Grain diets are not ecologically sound.

It shortens life, increasing the proportion of humans in the vulnerable stages of early childhood and old age, where mobility requires more energy.

It increases body fat levels to the point where as we see now, most people are at least 20% heavier, that's 20% more fuel to move them around.

And due to that fat and the excess load on the cardiovascular system, more people travel via powered vehicles.

In contrast, people eating proper diets of meat and non grain veg and fruit, they are lighter, more autonomous, live longer, and travel more on foot and bicycle than the fat ass grain eaters.