Junkfood Science: No support for finger pointing teens

July 17, 2008

No support for finger pointing teens

Teens are different from little kids. Little kids are little bees of nonstop activity. The teen years are spent focused more towards studies, relationships, and less physically active pursuits, especially among girls. While this would seem a no-brainer, it’s been reported this week as a crisis, requiring immediate action on a major public health scale.

A new study is said to have found that teens are sluggish. According to a University of California San Diego press release, “the increase in childhood obesity” may be partly due to this drop in physical activity as kids age. “People don't recognize this as the crisis that it is,” said the lead author.

This is a case of people seeing what they believe to be true, rather than reading what a study actually found. Reporters also seem to have fallen for basic fallacies of logic. While this study is being used to support a current obesity epidemic, did it show that: 1). teens are any less active today than in past generations, or 2). that young people’s activity levels are in any way related to obesity?

You may be surprised to learn that the answer to both questions is “no.” Nor did the study show that the drop in physical activity levels is as impressive as is being reported, that teen activity levels continue through adulthood, or that their activity levels have any relationship to future health problems. Everyone has just assumed all of it to be true.

Let’s take a critical look at the study.

Early Child Care and Youth Development Study

The paper was published in the Journal of the American Medical Association and done by the Steering Committee of the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care and Youth Development. This study had recruited and screened 8,985 families from ten university sites across the country beginning in 1991 and was designed to follow the kids from birth to 15 years of age.

Right off, the study excluded all those children born to teen mothers, of multiple births, non-English speaking mothers, any medical complications or babies who stayed in the hospital more than a week, children being adopted, anticipated family move, families living in unsafe neighborhoods, or those refusing to participate. They ended up with 1,526 families which were scheduled for the first home visit at 1-month of age. Another 162 were eliminated from this group for incomplete home visits.

So, 15% of the kids (1,364) from the original 8,985 families recruited were enrolled in the study. They were of similar ethnicity and income levels to the U.S. population.

Physical activity data and attrition

The children’s heights and weights were measured at ages 9, 11, 12 and 15. Physical activity was measured by asking them to wear accelerometers on a belt around their waists for 7 days (continually except when bathing) at each assessment age. At 9 years of age, of the 895 kids who agreed to wear the monitors, 879 kids turned in valid readings with at least 4 days of data. The number of kids who agreed to wear the bulky monitors dropped as the study went along, as did the number who complied sufficiently to turn in valid data. By age 12, only 752 agreed to wear the accelerometers with only 699 kids turning in usable data. And by age 15, only 695 had agreed to wear the monitors but only 604 actually did.

In other words, by the end of the study, the study authors had physical activity data on only 44% of the kids who began the study. The reasons why fewer than half of the kids in the study agreed to wear the monitors, as well as for the drop in compliance, lend caution in interpreting the precipitous drop in activity levels reported during these pre-teen and teen years.

“The 2 reasons most often given for refusing to wear the monitors were inconvenience and concerns for the appearance around the waist of the 1.5x1.5 inch monitor,” said lead author, Dr. Philip Nader, M.D. at the University of California San Diego, and colleagues. The monitors were worn continuously, but could be removed during contact sports or water sports, at the discretion of the kids or their coaches, for safety concerns (bruising or injuries) or, obviously, concerns the monitors could affect their performance.

“An unavoidable bias is due to the fact that accelerometers tend to underestimate activity of youth who frequently engage in contact sports or swimming because the accelerometer is not worn at these times,” said the authors. While they stated they believed the effect on underestimations of overall activity was minimal, there was no data presented on how many young people — both those who agreed to wear accelerometers and those who didn’t — were involved in sports. Another caution with accelerometry measurements is that they don’t record any upper body activity.

The young people with accelerometer data included in this analysis differed from those not included. They were slightly more likely to be girls, which is compatible with the fact more boys are involved in contact sports.

Dr. Nader and colleagues looked at levels of moderate-to-vigorous physical activity (MVPA) at each age. The counts recorded by the accelerometers were used to estimate the energy expended at various intensities of activity using an age-specific equation, and mean minutes were then computed for each day. Accelerometers are a new method to measure physical activity for public health surveillance, said the authors, and the technology is still developing. “There is no single accepted protocol across studies for setting accelerometer-based cutoff points for MVPA,” they wrote. For this study, they used a liberal 3.0 MET cutoff to determine activity levels of sufficient intensity.


Overall, as was reported in the news, activity levels dropped as kids entered puberty. The average time kids spent engaged in vigorous to moderate levels of activity each weekday, for example, was 3.03 hours at age 9, 2.06 hours at age 11, 1.6 hours at age 12, and dropped to 49.2 minutes at age 15. It’s doubtful most parents or medical professionals would find these levels especially worrisome, especially given the measurements may have come from the least active teens as they grew older.

The authors found that children from low-income families were more active at age 9 on weekdays than were those from higher-income families, with no differences for weekend activity. This study found no support for the popular belief that low-income kids are more sedentary. The authors also reported that income status was unrelated to changes in activity levels.

When all confounding factors were included in their modeling, they found children from low-income families were significantly more active at 9 years on both weekends and weekdays than kids from higher incomes. Gender, though, was the strongest determiner of activity levels, nearly twice as significant as income or BMI. Boys in general spent 21 minutes more than girls each weekday engaging in moderate-vigorous physical activity, and those from low-income families spent 9 minutes more than higher-income children. Among the girls, there was no relationship between activity levels and BMI, and a difference of only 2 minutes among boys of various BMIs.

The correlations between income levels, geographic regions or BMIs and activity levels as the kids grew up were “quite small,” said the authors, with a difference each year of about 1 minute on weekdays and 5 minutes on weekends among the various income levels, and less than a minute difference among the BMIs. Put another way, the changes in moderate-to-vigorous activity between ages 9 and 15 on weekends and weekdays, accounted for an effect on BMIs of 0.09 and 0.12, respectively. Clinically meaningless.

As the body of evidence has continued to show, physical activity does not explain the natural diversity in sizes among young people.

Putting results into perspective

This study added no new information to the body of research on young people’s activity levels. As Michael Gard, an physical education instructor at Charles Sturt University, Australia, has written, the research continues to find no support for the popular belief that children or adults are less physically active today than in past generations. In fact, the evidence suggests people in industrialized countries are more active and actively pursuing exercise and sports. The fact that time spent in vigorous activity is greatest among kids and drops during adolescence, especially among girls, is nothing new, either. [Research, using a wide range of measuring methods and populations around the world, addressing the myths of couch potato-telly tubby kids and sloth were covered previously here, here and here.]

As Dr. Gard concluded:

It is well known that young people are the most physically active section of Western populations. However, research shows that physical activity [and our diet] is not like a ‘habit’ one forms in childhood and then repeats for the rest of one’s life... In other words, physical activity seems to be context dependent and research suggests that childhood physical activity level is a poor predictor of adult activity level. In other words, children are not wind-up dolls.

This was even shown in a the latest research referenced by Dr. Nader and colleagues. Just published in the January issue of Medicine & Science in Sports & Exercise, that study had used the first data on a sampling of the population from the 2003-2004 National Health and Nutritional Examination Survey (NHANES). Based on accelerometry data, its authors from the National Cancer Institute had reported that among kids 6-11 years and adolescents 12-19 years of age, there was a drop in average accelerometry counts during the time the monitors were worn, moreso among the girls.

Recorded activity levels in the NHANES study then stabilized or rose among adults during their 20s and 30s, with subsequent gradual decreases with aging. But what was consistently shown was that black and Hispanic minorities had higher levels of overall activity by accelerometry counts, among both males and females at nearly every age, attributed to the fact this data included job and transportation activity rather than just leisure time used in other population surveys.

The NCI authors had attempted to correlate their data with government exercise recommendations, however even Dr. Nader and colleagues urged caution in interpreting the NHANES study for this purpose. Its sampling data had represented very small response rates. Examining that study finds, for example, that it only had valid data for four days on 5.9% to 11.2% of the adults surveyed and 11% to 13% of the kids. In addition to sparse levels of valid data available, the NCI authors had also arbitrarily made the definitions for moderate and vigorous levels of activity using METS nearly double those in this study and, not surprisingly, went on to say that few people met government physical activity guidelines. Lending further caution in interpreting this data as related to obesity, they reported no correlations between activity levels and BMIs.

As Dr. Gard has cautioned: “We should never lose sight of the fact that Western populations are, by and large, as healthy as they have ever been.” The current discourse on the dire health consequences of inactivity among children is “not only untrue, but also burdens children’s physical activity with an unnecessary and potentially counter-productive seriousness.”

So why the talk of a crisis?

This week’s study found no correlation between changing activity levels and BMIs. Nor did it find that young people are less active today than they’ve ever been. Nor did it find poorer kids were more sedentary than richer kids. Nor did it even look at if activity levels during adolescence were related to adult levels or had any relationship to future health problems.

In their conclusions discussing government recommendations for physical activity levels, Dr. Nader and colleagues also noted “there is limited data substantiating the 60-minute recommendation in terms of important clinical outcomes.” It is unknown, “the amount of MVPA required to affect child health outcomes.” Yet, four sentences later, the study authors concluded:

Consequently, there is need for program and policy action as early as possible at the family, community, school, health care, and governmental levels to address the problem of decreasing physical activity with increasing age.

The press releases from both the NICHD and UC San Diego also quoted the study authors as calling this study evidence of a major threat to public health, requiring immediate action. Dr. Nader called the magnitude of decline in activity among teens “dramatic” and a crucial factor in the obesity epidemic and one that warrants population-wide interventions to “create a culture and environment of activity.” As co-author, Renate Houts, Ph.D., from RTI International [background information on RTI here] was quoted as saying:

Program and policy action are needed immediately at the family, community, school, health care, and governmental levels to find ways to encourage children to remain active as they get older. Unless parents, elected officials, and volunteer and philanthropic organizations begin creating new opportunities for children’s activities, childhood obesity rates will likely continue to rise.

The study authors reported no financial disclosures, but there’s a bit more to the story that might help explain the disconnect between their own evidence and their calls for widespread interventions all the way to the government level, and of the pressing need for a program to get families active and trimmed down.

Dr. Nader is one of developers of the CATCH (Coordinated Approach to Child Health) program that has been adopted by more than 7,000 elementary schools across the country. According to the press release: “Students use a traffic-light system to identify foods that are ‘go,’ ‘slow,’ or ‘whoa’ and school P.E. and family activities are designed to keep every child moving.” Yes, it’s like all the studies in the evidential review by the U.S. Preventive Services Task Force found to be ineffective for reducing obesity, improving health outcomes or changing children’s diets or behaviors. Dr. Nader also just authored a new book called You CAN Lose Your Baby Fat, telling parents that they can prevent obesity in their children using the same techniques as in the program of the study’s authors and sponsor.

The Eunice Kennedy Shriver NICHD funded the study and the study authors (who were paid through NICHD or RTI International). As sponsor, NICHD’s role was inclusive. “The NICHD Study of Early Child Care and Youth Development Steering Committee, which includes an NICHD project director, was responsible for the design and conduct of the study, the collection, management, analysis, and interpretation of the data, and the preparation, review and approval of the manuscript.”

NICHD is one of the four NIH agencies which developed and runs the We Can! (Ways to Enhance Children's Activity and Nutrition) program geared for kids 8-13 to achieve a “healthy weight.” It produces resources for parents and community programs to encourage “families to make better food choices, increase physical activity, and reduce recreational screen time.” Yes, all of the tried and failed techniques for addressing the calories in-calories out equation. And their handbook for parents makes it very clear what they mean by the term “healthy weight,” the cutoff where they claim, despite even government data to the contrary, that serious health risks begin:

People have different ideas about what a “healthy” weight is. Some think that a model–thin physique is a healthy weight; others think that they can have some extra padding around the middle and still be at a healthy weight. That’s why health experts have developed standards that define normal weight, overweight, and obesity. For adults, a normal, or healthy weight, is defined as an appropriate weight in relation to height. This ratio of weight to height is known as the body mass index (BMI)... Normal weight is BMI of 18.5 to 24.9.

So, that program the study authors at NICHD said was of such urgent importance... is their own.

The evidence from this study and the medical literature finds no cause for parents to panic that their children and teens are “out of balance” or headed for the morgue. Rumors of a crisis have been greatly exaggerated.

© 2008 Sandy Szwarc. All rights reserved.

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