People who live past age 100 often have 1 trait in common: a penchant for routine, even unhealthy ones

• Elizabeth Sullivan lived to 106 after drinking three Dr. Peppers daily for years.
• Other super-agers swear by unhealthy habits like daily whiskey, tobacco, ice cream, and bacon.
• A new book on "routineology" argues the structure of habits are more important than their content.

For her 106 birthday, which was also her last, Elizabeth Sullivan received a gift basket stuffed with Dr. Pepper cans and a cake shaped like the soda from Dr. Pepper's parent company itself.

The brand owed her. For years and maybe decades, Sullivan, a retired math teacher in Fort Worth, Texas, drank three cans of Dr. Pepper daily.

"People try to give me coffee for breakfast. Well, I'd rather have a Dr. Pepper," she said in a 2015 interview with a local TV station. "Every doctor that sees me says they'll kill you," she continued. "But they die and I don't, so there must be a mistake somewhere."

Sullivan is one of several centenarians author Dr. Angel Iscovich, a former emergency medicine physician, highlights in his new book, "The Art of Routine: Discover How Routineology Can Transform Your Life."

He told Insider their stories demonstrate how important habits are - even if they're considered "bad" ones. The routine, he said, matters more than the habit itself.

Other super-agers have sworn by daily whiskey, ice cream, and bacon

Agnes Fenton was New Jersey's oldest living resident until she died at age 112 in 2017. Her secret? Three Miller High Lifes every day and a shot of Johnnie Walker Blue daily, her obituary says.

Her habit began after her doctor prescribed her alcohol in 1943 to treat a benign tumor. She maintained it until a few years before her death.

Then there's Virginia Davis, who was featured by the BBC and the Huffington Post in 2015 at age 108. She ate a bowl of ice cream every night. "Maybe that's her secret," the centenarian's caregiver told the HuffPost. "She finishes one gallon in a week."

While Davis said she never drank alcohol or smoked, other super-agers have credited those habits as their key to longevity. Fredie Blom, a South African man who in 2018 at age 114 was featured on the BBC, said smoking was his vice. "Every day I still smoke two to three 'pills,'" he told the station, referencing tobacco rolled in newspaper. He reportedly died last year.

And Susannah Mushatt Jones, who was the oldest living person until dying in 2016 at the age of 116, maintained a breakfast routine of eggs, grits, and bacon. "She'll eat bacon all day long," her aide at the Brooklyn facility where she lived told Page Six.

Only a fraction of a percent of people are 100 or older

The current life expectancy is 75.6 for women and 70.8 for men. Out of the 7.8 billion people in the world, they are only about 316,600 - or 0.004% - centenarians living today, Iscovich reports.

And while some researchers have emphasized the healthy habits they have in common, like eating a plant-based diet, maintaining strong relationships, and moving regularly, Iscovich features the unhealthy ones to make his point.

"The content of our days is not as important for longevity as doing things regularly to support stability," he says in the book.

That's not to say you should never be spontaneous or seek novel experiences - those are important for happiness and fulfillment throughout your life. But humans are wired to live rhythmically, beginning in the womb where the fetus develops a routine and extending to the field of chronobiology, or the 24-hour cycle that affects your sleep, focus, and sociability.

Riding those cycles, Ischovich and his co-author Michael Ashley argue, is the key to finding purpose, meaning, joy - and longevity.

"Pursuing those activities that reduce fear of the unknown and mitigate stress can lead to a more controlled environment, offering stability and aiding in longevity," they write. "Coupled with the importance of consistent activities is the need for purpose."