermittent fasting could help with weight loss, help slow the effects of aging, improve cognition and possibly protect the brain against neurodegenerative diseases, overall, experts agree that there needs to be more research done on humans to determine whether it's safe and maintainable.

As for the ketogenic diet, it is a high-fat, extremely low carbohydrate diet that was originally intended to treat epilepsy. The point of this diet is to put your body into a state of "ketosis," which means that you're burning fat instead of carbs for energy. Some mice studies suggest that a low-carb diet could improve brain function, and many people use the "keto" diet to lose weight. However given how restrictive it is, it can be challenging to follow and could lead to nutritional deficiencies.

You have to think "...is it sustainable? Is it foods you like? And are you fun to be around?" Musk says of restrictive diets.

Gluten-free diets have been popular but are really only a medical necessity for people with Celiac disease.

"I've had to teach myself in social situations not to talk about anything relating to diet because everybody has their own theory," Musk says.

But there are a few healthy diets Musk approves of, including the Mediterranean diet, which emphasizes vegetables, whole grains, legumes and olive oil; the DASH diet, a low sodium diet aimed at lowering blood pressure; and the flexitarian diet, a mostly plant-based diet.

Musk's own diet is closest to the flexitarian diet: Musk says she typically has high-fiber cereal and 1% milk for breakfast. Then she'll have a snack of yogurt and fruit, and a salad and whole wheat bread, or sushi for lunch. Her afternoon snack is a latte and fruit, and for dinner, she likes to "keep my evenings very light" with a bean soup, or cooked vegetables with protein and a starch like potatoes, rice or whole wheat bread.

"Healthy eating is easy with a knowledge, but avoiding poor eating is hard," Musk says. She loves sweet foods, such as chocolates and cookies, but says that she doesn't like to keep them in her home "because I will finish them off."

The best thing people can do to improve their diet is pretty simple, Musk says. "You have to really use common sense and enjoy the foods you eat, and you have to be aware," she says.

Reporting by Cat Clifford and Jessica Leibowitz

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Maye Musk was a dietitian for 45 yearswhy she doesn't believe in fad diets like keto or intermittent fasting - CNBC