I heard it that the Mediterranean diet is the most studied of them all, and it seems [1] that all studies are within 5 years of length.

So yeah. No long-term studies. It's not even clear if we have hope for such. The military could summon the discipline, but soldiers do retire early. Maybe the officers, I don't know.

[1] https://www.healthline.com/nutrition/5-studies-on-the-medite...