What economic impediments exist that prevent adoption of better food habits?

The most important impediment to good diets among the poor
is the price of high-quality food.  Studies have shown that the price of a given number
of calories of "junk food" is much lower than the price of a given number of calories of
healthier food.

The poor often cannot afford or do not have
access to the kinds of healthy fresh foods that nutritionists recommend that people
should eat.  Poor areas often lack the sorts of large grocery stores that carry many
healthy choices.  Even if poor people have access to such stores, they often cannot
afford to pay the higher prices for the healthier foods.  As the ajcn.org link below
says:

...
there is an inverse relation between energy density (MJ/kg) and energy cost ($/MJ), such
that energy-dense foods composed of refined grains, added sugars, or fats may represent
the lowest-cost option to the
consumer.

In this way,
economic impediments can have a great deal to do with preventing poor Americans,
especially, from adopting better eating habits.