Scientists found a link between iron and neurodegenerative diseases years later, he says, but at the time red meat had already been associated with shorter life spans and cardiovascular diseases.

As a result, he minimized the amount of red meat he ate.

While the primary motivation for writing the book was to help people enjoy their lives, Estep says, he wanted to deliver a message to readers that genes are really important for longevity, but "not in the way that people normally think".

"It's not like they just set the program. They (the genes) age you at a specific rate. (But) it is extremely important to know your genetic variants, to decide what your diet and other environmental factors should be. Then you can set them to complement your particular genetics to achieve physical and mental longevity. So we can control how slow or quickly we age," he says.

As he puts in the book, currently, most experts agree that genes are responsible for between 20 and 35 percent of extreme longevity, and the rest is due to environmental factors, such as diet, sleep, mental stimulation, mood and exercise.

"It's critical that we don't think that there is a specific fixed diet that's best for all of us," he says of food.

"We need to know our genetic variants to find out what kind of diet suits us to live a long life, physically and mentally," he adds.

People can get this information through genetic tests. Besides, he emphasizes that it is also very important to test one's blood iron level.

In the book, Estep focuses primarily on iron that is found to be associated with neurodegenerative diseases, diabetes and cardiovascular conditions.