This is not true. An American can obtain and hold citizenship to another country. (Tom Hanks and Rita Wilson, just recently did this. The US government doesn’t recognize your second citizenship in terms of legal issues. But there’s nothing stopping an American from gaining another country’s citizenship. I know this because I’m a US immigration officer.

On the reverse, if a Greek citizen becomes an American citizen, they must renounce their Greek citizenship.

Thanks for comment but you must have not read all of it. What the research is on is what the original Ikarian diet was – e.g. WWII era. Yes, for the generation there today, it is totally a westernized diet just like you mention.

Nope American laws don’t allow dual citizenship. To become Greek citizens must have legally lived in Greece for a period of ten years within the last twelve years in order to apply for the citizenship. The applicant must be an adult at the time of the lodging of the application and must not have been convicted of any of a range of crimes — including drug trafficking, money laundering, international economic crimes, crimes committed with the use of modern technology, monetary crimes, crimes against adolescents, smuggling and so on — or any crime receiving a sentence of a year or more in prison.