2 Chapter Two: Measuring Obesity and Balancing Calories

Authored by Sherri M. Cirignano, MS, RDN, LDN & Karen Ensle, EdD, RDN, FAND, CFCS

 

Body Mass Index (BMI)

Body mass index (BMI) is commonly used to determine weight status. BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters. For children and teens, BMI is age-and-sex-specific and is often referred to as BMI-for-age. A child’s weight status is calculated differently from adult BMI categories. Children’s body composition varies as they age and varies between boys and girls. Therefore, BMI levels among children and teens need to be expressed relative to other children of the same age and sex.

CDC Growth Charts are commonly used to measure the size and growth patterns of children and teens in the United States. BMI-for-age weight status categories and the corresponding percentiles, are based on expert CDC committee recommendations, and are listed in Table 1 for an eight-year-old child as an example:¹

Table 1. Child BMI categories and corresponding percentiles

Adapted from the Centers for Disease Control and Prevention. Defining Childhood Weight Status.

https://www.cdc.gov/obesity/basics/childhood-defining.html

In children, BMI percentile cutoffs for obesity are intended to reliably define a level above which a child is more likely to have or be at risk of developing obesity-associated adverse health outcomes or diseases. For more information, see the National Collaborative on Childhood Obesity’s page on Measures for Children at High Risk for Obesity.²

What is the Difference Between Overweight, Obesity and Morbid Obesity?

In adults, obesity is defined as having a body mass index (BMI) of greater than 30. Morbid obesity is when a person has excessive weight with a body mass index of 35 to 40 or more. Table 2 defines the four categories of BMI.

Table 2. Four categories of obesity

Overweight (not obese), if BMI is:	25.0 to 29.9
Class 1 (low risk) obesity if BMI is:	30.0 to 34.9
Class 2 (moderate risk) obesity if BMI is:	35.0 to 39.9
Class 3 (high-risk) obesity if BMI is equal to or greater than:	40.0

Morbid obesity, which is also termed “clinically severe obesity,” is typically defined as being more than 100 pounds overweight or having a BMI of 40 or higher. A clear definition of morbid obesity is very important because this definition is used to guide doctors in the selection of treatment options for people who are overweight. Individuals are usually considered morbidly obese if their weight is more than 80 to 100 pounds above their ideal body weight.

Measuring Obesity³

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Additions to BMI Measures

Measurement of BMI has long been a consistent outcome of obesity surveillance systems, owing to the ease and low cost of directly measured height and weight or even obtaining these data via self-report. BMI misclassifies as nonobese one-quarter of adult males and nearly half of adults whose dual energy x-ray absorptiometry (DXA) measurements classify them as obese.⁴ BMI has similarly poor sensitivity as a measure of adiposity in children.⁵ In addition, BMI does not detect an accumulation of abdominal fat, which is known to correlate with insulin resistance even in lean individuals.⁶ Surveillance science has been slow to take advantage of research that identifies alternative anthropometric measures of obesity.⁷ Combining two or more different anthropometric measures, such as waist-to-hip ratio and waist-circumference-to-height ratio, works well and may be more sensitive to the accumulation of abdominal fat,^7-9 although these measurements are more invasive and require additional privacy.

Addition of longitudinal data include methods for longitudinal population-based analyses which are essential to assess the impact of policy-led interventions on the incidence of obesity during crucial developmental time periods, such as childhood or transitions from high school to college.¹⁰ Developing cohorts with regularly assessed obesity measures over the lifespan would provide data on incidence of obesity that would coincide with obesity prevention initiatives. Longitudinal BMI measure analyses are uncommon, particularly among low-resource populations, which are at greater risk of obesity. More research on longitudinal BMI studies are needed on low-income individuals of all age groups and backgrounds.

Food and Metabolism¹¹

The carbohydrates, lipids (or fats), and proteins in the foods you eat are used for energy to power molecular, cellular, and organ system activities. Importantly, the energy is stored primarily as fats. The quantity and quality of food that is ingested, digested, and absorbed affects the amount of fat that is stored as excess calories. Diet—both what you eat and how much you eat—has a dramatic impact on your health. Eating too much or too little food can lead to serious medical issues, including cardiovascular disease, cancer, obesity, anorexia, and diabetes, among others. Combine an unhealthy diet with unhealthy environmental conditions, such as smoking, and the potential medical complications increase significantly.

The amount of energy that is needed or ingested per day is measured in calories. The nutritional Calorie (C) is the amount of heat it takes to raise 1 kg (1000 g) of water by 1 °C. This

is different from the calorie (c) used in the physical sciences, which is the amount of heat it takes to raise 1 g of water by 1 °C. When we refer to “calorie,” we are referring to the nutritional Calorie.

On average, a person needs 1500 to 2000 calories per day to sustain (or carry out) daily activities. The total number of calories needed by one person is dependent on their body mass, age, height, gender, activity level, and the amount of exercise per day. If exercise is a regular part of one’s day, more calories are required. As a rule, people underestimate the number of calories ingested and overestimate the amount they burn through exercise. This can lead to ingestion of too many calories per day. The accumulation of an extra 3500 calories adds one pound of weight. If an excess of 200 calories per day is ingested, one extra pound of body weight will be gained every 18 days. At that rate, an extra 20 pounds can be gained over the course of a year. Of course, this increase in calories could be offset by increased exercise. Running or jogging one mile burns almost 100 calories.

The type of food ingested also affects the body’s metabolic rate. Processing of carbohydrates requires less energy than processing of proteins. In fact, the breakdown of carbohydrates requires the least amount of energy, whereas the processing of proteins demands the most energy. In general, the amount of calories ingested and the amount of calories burned determines the overall weight. To lose weight, the number of calories burned per day must exceed the number ingested. Calories are in almost everything you ingest, so when considering calorie intake, beverages must also be considered.

Energy Requirements

To determine the exact energy requirements needed to achieve a healthy weight, it is necessary to consider an individual’s age, gender, height, weight, and physical activity level, as well as goal weight if appropriate. Although determining energy requirements comes down to a matter of energy in through food eaten and energy out through both biological activities such as breathing and physical activities, it is not as easy as it sounds. There are mathematical formulas and other more precise methods that are used by registered dietitians and other healthcare professionals to determine an individual’s everyday energy needs, but these are not practical for use by the general public. For a consumer to estimate their energy requirements, there are sources such as the Dietary Guidelines for Americans, (DGA) that provide a generalization of estimated needs throughout life.¹² This resource includes changes in daily calorie needs based on an individual’s activity level whether it be sedentary, moderately active or active.

In general, energy requirements are higher for males than females, and according to the DGA, range from 1,000 to 2,000 calories per day for young children; up to 3,200 calories per day for adolescents; 2,600 to 3,000 calories per day for adult men; 2,000 to 2,400 calories per day for adult women; 2,000 to 2,600 calories per day for older adult men; and 1,600 to 2,000 calories per day for older adult women.¹² Keep in mind that calorie needs can be increased during the greatest periods of growth throughout life including during childhood and adolescence as well as during pregnancy and lactation. An increase in calories may also be needed when the body is experiencing stress as a result of major injuries and/or illnesses.

Balancing Calories¹³

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Summary

Body mass index, (BMI) calculated by dividing a person’s weight in kilograms by the square of their height in meters, is commonly used to determine weight status. For children, age and gender are also taken into consideration when determining BMI, and CDC Growth Charts are commonly used to measure the size and growth patterns of children and teens in the United States. Although BMI does not measure body fat directly, it has been found to correlate with more direct measures of body fat and because it is a non-invasive and inexpensive method, it has been beneficial and long been of use in national and other obesity surveillance systems.

The food we eat, in the form of carbohydrates, fats and protein, are used as the energy source for all metabolic processes in our bodies, with excess energy stored as fat. Determining the exact amount of food needed to meet our energy needs and maintain, gain or lose weight, dependent on individual specifications, can be estimated through a variety of precise calculations, web-based information or phone apps. Balancing intake is important for maintaining a healthy weight.

Resources

Centers for Disease Control and Prevention BMI Percentile Calculator for Child and Teen

Centers for Disease Control and Prevention Adult BMI Calculator

References

1. Centers for Disease Control and Prevention. Defining childhood weight status. Accessed December 2022. https://www.cdc.gov/obesity/basics/childhood-defining.html
2. National Collaborative on Childhood Obesity Research. Measures for children at high risk for obesity. Accessed December 2022. https://www.nccor.org/projects/measures-for-children-at-high-risk-for-obesity/
3. Falcone K. Introduction to Health. Weight management: measuring obesity. LibreTexts; 2021. Accessed December 2022. https://batch.libretexts.org/print/url=https://med.libretexts.org/Bookshelves/Health_and_Fitness/Book%3A_Introduction_to_Health_(Falcone)/04%3A_Weight_Management/4.03%3A_Measuring_Obesity.pdf
4. Shah NR, Braverman ER. Measuring adiposity in patients: the utility of body mass index (BMI), percent body fat, and leptin. PLOS ONE. 2012;7:e33308.
5. Javed A, Jumean M, Murad MH, Okorodudu D, Kumar S, et al. Diagnostic performance of body mass index to identify obesity as defined by body adiposity in children and adolescents: a systematic review and meta-analysis. Pediatr. Obes. 2015;10:234–44.
6. Cnop M, Landchild MJ, Vidal J, Havel PJ, Knowles NG, et al. The concurrent accumulation of intra-abdominal and subcutaneous fat explains the association between insulin resistance and plasma leptin concentrations: distinct metabolic effects of two fat compartments. Diabetes. 2002;51:1005–15.
7. Blundell JE, Dulloo AG, Salvador J, Fruhbeck G, EASO SAB Work. Group BMI. Beyond BMI—phenotyping the obesities. Obes. Facts. 2014;7:322–28.
8. Carmienke S, Freitag M, Pischon T, Schlattmann P, Fankhaenel T, et al. General and abdominal obesity parameters and their combination in relation to mortality: a systematic review and meta-regression analysis. Eur. J. Clin. Nutr. 2013;67:573–85.
9. Wolfgram PM, Connor EL, Rehm JL, Eickhoff JC, Zha W, et al. In nonobese girls, waist circumference as a predictor of insulin resistance is comparable to MRI fat measures and superior to BMI. Horm. Res. Paediatr. 2015;84:258–65.
10. Cunningham SA, Kramer MR, Narayan KM. Incidence of childhood obesity in the United States. N. Engl. J. Med. 2014;370:403–11.
11. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P. Anatomy and Physiology. 2e. Houston, TX: Open Stax;2022. Accessed December 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/24-7-nutrition-and-diet
12. U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. 9th Edition. 2020. Accessed September 2022.
13. DietaryGuidelines.gov
14. Falcone K. Introduction to Health. Weight management: balancing calories. LibreTexts; 2021. Accessed December 2022. https://batch.libretexts.org/print/url=https://med.libretexts.org/Bookshelves/Health_and_Fitness/Book%3A_Introduction_to_Health_(Falcone)/04%3A_Weight_Management/4.02%3A_Balancing_Calories.pdf