Genetic isolation. Japan and Okinawa are among the most insular populations in the world, remarkable considering their relative openness to international commerce, tourism and the US military.
Many small indigenous tribes worldwide have similar adaptations to diets. Doesn't mean those diets are good for anyone else.
Eat a balanced diet. Enjoy what you eat. Avoid sugar and junk carbs and excess alcohol. Be careful with most modern fruit -- it's genetically designed to be sugar in fiber.
Use sugar and sugary fruits and foods as fuel. I love me some chocolate donuts. My reward to myself is to eat a couple of donuts on my bicycling days. If I'm doing a 30 minute HIIT session or riding 30 miles or longer, I earned and burned that sugar.
Eat some meat. Enjoy that crispy bbq chicken skin and fatty brisket and hamburger. It's an efficient source of complete proteins and amino acids. A little goes a long way. It helps break the cycle of bipolar blood sugar peaks and troughs. Use butter and olive oil.
Avoid extremes -- the keto kraze being the latest in a long history of bad diets for most people. If you're not a serious athlete aspiring to up your game a notch, the keto diet is practically pointless. It's been around for decades, resurfacing occasionally. Google around for complications from ketoacidosis, etc., and it'll become clear why it's a highly specialized diet that should be undertaken very carefully and under medical supervision.
Ditto some of the vegan diets I see my woke friends struggling with. Two of my local cycling friends have either gained weight or look unhealthy from bad weight loss after going vegan. They won't admit it yet, but they're struggling with the unbalanced diet by eating too many junk carbs and sugar. They struggle with feeling unwell, hungry all the time, feeling less fit. In between humble-brag Facebook posts about their latest healthy meals, they're binging on sugar to compensate for feeling famished and lacking energy. I've seen it many times among family and friends.
In six months they'll be posting on FB about returning to "a little" meat via fish, and feeling much better. They'll post some sort of pseudo-science article explaining why they have a special genetic makeup that requires a little meat. Their special genetic makeup is what the rest of us call being human. Pretty soon they'll be eating burgers and brisket and feeling much better and return to their pre-diet decent baseline health and weight.
Knock it off with the gluten boogeyman. Only a tiny handful of people are genetically incapable of digesting glutens. For most of us, especially if we have some European lineage, gluten is a good source of fuel. In my wide circle of family and friends of diverse ethnicity I know exactly one person who's genetic makeup means she actually does have gluten intolerance.