Are we exercising less or eating more?

There are many compelling reasons to exercise. Regular physical activity has been shown to reduce the incidence of heart disease, diabetes and hypertension. It improves mental health, fosters strong muscles and strong bones. It also makes most participants happier and improves our looks.

Physical activity also burns calories. In the past few decades exercise has therefore become one arm of the two pronged approach to combat overweight and obesity. Exercise is the calories expended in the equation; food intake is the calories we take in. Any change in the balance of calories in and calories out that leaves us calorie depleted would, assumingly, lead to weight loss. The best approaches to overall health and weight maintenance employ both increased exercise and calorie restriction.

Inactivity has also been blamed for our weight gain. Our life has indeed become much more sedentary, as the car replaced walking, as convenience and mechanization crept into every aspect of our lives and as we started spending hours without end watching TV.

But blaming inactivity has also become a way for the fast food and processed food industry to escape any responsibility for the highly palatable and calorie-dense foods they produce and promote. It’s of course inconvenient for food manufacturers and for us individually to admit that we’re eating too much; it seems we’d all like to believe that we can just burn a lot more calories through physical activity and allow the party to go on.

Blame games are generally useless, but if we are to address the obesity crisis rationally perhaps it's worthwhile to look at the relative contribution of inactivity and overeating to our weight gain.

Surprisingly enough, there are now serious questions emerging about whether inactivity has contributed to the expanding American waistline in any significant way.

A new study in Obesity Reviews looked at adolescents’ activity levels and asked two questions:


Are American adolescents getting enough physical activity?

And

Have adolescents become less active over the past fifteen years or so?


The reviewers examined nationally representative Youth Risk Behavior Surveillance Surveys taken since 1991, which provide information on adolescents’ physical activity (such as swimming, soccer, running, etc.), physical education at school and sedentary behaviors, such as watching TV and playing video games. Sample sizes were large, ranging from about 11,000 to 16,000 participants each year.

Let’s start with the not-so-great news:


• Only a minority—about a third—of our adolescents met the recommended daily levels for physical activity. (Optimal physical activity is any activity that makes a person increase their heart rate and breathe hard for at least 60 minutes, at least five days a week.) More boys met these criteria than girls, as did more Caucasians compared to minority youths.

• Most participants—about two-thirds of the study population—reported what this study defines as “sufficient vigorous physical activity”—activity that makes a person increase their heart rate and breathe hard for at least 20 minutes, at least three days a week.


On the other hand, the data shows that American adolescents have not decreased their level of physical activity; physical activity levels have been quite stable over the past 17 years. During the same period of time studies show that obesity rates in kids had close to doubled.

There were also a few encouraging trends:


• Physical education class attendance and engagement have improved.

• TV viewing time had decreased significantly in the past few years (although that doesn’t mean TV hasn’t been replaced by other media, such as the Internet and computers).



The authors, led by Youfa Wang, summarize by saying: “Reduced PE (physical activity) is not likely the major explanation of the recent increase in obesity among U.S. adolescents.”

Another recent study in the American Journal of Clinical Nutrition looked at the relative contributions of food and exercise habits to the development of the obesity epidemic, and concluded that the rise in obesity in the United States in the last three decades was virtually all due to increased energy intake.

These studies, and several others, suggest that the main driver of the obesity epidemic is increased food intake. Increasing physical activity has to be part of the solution because exercise is so very good for health and helps us lose weight, but in order to address the obesity crisis head-on, we have to face the fact that the main target of our efforts needs to be reducing our caloric intake.

We probably can’t—as a society—increase our energy expenditure to compensate for our levels of food consumption. We’d better be realistic about this.

The American Journal of Clinical Nutrition study cited above suggests that to get U.S. kids back to the mean weights we saw in the 1970s, kids would need to decrease their average daily intake by 350 calories/day, or walk an extra two and a half hours daily (while eating the same amount, which is no easy task since exercise drives hunger). Both options seem difficult, but the latter appears totally implausible.

Please don’t misunderstand me; I know the importance of physical activity and truly believe in its critical value. Exercise does help in weight loss and improves health, even if no weight is lost.

But I don’t think physical activity without a serious look at what and how we eat will likely get us and our kids out of the obesity crisis.

Dr. Ayala
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