The main tenets of the longevity diet—eating more plants, less protein, and fasting—are based on Dr. Longo's research of long-lived populations around the world, including in Greece, Italy, and Japan, as well as studies he was involved in in the United States.

One of these, published in October 2016 in JAMA Internal Medicine, assessed the dietary habits and health of more than 130,000 healthcare professionals.

It found that "high animal protein intake was positively associated with cardiovascular mortality and high plant protein intake was inversely associated with all-cause and cardiovascular mortality, especially among individuals with at least one lifestyle risk factor. Substitution of plant protein for animal protein, especially that from processed red meat, was associated with lower mortality, suggesting the importance of protein source."

Dr. Longo has also researched the benefits of fasting-mimicking diets. His research found that such diets improve factors that have a significant impact on health, particularly as people age, including body weight, blood pressure, and levels of glucose (sugar) in the blood.