Dan Buettner has revealed that a cup of this particular food is associated with an additional four years of life expectancy. 

Buettner, 64, is the founder of Blue Zones and the presenter of the popular Netflix documentary 'How to Live to 100'.

The National Geographic fellow studies the world's 'blue zones' - otherwise known as areas on our planet where a high proportion of people live well into their old age. 

The author analyses everything about these areas including their diets and exercise routines to their work habits and relationships within their communities. 

Buettner observed that regardless of where the longest-living people in the world are based ( in Europe, Asia, Latin America, or elsewhere), they eat 90%-100% whole food and plant-based diets.

"The five pillars of every longevity diet in the world are whole grains, greens, tubers like sweet potatoes, nuts and beans," the expert explained.

Buettner added: "In fact if you are eating around a cup of beans a day, it's probably adding up about four years to your life expectancy."

That being said, the New York Times best-selling author did note some of the foods that were less common in blue zone diets.

For instance, he noted that they do eat some meat but probably about five times a month on average.

"Way less fish and fewer eggs than you would think," Buettner continued. 

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The expert also observed that there's no cow's dairy in Blue Zone diets.

Additionally, Buettner noted that people in these areas drink mostly water, teas and coffee.

In fact, they will consume around six glasses of water a day on average.