Sometimes I think we forget that the human gene pool is pretty diverse. Until relatively recently we have survived in many different extremes, desserts, ice belts, tropics, temperate climates. Places where food is scarce, places it's everywhere. Thus our gene pool should contain abilities to survive on many different diets. In fact I think it's more likely that many different peoples would have subsisted on very varied diets and would have quickly adapted to this.
I've read that many asian people cannot stomach milk because they have no lactase in their bodies to break down the milk sugar lactose. Europeans rarely suffer from this. That's because European lifestyles involve dairy produce and asian people don't. That's a generalisation but it shows the effects each culture can have on its population. Now that, as people, we mix much more than before it could be that all these genes will eventually merge but I don't think enough time has passed and there's no proof it could all merge.
What I'm saying is that there is no evidence that we're all exactly the same. In fact all the evidence is to the contrary. We're all slightly unique in our own way and that should mean that the perfect diet for each individual will be slightly unique too. In fact it means that the perfect lifestyle as a whole should be slightly unique. Maybe now it seems a bit silly that we should all be encouraged to eat the same things. Especially when no one can agree.