Gerald Hurowitz, M.D.·Published 5 November 2004·10 min read
Key points
The definition of obesity varies depending on what one reads, but in general, it is a chronic condition defined by an excess amount body fat.
Overweight and obesity are both labels for ranges of weight that are greater than what is generally considered healthy for a given height.
As in adults, obesity is also a growing problem in children and adolescents.
One important category of obesity not captured by BMI is so-called "abdominal obesity" - the extra fat found around the middle that is an important factor in health, even independent of BMI.
What is obesity?
The definition of obesity varies depending on what one reads, but in general, it is a chronic condition defined by an excess amount body fat. A certain amount of body fat is necessary for storing energy, heat insulation, shock absorption, and other functions. The normal amount of body fat (expressed as percentage of body fat) is between 25-30% in women and 18-23% in men. Women with over 30% body fat and men with over 25% body fat are considered obese.
Defining Overweight and Obesity
Overweight and obesity are both labels for ranges of weight that are greater than what is generally considered healthy for a given height. The terms also identify ranges of weight that have been shown to increase the likelihood of certain diseases and other health problems.
Healthy BMI Ranges for Adults and Children
What's considered a healthy BMI?
For adult men and women, a BMI between 18.5 and 24.9 is considered healthy.
Overweight is defined as a BMI between 25.0 and 29.9; and a BMI of 30 or higher is considered obese.
As in adults, obesity is also a growing problem in children and adolescents. Because children grow at different rates, depending on their age and gender, the definitions of overweight and obesity in children and adolescents differ from those in adults.
In the U.S., for example, the definition is based on standard growth charts developed by the Centers for Disease Control and Prevention. In children and adolescents age 2 to 20 years old, a BMI in the 85th to 94th percentiles for age and gender is considered overweight; a BMI in the 95th percentile or higher is considered obese.
It is important to remember that although BMI correlates with the amount of body fat, BMI does not directly measure body fat. As a result, some people, such as athletes, may have a BMI that identifies them as overweight even though they do not have excess body fat. For more information about BMI, visit Body Mass Index.
Other methods of estimating body fat and body fat distribution include measurements of skinfold thickness and waist circumference, calculation of waist-to-hip circumference ratios, and techniques such as ultrasound, computed tomography, and magnetic resonance imaging (MRI).
Waist Size Matters: Abdominal Obesity
One important category of obesity not captured by BMI is so-called "abdominal obesity" - the extra fat found around the middle that is an important factor in health, even independent of BMI.
Written and revised by
Gerald Hurowitz, M.D.
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The simplest and most often used measure of abdominal obesity is waist size. Guidelines generally define abdominal obesity in women as a waist size 35 inches or higher, and in men as a waist size of 40 inches or higher.
Measuring Body Fat
There are a number of ways to measure body fat. Some are well suited to the doctor's office, such as calculating a person's BMI. Other, more complex methods require specialized equipment, such as magnetic resonance imaging or dual energy X-ray absorptiometry machines; while these machines can measure body fat very accurately, they are typically only used for this purpose in research settings.
Globally, there are 1.5 billion adults who are either overweight or obese, a number expected to increase to 3 billion by 2030. The epidemic is reaching catastrophic proportions, and one of the key "if small" steps to bringing it under control is to have a common language to describe the problem.
Facts about overweight and obesity
Some recent WHO global estimates follow.
In 2014, more than 1.9 billion adults, 18 years and older, were overweight. Of these over 600 million were obese.
Overall, about 13% of the world's adult population (11% of men and 15% of women) were obese in 2014.
In 2014, 39% of adults aged 18 years and over (38% of men and 40% of women) were overweight.
The worldwide prevalence of obesity more than doubled between 1980 and 2014.
In 2013, 42 million children under the age of 5 were overweight or obese. Once considered a high-income country problem, overweight and obesity are now on the rise in low- and middle-income countries, particularly in urban settings. In developing countries with emerging economies (classified by the World Bank as lower- and middle-income countries) the rate of increase of childhood overweight and obesity has been more than 30% higher than that of developed countries.
Overweight and obesity are linked to more deaths worldwide than underweight. Most of the world's population live in countries where overweight and obesity kill more people than underweight (this includes all high-income and most middle-income countries).
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Obesity Causes
Many factors influence body weight - genes, though the effect is small, and heredity is not destiny; prenatal and early life influences; poor diets; too much television watching; too little physical activity and sleep; and our food and physical activity environment.
What Tips the Scales Toward Excess Weight?
The causes of obesity are as varied as the people it affects.
At its most basic, of course, obesity results when someone regularly takes in more calories than needed. The body stores these excess calories as body fat, and over time the extra pounds add up. Eat fewer calories than the body burns, weight goes down. This equation can be deceptively simple, though, because it doesn't account for the multitude of factors that affect what we eat, how much we exercise, and how our bodies process all this energy. A complex web surrounds a basic problem.
What are some of the factors that increase the risk of obesity?
Genes Are Not Destiny
Heredity plays a role in obesity but generally to a much lesser degree than many people might believe. Rather than being obesity's sole cause, genes seem to increase the risk of weight gain and interact with other risk factors in the environment, such as unhealthy diets and inactive lifestyles. And healthy lifestyles can counteract these genetic effects.
Prenatal and Postnatal Influences
Early life is important, too. Pregnant mothers who smoke or who are overweight may have children who are more likely to grow up to be obese adults. Excessive weight gain during infancy also raises the risk of adult obesity, while being breastfed may lower the risk.
How can overweight and obesity be reduced?
Overweight and obesity, as well as their related noncommunicable diseases, are largely preventable. Supportive environments and communities are fundamental in shaping people's choices, making the healthier choice of foods and regular physical activity the easiest choice (accessible, available and affordable), and therefore preventing obesity.
At the individual level, people can:
limit energy intake from total fats and sugars;
increase consumption of fruit and vegetables, as well as legumes, whole grains and nuts;
engage in regular physical activity (60 minutes a day for children and 150 minutes per week for adults).
Individual responsibility can only have its full effect where people have access to a healthy lifestyle. Therefore, at the societal level it is important to:
support individuals in following the recommendations above, through sustained political commitment and the collaboration of many public and private stakeholders;
make regular physical activity and healthier dietary choices available, affordable and easily accessible to all - especially the poorest individuals.
The food industry can play a significant role in promoting healthy diets by:
reducing the fat, sugar and salt content of processed foods;
ensuring that healthy and nutritious choices are available and affordable to all consumers;
practicing responsible marketing especially those aimed at children and teenagers;
ensuring the availability of healthy food choices and supporting regular physical activity practice in the workplace.
Unhealthy Diets
What's become the typical Western diet - frequent, large meals high in refined grains, red meat, unhealthy fats, and sugary drinks - plays one of the largest roles in obesity. Foods that are lacking in the Western diet - whole grains, vegetables, fruits, and nuts - seem to help with weight control, and also help prevent chronic disease.
Too Much Television, Too Little Activity, and Too Little Sleep
Television watching is a strong obesity risk factor, in part because exposure to food and beverage advertising can influence what people eat. Physical activity can protect against weight gain, but globally, people just aren't doing enough of it. Lack of sleep - another hallmark of the Western lifestyle - is also emerging as a risk factor for obesity.
Toxic Environment - Food and Physical Activity
As key as individual choices are when it comes to health, no one person behaves in a vacuum. The physical and social environment in which people live plays a huge role in the food and activity choices they make. And, unfortunately, in the U.S. and increasingly around the globe, this environment has become toxic to healthy living: The incessant and unavoidable marketing of unhealthy foods and sugary drinks. The lack of safe areas for exercising. The junk food sold at school, at work, and at the corner store. Add it up, and it's tough for individuals to make the healthy choices that are so important to a good quality of life and a healthy weight.
Obesity and its causes have, in many ways, become woven into the fabric of our society. To successfully disentangle them will take a multifaceted approach that not only gives individuals the skills to make healthier choices but also sets in place policy and infrastructure that support those choices.
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How common is obesity?
Obesity has reached epidemic proportions in the United States. One in three Americans is obese. Obesity is also increasing rapidly throughout the world, and the incidence of obesity has nearly doubled form 1991 to 1998.
What are the health risks associated with obesity?
Obesity is not just a cosmetic consideration; it is a dire health dilemma directly harmful to one's health. In the United States, roughly 300,000 deaths per year are directly related to obesity, and more than 80% of these deaths are in patients with a BMI (body mass index, which will be discussed later in this article) over 30. Obesity also increases the risk of developing a number of chronic diseases including:
Insulin Resistance. Insulin is necessary for the transport of blood glucose (sugar) into the cells of muscle and fat (which is then used for energy). By transporting glucose into cells, insulin keeps the blood glucose levels in the normal range. Insulin resistance (IR) is the condition whereby the effectiveness of insulin in transporting glucose (sugar) into cells is diminished. Fat cells are more insulin resistant than muscle cells; therefore, one important cause of IR is obesity. The pancreas initially responds to IR by producing more insulin. As long as the pancreas can produce enough insulin to overcome this resistance, blood glucose levels remain normal. This IR state (characterized by normal blood glucose levels and high insulin levels) can last years. Once the pancreas can no longer keep up with producing high levels of insulin, blood glucose levels begin to rise, resulting in type 2 diabetes, thus IR is a pre-diabetes condition. In fact scientists now believe that the atherosclerosis (hardening of the arteries) associated with diabetes likely develops during this IR period.
Type 2 (adult-onset) diabetes. The risk of type 2 diabetes increases with the degree and duration of obesity. Type 2 diabetes is associated with central obesity; a person with central obesity has excess fat around his/her waist, so that the body is shaped like an apple.
High blood pressure (hypertension). Hypertension is common among obese adults. A Norwegian study showed that weight gain tended to increase blood pressure in women more significantly than in men. The risk of developing high blood pressure is also higher in obese people who are apple shaped (central obesity) than in people who are pear shaped (fat distribution mainly in hips and thighs).
High cholesterol (hypercholesterolemia)
Stroke (cerebrovascular accident or CVA)
Heart attack. The Nurses Health Study found that the risk of developing coronary artery disease increased 3 to 4 times in women who had a BMI greater than 29. A Finnish study showed that for every one kilogram (2.2 pounds) increase in body weight, the risk of death from coronary artery disease increased by one percent. In patients who have already had a heart attack, obesity is associated with an increased likelihood of a second heart attack.
Congestive heart failure
Cancer. While not conclusively proven, some observational studies have linked obesity to cancer of the colon in men and women, cancer of the rectum and prostate in men, and cancer of the gallbladder and uterus in women. Obesity may also be associated with breast cancer, particularly in postmenopausal women. Fat tissue is important in the production of estrogen, and prolonged exposure to high levels of estrogen increases the risk of breast cancer.
Gallstones
Gout and gouty arthritis
Osteoarthritis (degenerative arthritis) of the knees, hips, and the lower back
Sleep apnea
Pickwickian syndrome (obesity, red face, underventilation, and drowsiness)
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