In 2024, America's CDC, established that roughly 42% of Americas population is obese, which was just 23% in 2005, meaning the obesity rate has nearly doubled in the recent two decades. Obesity is defined by BMI, or body mass index, of 30 or higher, and is associated with numerous health conditions.
Understanding the Obesity Pandemic: Thermodynamics, Sedentary Lifestyles, and Caloric Consumption
Simply put thermodynamics is the study of energy transfer, and in terms of human physiology, the principle can be described as the following equation:
Calories in (Calories(From food) Consumed) = Calories Out (Calories Burned (by the body))
When calories consumed, is greater than calories expended, this excess amount is stored weight, typically body fat. Conversely, when the amount of calories expended are greater than that which are consumed, you result in a deficit resulting in fat loss.
Lets begin with the left side of this equation, calories in.
In the 20th century, the average caloric intake was roughly 2200-2500 calories, composed of simple whole foods. (Smith 1995)
Fast forward to the 21st century and the daily caloric intake on average for a male, (who from the standards of the 20th century is significantly less active, meaning less calories expended), is anywhere from 2700 to 3000 calories. (Wells & Buzby, 2021)
Now lets tie the right side of the equation in. Calories out, which simply put is calories expended, this is a summation of all the calories a human being burns in a 24hour span (accounting for everything from a single breath to running).
Harvard researchers found that in todays world Americans participate in roughly 30 minutes of less physical activity than Americans in the 20th century. (Yegian, Heymsfield, & Lieberman, 2021). Using Harvard's chart of "Calories burned in 30 minutes for people of three different weights", this can range from 100-400 additional calories burned per day.
Using these rough numbers, you can see that not only did the American Diet increase caloric consumption by 500 calories, but decreased caloric expenditure by about 250 calories as well, resulting in a net excess of 750 calories a day. Although these numbers are theoretical, and not accurate, they showcase the mechanism behind the ongoing obesity pandemic in our nation.
References:
Brownson, R. C., Boehmer, T. K., & Luke, D. A. (2005). Declining rates of physical activity in the United States: What are the contributors? Annual Review of Public Health, 26, 421-443.
Robinson, T. N., Hammer, L. D., Killen, J. D., Kraemer, H. C., Wilson, D. M., Hayward, C., & Taylor, C. B. (1993). Does television viewing increase obesity and reduce physical activity? Cross-sectional and longitudinal analyses among adolescent girls. Pediatrics, 91(2), 273-280.
Smith, D. F. (1995). Nutrition in Britain: Science, scientists, and politics in the twentieth century. Routledge.
Steeves, J. A., Bassett, D. R., Thompson, D. L., & Fitzhugh, E. C. (2012). Relationships of occupational and non-occupational physical activity to obesity. Obesity, 20(9), 1980-1985.
Tudor-Locke, C., Ainsworth, B. E., & Popkin, B. M. (2001). Active commuting to school: An overlooked source of children's physical activity? Sports Medicine, 31(5), 309-313.
Tudor-Locke, C., & Bassett, D. R. (2004). How many steps/day are enough? Preliminary pedometer indices for public health. Sports Medicine, 34(1), 1-8.
Wells, H. F., & Buzby, J. C. (2021). Dietary assessment: Food and nutrient intakes from national surveys. Agricultural Economic Reports.
Yegian, A. K., Heymsfield, S. B., & Lieberman, D. E. (2021). Historical body temperature records as a population-level ‘thermometer’ of physical activity in the United States. Current Biology, 31(20), R1307-R1313. https://doi.org/10.1016/j.cub.2021.09.014
Young, L. R., & Nestle, M. (2002). The contribution of expanding portion sizes to the US obesity epidemic. American Journal of Public Health, 92(2), 246-249.
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