Or, again, we may choose brick for walls, floors, and chimneys but it would not do any better than wood for windows, would be rather unsatisfactory for ceilings, and impossible for doors.

In other words, we could not build a modern house from one kind of material only and we really need at least four to carry out even a simple plan.
In a similar fashion, diet is constructed from fuel material, body-building material and body-regulating material.

No diet is perfect in which these are not all represented.

Now, foods are like sections of houses.

Some correspond to single parts, as a floor or a window or perhaps a chimney; others to a house complete except for windows and roof; still others to a house lacking only a door or two.