You could measure by calories, since there will not be a huge difference in the number of calories consumed by people with different diets.

Giving an incredibly quick example, wheat and ground beef have roughly the same caloric density

Wheat: http://www.fatsecret.com/calories-nutrition/usda/whole-grain...

Beef: http://www.fatsecret.com/calories-nutrition/usda/ground-beef...

But since it requires numerous pounds of wheat to create one pound of beef, you are losing overall caloric density.