The progressive increase in adult obesity is more complicated than we think

All of us are aware of what has been termed our “obesity epidemic.” The current prevalence of obesity among adults in the U.S. is around 40 percent, a dramatic increase over the past 50 years; it was about 15 percent in 1970. Rates are also increasing across first world countries, so we are not alone in this.

Obesity is defined as a body mass index (BMI) greater than 30. Values of 25 to 30 are termed overweight. BMI is weight in kilograms divided by height in meters squared.

The graph shows the trends over the past decades and has some interesting features. Note that the percent of the population that is obese or extremely obese (BMI > 40) has increased, but the percent classified as overweight has not. This suggests to me, although I haven’t seen anything written about it, that overweight and obese patients are two separate groups; the overweight are not destined to become obese. There are even some recent data that suggest being mildly overweight may actually be a good thing as you age.

Many explanations have been offered for the progressive increase in adult obesity, including increased intake of calories, often in the form of soft drinks, and a sedentary lifestyle. The simple calculation of excess calories consumed versus calories burned offers a partial explanation, and certainly, that’s what I was taught in medical school in the 1970s; obesity was simple arithmetic. It turns out things are more complicated than that. Genetics, for example, plays a large role, as do various hormonal systems.

I don’t follow the enormous medical literature on obesity closely, but this recent study really intrigued me. It was in a journal I haven’t seen before, Economics and Human Biology. This seems appropriate since the economic effects of the obesity epidemic are massive and getting larger all the time. The authors studied annual sugar consumption in the U.S. population and compared it with obesity rates later. Now, that approach is pretty reductionist in that it ignores many other kinds of calories that aren’t sugar, but the results are interesting. Their findings suggest that, among today’s adults, obesity correlates with global sugar intake during their childhood years in the 1970s and 1980s. If this is the case, one would predict a decrease in obesity among adolescents and young adults now because sugar intake in the US has decreased by 25 percent in the last decade. In fact, adolescent obesity prevalence, after a steady and seemingly inexorable rise, may actually have plateaued over the past five years or so.

The usual caveat of correlation not indicating causation needs to be kept in mind, of course. Yet it makes biological sense to me. I think our metabolic state could have a certain kind of “memory” about the milieu it experienced during early growth and development and have responded to that in ways that could persist for many years.

Christopher Johnson is a pediatric intensive care physician and author of Keeping Your Kids Out of the Emergency Room: A Guide to Childhood Injuries and IllnessesYour Critically Ill Child: Life and Death Choices Parents Must FaceHow to Talk to Your Child’s Doctor: A Handbook for Parents, and How Your Child Heals: An Inside Look At Common Childhood Ailments. He blogs at his self-titled site, Christopher Johnson, MD.

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