On his blog "On the Table" New York Times contributing writing Michael Pollan provides a compelling take on people's dietary selections, citing low income levels and high cost of fresh produce as major reasons why people eat poorly. Research indicates junk food is more cost effective:
A 2004 article in the American Journal of Clinical Nutrition by Adam Drewnowski and S.E. Specter offers some devastating answers. One dollar spent in the processed food section of the supermarket — the aisles in the middle of the store — will buy you 1200 calories of cookies and snacks. That same dollar spent in the produce section on the perimeter will buy you only 250 calories of carrots. Similarly, a dollar spent in the processed food aisles will buy you 875 calories of soda but only 170 calories of fruit juice. So if you're in the desperate position of shopping simply for calories to keep your family going, the rational strategy is to buy the junk.
Mr. Drewnowski explains that we are driven by our evolutionary inheritance to expend as little energy as possible seeking out as much food energy as possible. So we naturally gravitate to "energy-dense foods" — high-calorie sugars and fats, which in nature are rare and hard to find. Sugars in nature come mostly in the form of ripe fruit and, if you're really lucky, honey; fats come in the form of meat, the getting of which requires a great expense of energy, making them fairly rare in the diet as well. Well, the modern supermarket reverses the whole caloric calculus: the most energy-dense foods are the easiest — that is, cheapest — ones to acquire. If you want a concise explanation of obesity, and in particular why the most reliable predictor of obesity is one's income level, there it is.