Before starting Purely Elizabeth, CEO Elizabeth Stein was a certified holistic nutrition counselor.

In 2007, Stein left a career in marketing, to complete a health coach training program at the Institute of Integrative Nutrition in 2007. She went on to launch her own practice, coaching clients one-on-one and teaching cooking classes on the side.

It was Stein's work as a nutrition counselor that helped launch her health foods brand just a year later in 2008. Now, Purely Elizabeth brings in $147 million a year.

At the time, Stein, an avid long-distance athlete, baked muffins filled with ingredients like chia seeds, quinoa, and coconut oil to promote her practice as a nutrition counselor at a local triathlon.

Stein launched a website for the muffin mixes and garnered some publicity from her email newsletter DailyCandy. She received $10,000 in orders and decided to work full-time on the brand that became Purely Elizabeth.

Though she has since discontinued her practice, nutrition remains a major part of Stein's life today — she calls her eating habits a "non-negotiable."

Here's how she eats every day for better health.

'When I eat better, I feel better'

Stein says she adheres to an anti-inflammatory diet focusing on "leafy greens, proteins, great fats, and great carbs." She also eats mostly gluten and dairy-free.

"I know that when I eat better, I feel better," she says. "When I'm not eating great foods, I don't feel my best."

Inflammatory foods can contribute to a greater risk of cardiovascular disease, type 2 diabetes, and some cancers, among other diseases, according to the Harvard School of Public Health. These include processed meats like hot dogs, sweetened beverages, fried foods, and saturated dairy fats.

Anti-inflammatory foods include fruits, vegetables, whole grains, and legumes like beans or peanuts.

The Mediterranean diet is one such anti-inflammatory diet. Ranked the no. 1 diet by 69 diet and nutrition experts for the U.S. News and World Report, it emphasizes fruits and vegetables, fish, and nuts, and staying away from red meat, sweets, and processed foods.

While Stein did not specify if she follows a specific dietary plan, she does allow some flexibility in her daily habits. She uses the "80/20 rule," which means she sticks with her stricter nutrition practices 80% of the time, while leaving 20% for unhealthier foods she enjoys, like pizza.

"We can't be perfect all the time," she says. "It's great to be able to indulge guilt-free."

Her rule aligns with what longevity experts typically say: Occasional cheat meals are fine.

"We only have one life to live on this planet," says Dr. Anthony Youn, a plastic surgeon and CNBC Make It contributor. "A tasty splurge is totally fine every once in a while!"

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