It is highly likely that you have seen weight loss ads in the newspaper, magazines, or in your social media feed. With about 50% of the U.S. population saying they tried to lose weight in the last 12 months, it is not surprising that the weight loss industry is estimated to be a $3 billion dollar industry.
Although this chapter will address those who are underweight, the predominance of information will relate to persons who are overweight or obese. Overweight and obesity are linked to more deaths worldwide than underweight; most of the world’s population live in countries where overweight and obesity kills more people than underweight. In the U.S., an estimated 1.5% of adults aged 20 and over are underweight, compared to about 42% of adults who are considered obese. The goal is to strive for a healthy weight since persons who are underweight or overweight have negative health implications.
This chapter focuses on body weight and body composition. Both body weight and body composition are important measurements to understand your health. Body weight is typically measured in pounds or kilograms and is simply your body’s total mass. Body composition is what your weight is made of (muscle, bone, water, and fat) and is typically measured as a percentage. For example, a person might have a body weight of 200 lbs and a percentage body fat of 25%, in this example, 50lbs of their body weight comes from fat. Although body weight is the measurement most commonly used to assess the health of our body size, body composition is a more accurate look at our body size by understanding what the body is made up of. It is quick and easy to step on a scale and measure your body weight, but measuring body composition is much more challenging, thus body weight is the standard used to address overweight and obesity.
Fat is not bad, we need fat for healthy cellular function, energy, cushioning for vital organs, insulation, long term energy storage, and absorption of fat-soluble vitamins. The minimum about of body fat needed by males is about 3% and females is about 12%. It is generally accepted that an overall range of 10-22 percent for men and 20-32 percent for women is considered satisfactory for good health. This necessary body fat for health is considered essential body fat. When the body stores additional body fat above the essential amount it is called non-essential body fat. A woman’s essential fat range is naturally greater than a man’s because of fat deposits in breasts, uterus and sex-specific sites.
In the Nutrition Chapter, you were introduced to the energy (calorie) balance equation, which simply means to compare energy in versus energy out, or how many calories we are eating each day compared to the amount of calories we expend. If a persons consumes an excess of 3,500 calories that is the equivalent of gaining one pound of weight. That means if you reduce your daily calorie intake by 500 calories or burn 500 more calories each day than you consume, you may be able to lose a pound a week. Caloric expenditure is most often associated with exercise, however most of our daily caloric expenditure is to maintain bodily functions, like breathing, circulating our blood, and digesting food.
The term used to describe the chemical processes in the body that convert calories into energy is metabolism. Your body needs a minimum number of calories to sustain these functions. Basal metabolic rate (BMR) and Resting metabolic rate (RMR) are measurements used to estimate how many calories your body burns when you’re not exercising. These two measurements are often used interchangeably, however they do have a slight difference. A BMR measurement is slightly more accurate because it is measured when you are doing nothing, just upon waking up, while laying down in a dark room after sleeping for for at least eight hours and fasting for 12 hours. RMR is taking without the strict conditions of the BMR; BMR is usually slightly lower than your RMR. Both basal metabolic rate (BMR) and resting metabolic rate (RMR) measure the amount of energy (Calories) that your body needs to stay alive and function properly. It is helpful to understand your BMR/RMR since it accounts for about 60-80% of total energy expenditure each day. Your BMR/RMR is effected by your weight, height, age, gender, and genetics. A key component of your weight that impacts BMR/RMR is the amount of muscle you have; muscle mass increases BMR/RMR.
Defining Overweight and Obese
The measurement used across the world to identify if a person is overweight or obese is called the Body Mass Index (BMI), which is based on a persons weight versus their height. BMI is used because it is an inexpensive and easy screening method to identify weight categories (underweight, healthy weight, overweight, and obesity). It is important to recognize that BMI does not assess body composition or body fatness, it is simply a measurement of body size and is used as a screening tool. Although it may not be accurate for some of the population, for example athletes with high muscle mass, it has been shown to be correlated to other more accurate measures of body fatness and to various weight related diseases.
BMI is a person’s weight in kilograms divided by the square of height in meters:
BMI= weight (kg) / [height (m)]2
Table (\PageIndex{1}\): BMI Indicators
| BMI |
Weight Status |
| Below 18.5 |
Underweight |
| 18.5 – 24.9 |
Healthy Weight |
| 25.0 – 29.9 |
Overweight |
| 30.0 and Above |
Obesity |
Table (\PageIndex{2}\): BMI Example for person who is 5’9″
| Height |
Weight Range |
BMI |
Weight Status |
| 5’9″ |
124 lbs or less |
Below 18.5 |
Underweight |
| 5’9″ |
125 lbs to 168 lbs |
18.5 to 24.9 |
Healthy Weight |
| 5’9″ |
169 lbs to 202 lbs |
25.0 to 29.9 |
Overweight |
| 5’9″ |
203 lbs or more |
30 or higher |
Obesity |
As noted previously, BMI is the most commonly used tool to assess health weight, but for many people it is not an accurate measurement because it does not assess your body composition. For example, two people can have the exact same height and weight, thus the same BMI, but one could be a professional athlete who is very active with low body fat and the other could be sedentary with high body fat. Using the example in Table (\PageIndex{2}\) , a person who is 5’9″ and 203 pounds is considered obese using the BMI, it does not matter whether that person is a 203 pound professional weight lifter or a sedentary person with high body fat.
Recently, the American Medical Association (AMA) highlighted the limitations of BMI, emphasizing its inability to consider variations across race/ethnic groups, sexes, genders, and age groups. The report acknowledged that BMI is an imperfect measure of body fat and discussed its associated harms and benefits. In response to these findings, the new policy aims to educate physicians about the issues related to BMI and advocates for alternative measures in diagnosing obesity.
Under the newly adopted policy in 2024, the AMA acknowledges the historical harm and racist exclusion associated with BMI. The policy recognizes that BMI is primarily based on data from non-Hispanic white populations of previous generations. Due to significant limitations in using BMI alone in clinical settings, the AMA recommends its use in conjunction with other valid measures of risk, including but not limited to, visceral fat, body adiposity index, body composition, relative fat mass, waist circumference, and genetic/metabolic factors.
The policy emphasizes that while BMI is correlated with fat mass in the general population, its predictability diminishes when applied on an individual level. The AMA also underscores the importance of considering relative body shape and composition differences across various demographic groups when using BMI as a measure of adiposity. Additionally, the policy advises against using BMI as the sole criterion for denying insurance reimbursement. The overarching goal is to encourage a more comprehensive and nuanced approach to assessing and addressing health risks associated with obesity.
Also, extremely muscular individuals are often quite metabolically healthy despite having BMIs on the upper end of the spectrum. Other methods of measuring body fat, like DEXA, or Dual-energy X-ray absorptiometry scans, may be more suitable than BMI. However, these alternative methods can be more challenging to obtain and interpret in clinical practice, as they require more time and training and/or special equipment.
