School officials count 39 fast food spots within National City's nine square miles.

The schoolchildren of more affluent parents not only do the best on standardized academic tests, they also do better on measures of physical fitness, a recent report on obese children in San Diego County suggests.

Meghann O'Connor: it's cheaper to fill up on junk food. (Photo of Popeye's on Plaza Blvd.in National City)

The study, from the county's Childhood Obesity Initiative, located the most severe problems in predominantly Hispanic school districts and found the National City elementary schools to have the most unfit kids — about half the fifth graders were deemed overweight or obese — among 42 school systems throughout San Diego County. Coronado tops the opposite end of the scale; leading all 42 districts in student fitness. There, only one in six children tested as overweight or obese.

Coronado entertains but one Burger King near the ferry landing.

In National City, a key administrator says students are more prone to dine at fast-food restaurants not only because of their ubiquity (school officials count 39 such spots within the city's nine square miles), but because it is tolerated by their financially hard-pressed parents. The reason, said Meghann O'Connor, director of student support services for the National School District, is that it's actually cheaper to fill up on junk food than have a nutritious and balanced home-cooked meal.

O'Connor pointed out the proliferation of fast-food restaurants in National City — a McDonald's, a Popeye's, a Wienerschnitzel, a Burger King, an El Pollo Loco and a Jack in the Box can be sampled on a short stroll south from 12th Street along Highland Avenue. Coronado entertains but one Burger King near the ferry landing. Rounding out the most-fit five districts are Rancho Santa Fe, San Dieguito, Del Mar, and the Julian Union High School District. Joining National City among the least-fit are Escondido, South Bay, Lemon Grove, and San Ysidro.

The fitness test itself looked at aerobic capacity, upper body strength, flexibility, and what's called body composition, which determines how much fat-free mass the body contains. A key component is the body-mass index, or BMI, which is derived from dividing one's weight in kilograms by the square of height in meters. For example, a 10-year-old boy would be obese at a BMI of 23 and overweight at a 21. The CDC says about one in five schoolchildren in the United States is overweight or obese.