Author Matt Fitzgerald summarized the finding, or rather, the lack thereof, in his new book Diet Cults:
"Science has not identified the healthiest way to eat. In fact, it has come as close as possible (because you can't prove a negative) to confirming that there is no such thing as the healthiest diet. To the contrary, science has established quite definitively that humans are able to thrive equally well on a variety of diets. Adaptability is the hallmark of man as eater. For us, many diets are good while none is perfect."
Further support for this notion comes from a simple glance back at the history of our species. Mankind has populated almost every corner of the earth, and in every diverse situation, humans were able to survive, even thrive, on whatever food their homes had to offer.
Even more convincing evidence has been found by observing those who have lived the longest. The University of California-Irvine's 90+ Study has tracked thousands of Americans who've made it to age 90 and beyond, yielding an unprecedented wealth of information about their lifestyle habits. For lead investigators Claudia Kawas and Maria Corrada, the most surprising finding they made is that most participants didn't seem to be too concerned with their health. Generally, the 90-year-olds said they didn't really keep to a restrictive diet. Nor did they abstain from alcohol, quite the opposite actually! The researchers found that up two drinks a day -- no matter the type -- was associated with a 10-15% reduced risk of death. They also discovered other things that might disturb ardent dieters. Vitamin supplements did not affect lifespan in any way, and being a little overweight starting in middle age positively affected longevity.
The entire article can be read here.