Definition and Overview

The World Health Organization (WHO) defines overweight and obesity as having “excessive fat accumulation that presents a risk to health.” There are various methods to calculate body fat, as each range in accuracy and can have limitations. Body mass index (BMI) is one tool used to screen for excessive body fat. A BMI value of more than 25 is categorized as overweight, and a BMI value of more than 30 is categorized as obese.

Why use BMI?

BMI is one of the most widely used screening tools for estimating excess body fat. However, it can be a controversial measure as it only accounts for one’s height and weight and not the amount of, location of, or types of body fat. It does not distinguish between fat mass and fat-free mass (lean body mass) that includes muscle and the body’s tissues and organs, which contribute to total body weight. That said, BMI is simple to measure and noninvasive, and research has shown that BMI often correlates with “gold standard” direct measures of fat mass and fat-free mass such as dual X-ray absorptiometry. Learn more about calculating BMI and other methods of measuring body fat.

Health Impacts

There are negative health impacts associated with excess body fat. The WHO estimates that in 2019, 5 million deaths from noncommunicable diseases such as cardiovascular disease and diabetes were caused by a high BMI, and rates of obesity continue to grow globally in children and adults. [1] According to the Centers for Disease Control and Prevention in the U.S., 1 in 5 children and 1 in 3 adults has obesity. [2] Children with obesity are more likely to have obesity as adults and the associated risks of heart disease, high blood pressure, type 2 diabetes, and certain types of cancer.

Weight gain of greater than 10 pounds in adulthood also increases disease risk (i.e., heart disease, high blood pressure, type 2 diabetes, gallstones) even in those whose BMI remains in the normal range, according to research from the Nurses’ Health Study and Health Professionals Follow-up Study. [15-19]

With the five leading causes of death – heart disease, cancer, chronic lower respiratory disease, cerebrovascular diseases such as stroke, and unintentional injuries – obesity is a major risk factor for the first four. [20] Obesity is also associated with other health conditions such as sleep apnea, fatty liver disease, gallstones, infertility, respiratory diseases, gastroesophageal reflux disease, and musculoskeletal disorders such as arthritis. No less real are the social and emotional effects of obesity which may include discrimination, lower quality of life, and susceptibility to depression.

Economic Impacts

Obesity costs the U.S. health care system almost $173 billion annually. [2] This includes money spent directly on medical care and prescription drugs related to obesity. In the workplace, research has shown that the number of sick days, short-term disability, and workers’ compensation days increase with increasing BMI. [21] Compared with an employee with a BMI of 25, an employee with a BMI of 35 has nearly double the risk of a disability or workers’ compensation claim. Perhaps one of the most surprising consequences of the current obesity epidemic in the U.S. is its impact on recruitment for the armed services, with data showing that 3 in 5 young adults carry too much weight to qualify for military service. [2]


According to the WHO, worldwide obesity rates are rising with 1 in 8 people, or more than 1 billion people, around the world living with obesity. [14] A Lancet review showed that global adult obesity doubled from 1990 to 2022, and adolescent obesity quadrupled. [22] As low to middle-income countries adopt unhealthy eating patterns and behaviors of industrialized nations such as less physical activity, so do their obesity rates. Specific regions including Polynesia, Micronesia, the Caribbean, the Middle East, and north Africa showed the greatest increases in obesity, as well as higher-income countries such as Chile. [22] Increased eating out of home, access to ultra-processed low-nutrient-dense foods, and sedentariness (e.g., sitting at work, driving instead of walking or bicycling) have contributed. Higher cost and limited access to healthy foods and decreased opportunities for play and sports may cause inequalities in obesity and could limit the impact of policies that target unhealthy foods. [22]

The NCD Risk Factor Collaboration visualizes the prevalence of obesity globally as well as changing obesity rates in various countries since 1990. Below are data summaries of obesity rates for both adults and children around the world:

Risk Factors

Various factors can influence body weight or greater weight gain in specific areas of the body. Some of these cannot be changed, but others may be modified:

  • Non-modifiable risk factors (risk factors you can’t change): age, gender, genes, ethnic origin, and sometimes medications that change how energy is processed in the body leading to weight gain. There’s also strong evidence that having obesity in infancy or childhood increases the chances of remaining obese in adulthood.
  • Risk factors that can be modified: food and beverages consumed, level of physical activity, daily screen time (increased screen time is associated with reduced physical activity time and also increases exposure to marketing of unhealthy foods/beverages), poor sleep hygiene, uncontrolled negative stress. While easier said than done, evidence has shown that addressing these factors as early as possible, even in childhood, may reduce the risk of developing obesity.

Obesity is complex and not just about being born with a certain body size, taking in more calories than the body needs, or burning extra calories through exercise to lose weight. It is often the result of a combination of several non-modifiable and modifiable risk factors. When looking at diet and food, which is often the focus with obesity, there are various factors to consider than just calories in/calories out:

  • Living in a food environment that lacks access to healthy food choices or income-related barriers to regularly consuming a variety of healthy foods.
  • Amount of low-nutrient ultra-processed foods and sugary beverages consumed, especially when displacing high-fiber whole foods, which can increase cravings.
  • Eating behaviors such as skipping meals and eating heavy meals or snacks at night before bed.
  • Overeating portions not from hunger but from stress or boredom.

The environments that surround us also play an important role, as they can make a modifiable risk factor a non-modifiable one. Examples are when someone does not have the ability to secure healthy food choices due to living in a food desert or a safe place to perform regular physical activity.

Is Prevention Possible?

Many factors contribute to rising rates of obesity in children and adults. Among them, the abundance of low-priced, high-calorie ultra processed foods and sugary drinks; incessant marketing driving people to eat more; and an environment that reduces the need for regular physical activity.

Although preventing weight gain over the years of life may not be possible for everyone due to a variety of circumstances, there are strategies to reduce the amount of weight change by increasing awareness of modifiable risk factors and working toward healthy lifestyle behaviors.

However, focusing only on the individual (e.g., nutrition and lifestyle education, weight loss medications) has little impact on global obesity prevalence. Prevention requires a broader scope that addresses food systems and an obesogenic environment, including policy changes such as regulating the marketing of ultra-processed low-nutrient-dense foods and taxing items such as sugar-sweetened beverages.

Indeed, what sometimes gets lost in the discussion is that obesity is preventable. We can turn around the obesity epidemic by collaboratively creating an environment where the default option is the healthy choice.

Looking for The Obesity Prevention Source?

The Obesity Prevention Source website was launched in 2010 as an in-depth resource for all who seek to understand the causes of obesity and to reverse the epidemic of obesity in children and adults. Core information from the website has been integrated into The Nutrition Source, including policy and environmental change recommendations crucial for obesity prevention.

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