American children are gaining weight in a period – summer – when they once did not. What or who is to blame?

Two things spike at this time of year: the heat and, apparently, our schoolchildren’s weight.

The first observation is obvious. The second comes from Ohio State University and Indiana University researchers who found that the increase in children’s body mass index, or BMI, during the summer was more than twice as fast as during the school year.

Many of us who are baby boomers or older remember summers as a time of unbridled joy and nonstop physical activity. The day’s possibilities propelled you outside on foot or on a bicycle. Having to return home to eat lunch or dinner was an annoying intrusion. There were no personal computers, and you could count the number of TV channels on one hand.

Fast-forward four decades. Idle time is largely spent on the Internet, playing video games, watching DVDs or cable TV in air-conditioned comfort while grazing on unhealthy food. It is no wonder that Duke University found this year that U.S. children age 6-11 are almost four times more likely to be obese than were children of similar ages in the 1960s. Few public health issues generate more anxiety than childhood obesity.

About one-third of the nation’s children are overweight or obese.

Physicians are treating elementary-school children with adult conditions such as high blood pressure, high cholesterol, diabetes, joint pain and even gallstones.

About 80 percent of obese teens turn into obese adults, and we are clearly headed toward large increases in heart disease, cancer and other weight-related chronic conditions. A group of scientists in 2005 projected an unprecedented drop in life expectancy of two to five years unless obesity is reversed.

An accompanying trend is an alarming lack of fitness among children, which may or may not be weight-related. About 2.6 million Texas students in grades three through 12 performed six fitness tests as a result of a legislative mandate passed last year. Fitness peaked in third grade and eroded every year thereafter. About 32 percent of third-grade girls were considered fit, compared with 8 percent of 12th-grade girls. The comparable numbers for boys were 28 percent and 9 percent.

A recent study in the Journal of the American Medical Association of more than 1,000 children in 10 U.S. cities produced similar results.

As with most issues, people want to find someone to blame and a simple solution. But child obesity is very complex, and there is plenty of blame to be shared.

THE CHILDREN: Many want to view them as innocent victims, but school-age kids learn the merits of exercise and good nutrition almost immediately – and consistently – throughout school. Based on the results of the Texas fitness test, the message is being ignored increasingly as children age.

THE PARENTS: They are not the best role models. About two-thirds of U.S. adults are overweight, and about 20 percent engage in exercise at least twice a week. They also do most of the grocery shopping, cooking and driving to restaurants.

THE SCHOOLS: They are everyone’s favorite whipping boy, with accusers pointing to suspect lunch offerings and vending machines with few healthy choices.

But schools have made significant progress in recent years, thanks to restrictions imposed by politicians wanting to please constituents: the parents.

FOOD MARKETERS: Vigilant parents fear strangers on the street harming their children.

What about the strangers luring them with harmful substances between cartoons on television? A landmark 2005 Institute of Medicine report stated it plainly: Food and beverage marketers spent $10 billion that year targeting children 12 and younger, leading them to request and consume high-calorie, low-nutrient products. A study in this month’s British journal Obesity Reviews said 89 percent of food marketed to children was nutritionally inferior. Companies have been scrambling to re-engineer their products and alter advertising practices amid threats of lawsuits.

THE REAL VILLAIN: If you have to pick a villain, pick “screen time” – the hours spent parked in front of a television or computer. The average child spends five to six hours staring, and often mindlessly munching. The American Academy of Pediatrics recommends that children watch TV or play on a computer no more than two hours a day, and that boys and girls take 11,000 and 13,000 steps daily, respectively. A study funded by the National Institute of Diabetes restricted screen time to two hours in 70 homes. The result was less sedentary children with healthier BMIs.

WHAT TO DO: Sigmund Freud, the father of modern-day psychoanalysis, supposedly said biology is destiny. He apparently was referring to gender roles, but it appears to hold true for weight. A 2004 Stanford University study showed that 48 percent of children with overweight parents became overweight, compared with 13 percent of those with normal-weight parents.

Scientists believe that we have a genetically determined weight toward which our body gravitates, and a weight range of about 30 pounds. How much you eat and exercise determines where you will fall within that range. That may be why your body fights back when you try to get below that weight range to what you consider your ideal weight. But you won’t get morbidly obese if it is not in your DNA.

If you – and your child – adopt a healthy diet and exercise enough to be reasonably fit, you will stay at the low end of your weight range.

Steve Jacob is publisher of the Star-Telegram/Arlington and Northeast Tarrant County and a master’s student in health policy and management at the University of North Texas Health Science Center in Fort Worth. E-mail sjacob@star-telegram.com.