10.7: Obesity

The obesity epidemic is the probably most modern public health concern. Body fat is not an inert blob. In fact, adipose tissue (fat storage cells), are metabolically active, pumping out hormones and cytokines (chemical messengers) associated with everything from satiety (that feeling of fullness) to inflammation (Foreman, 2020). Excess stored fat on the body is associated with the leading causes of death, including CVD, stroke, diabetes, several cancers, pancreatic, gallbladder, and kidney diseases, and breathing problems like sleep apnea and asthma. Carrying excess weight throughout life can impede physical activity, and when people do move it causes more stress to joints - which are then more likely to develop osteoarthritis (inflammation and breakdown of cartilage). Although infectious diseases are caused by pathogens, people with excess body fat can often experience more severe illness from infections. Social stigma around obesity can impede career, academic, and social function, and these factors combined with obesity can lead to poor mental health (NIDDK, 2023). Of particular concern is visceral, or central adiposity, which is the fat that accumulates around the internal organs in the abdomen and is more strongly associated with chronic health risks due to its metabolic activity. Central adiposity is a more common weight distribution pattern in men and postmenopausal women, putting them at higher risks for cardio-metabolic diseases like diabetes, hypertension, and cardiovascular disease (Juppi et al., 2022).

It would be optimal to estimate body fat accurately with tools like a dual energy X-ray absorptiometry scan (DEXA scan) or even bioelectrical impedance analyzer (BIA), but these are often costly machines. The DEXA requires trained personnel to operate, and results from the BIA can be influenced by other physiological factors (like hydration status), so specific procedures must be followed to ensure accuracy. Therefore, these tools are not typically used to estimate fat mass to body weight ratios, or body fat percentage (BF%), for large populations. Instead, epidemiologists have long used a controversial calculation of weight-to-height ratio instead, called body mass index (BMI). There are several reasons that BMI is considered less accurate than BF%, one of them being that BMI will be higher in those individuals with more muscle mass or bone density, and there may be genetic variance in body density that is not accounted for by BMI. For example, Asian populations tend to have lower rates of obesity as classified by currently accepted cutoffs (see below), and yet have higher rates of central adiposity (fat around the internal organs in the abdomen) and higher levels of obesity-related disease factors at lower BMIs (Li et al., 2023). Even with these complications, BMI is easy to calculate using self-reported weight and height, and so large epidemiological surveys still employ this measurement to indicate health-related weight status for populations.

BMI is calculated using either of the equations below:

BMI = weight (kg) / height (m)² BMI = weight (lbs) / [height (in)]² x 703

Using BMI levels as criterion values for weight status provides the following classifications:

• Underweight = BMI <18.5
• Normal weight = BMI 18.5-24.9
• Overweight = BMI 25-29.9
• Obesity = BMI 30+
• Severe or “morbid” obesity = BMI 40+

Although we have been calling obesity an epidemic for what seems like a long time, it is still a growing problem. According to the CDC, the prevalence of adult obesity was 30.5% in 1999-2000, and had increased to 41.9% in 2017 - March of 2020 (at the beginning of the COVID-19 pandemic response). Severe obesity increased from 4.7% to 9.2% in the same timeframe. In terms of healthcare, obesity costs over $1,861 excess 2019 dollars per person, and severe obesity over $3000 per year. Obesity also disproportionately impacts people of color, with the highest rates of obesity in Non-Hispanic Black populations (49.9%), and Hispanic (45.6%) populations. Interestingly, those populations who had a high school diploma and some college had the highest rates of obesity (46.4%) compared to those without a high school diploma (40.1%) and those with a college degree or higher (34.2%). As we might expect, obesity rates do increase with age up to the 40-59 year old range, then decline slightly for those over 60 years of age (CDC, 2024c). Some researchers project that the rates of obesity in U.S. adults will reach almost 50% by 2030, and half of that will be severe obesity (Ward et al., 2019).

Obesity is not only a problem in America either. According to the WHO, obesity rates have doubled worldwide since 1990: with nearly 1 in 8 adults, and 8% of children between the ages of 5-19 living with obesity in 2022. In medium and low income countries in particular, obesity co-exists with malnutrition. Food insecurity may include not having enough food to eat or only having access to low quality food sources, often high in sugar and fat, which are associated with the development of obesity. Thus, undernutrition and obesity may both be affecting impoverished areas around the globe (World Health Organization: WHO, 2024a).

In a higher-income country like the U.S., obesity has increased in both low and high socio-economic strata. However, poverty is still considered a driver of obesity, as well as other factors associated with poverty such as stress and “obesogenic” environments. The link between obesity and socio-economic status may be bi-directional. Having obesity may limit economic success via weight stigma in the workplace, while conversely, living in a wealthier neighborhood with plenty of greenspace and opportunities for physical activity, as well as access to high-quality healthy food sources (more supermarkets, fewer convenience stores), is associated with having a lower BMI (Anekwe et al., 2020).