Children who consume fast food eat more calories overall than those who do not (either regularly or on particular days) because these low-fiber “empty calories” leave people hungry later. One study found that kids who eat fast food consume an average of about 15 percent more calories than those who do not, and gain about an extra six pounds per year as a result if they do not burn those excess calories off through exercise. Fast food was also the main food source for 29 to 38 percent of the randomly-chosen subjects in this study, and it typically replaced healthier options like fresh fruits and vegetables in their diets. 
African-Americans, Latinos and other people of color most likely to live in food deserts suffer disproportionately from higher rates of obesity (and therefore other diet-related disorders) than whites—and fast food is one of the main causes of this deadly disparity. Residents of food deserts typically have a plethora of fast food restaurants to choose from within walking distance of their homes, but the nearest supermarket or grocery store may be miles away, and many low-income individuals do not have access to private transportation and must work two jobs just to make ends meet. Feeding their families fast food is therefore usually quicker, easier and less expensive than shopping for and preparing home-cooked meals. However, reliance on fast food as a dietary staple (especially over long periods of time) causes dangerously unhealthy weight gain and other physical problems resulting from poor nutrition.
Consumer Behavior: The Psychology of Marketing
 Smith, Stephen. “Food, fun — and fat: The battle to shrink the waistlines of America’s children focuses increasingly on how food is marketed, including the use if toys as lures.” The Boston Globe. July 19, 2010. (7/4/11)