Obesity has become one of the greatest health scourges of our time. Around the world, it contributes to 3 million deaths each year. There are more than a billion adults who classify as overweight or obese, and the epidemic has hit developing countries, where starvation and malnutrition are still widespread. As researchers learn more about the social and economic impact of obesity, a number of basic unanswered questions remain — such as why obesity rates have soared in the past three decades, and how to turn that trend around.
What is obesity, and why are we so obsessed about it?
The word obesity comes from the Latin "obesitas," meaning fat or plump. "Body mass index," or BMI — a calculation of a person’s height and weight — is the most widely used method for measuring obesity, though it is by no means perfect. BMI will put really muscular folks — including actors like Tom Cruise — in the overweight and obese categories because it cannot distinguish between lean muscle mass and fat mass.
Most people aren't athletes or Tom Cruise, however, so BMI remains a pretty good indicator. You can find your BMI using this online calculator or the chart here. A BMI of 25 or more puts a person in the overweight category; 30 is the threshold for obesity.
As for the attention on obesity, the reason is simple: It's become one of the greatest health scourges of our time in developed and developing countries alike.
Studies have found that excess weight — especially obesity and morbid obesity — is a major risk factor for death and many, many diseases. Obesity is linked with diabetes, high blood pressure, stroke, heart disease, asthma, sleep apnea, kidney stones, and several types of cancer. Around the world, it's thought to contribute to 3 million deaths each year. There are also more than a billion adults who now classify as overweight or obese, and, again, the epidemic has hit low- and middle-income countries, where starvation and malnutrition are still widespread.
Yet we don't know precisely what has caused rates of obesity to rise, so there are big uncertainties at the center of the science. A lot of the data can only demonstrate correlations, because it's observational and not experimental. In other words, researchers can't randomly assign people to obesity in an experiment and make them gain weight; they must observe health outcomes in people who are already overweight or obese. (Read more about the limitations of different types of scientific evidence here.)
Those uncertainties don't make the problem of obesity any less real. "Obesity costs the world nearly as much as war, armed violence and terrorism," one Financial Times writer put it. He also wrote: "Yet in dealing with obesity, we are like 16th century navigators using maps with missing islands and misshapen continents."
What does obesity look like?
You can see BMI visualized by watching this figure grow from a normal bodyweight to overweight and then obese.
You’ll notice the waistline here puff up; that excess fat around the midline is another important measure of obesity that’s not reflected in BMI. Larger waists are associated with an increased risk of Type 2 diabetes, cardiovascular disease, and death, even after controlling for BMI. You can easily tell whether your waist size puts you in the danger zone: Abdominal obesity in women is a waist size that’s 35 inches or more, and in men it’s 40 inches or more.
People are really bad judges of their own weight
In a recent study, researchers at the Rush University Medical Center in Chicago set out to see how African-American women thought about their bodyweight. They used this "body image scale" to determine what women deemed normal weight, overweight, and obese.
"Overweight figures were not considered too fat," the researchers found. So even though medical professionals consider the women in pictures 2, 3, and 4 to be normal weight — and all the other figures to the right to be overweight or obese — the participants in the study only thought the last two drawings (8 and 9) were tipping the scales. White women, on the other hand, tend to place themselves in a higher weight group than they actually belong in.
Excess weight has a hefty price tag
"Excess weight, especially obesity, diminishes almost every aspect of health," according to the Harvard School of Public Health, "from reproductive and respiratory function to memory and mood." Being obese raises one’s risk of death from a range of diseases including diabetes, high blood pressure, stroke, heart disease, asthma, sleep apnea, kidney stones, and several types of cancer. One study estimated that in 2012, 3.6 percent of all new cancers worldwide were attributable to high body mass index.
Obesity is also associated with shorter lifespans, and the higher one’s BMI, the more years of life lost. For example, a 20-year-old man with a BMI of 40 will live six years less than a non-obese man the same age.
In the US, as you can see below, obesity — more than other risk factors — has the most negative impact on life expectancy.
In addition to the personal health costs of obesity, there are economic costs, too. Researchers at George Washington University tried to quantify the economic impact of obesity on men and women. They found that obesity is associated with almost twice the cost for women as compared to men. And 38 percent of this cost for women came from lower wages, while the incomes of obese men barely suffered.
They weren't the first group to come to this conclusion. A seminal 2004 study, for example, found that a 65-pound increase in a woman's weight is linked with a 9 percent cut in income.
It's not clear why obesity and lower incomes are linked, which condition comes first, and why women seem to be especially burdened by this correlation. On the last question, however, there is a growing consensus that it has to do with discrimination against larger women.
"This [gender] disparity around wages represents the stigmatization and discrimination against obese individuals, and particularly Caucasian females," said William Dietz of the Centers for Disease Control and Prevention. "This stigmatization is pervasive and longstanding."
At the country level, there are obesity costs, too. Nearly 10 percent of health-care spending goes to obesity each year in the US. In 2008, obesity-related costs totaled about $147 billion. That’s because compared with normal-weight people, obese patients have been estimated to spend 46 percent more on hospital visits, 27 percent more on doctor exams, and 80 percent more on prescription drugs.
Americans have only gotten fatter over time
This data from the Centers for Disease Control and Prevention shows the red tide of obesity spreading across the United States over the past 30 years. Thirty percent of the adult population is now obese. Since 1980, obesity rates more than doubled among adults and tripled among children.
In every state, at least one-fifth of adults are obese. In the Southern and Midwestern parts of the country, the problem is even more acute, and Mississippi and West Virginia have the highest obesity rates in the country at over 35 percent. Only seven states — Massachusetts, Hawaii, Colorado, Vermont, Montana, Utah, and California — and the District of Columbia have an obesity prevalence lower than a quarter. Colorado was the least obese state, with a prevalence of 21.3 percent, followed by Hawaii at 21.8 percent.
Scientists use the term "obesity epidemic" to describe what's happening here. It might sound like a weird way to refer to a non-infectious condition, but obesity is classified as a "disease" in America, and its prevalence has now hit epidemic proportions.
The scary part is that despite the clear and dramatic increase in obesity rates, no one has been able to figure out exactly what’s driving this trend. As CDC researcher Katherine Flegal put it, "There’s a lot of speculation, but we don’t really know the answer."
Obesity is much more prevalent among racial minorities than among white people
Your skin color relates to how obese you're likely to be. Black people reported the highest obesity prevalence (37.6 percent), followed by Hispanic people (30.6 percent) and then white people (26.6 percent). So there was a difference of more than 10 percentage points between obesity in blacks and whites.
This map shows the obesity prevalence by state among white people for the years 2011 to 2013:
Compare that to this map, which shows obesity rates among black adults for the same years:
There are many more states reporting the highest level of obesity for this population. According to the CDC, nearly 40 percent of black men 20 years of age and older are obese. No population in the US has an obesity rate as high as black adult women: 58 percent are obese.
Less educated and poorer people report more obesity
The Robert Wood Johnson Foundation's State of Obesity study found a strong correlation between obesity and education and income levels. Of adults who didn’t graduate high school, more than 35 percent were obese compared with 26 percent of those who finished college.
Meanwhile, 33 percent of adults who earn less than $15,000 per year were obese, while a quarter of those who earned at least $50,000 were obese. Again, there's the chicken-and-egg problem here: it's not clear whether the lower income and earning potential result in obesity, or whether being obese leads to a lower socioeconomic status.
Some people believe in the "obesity paradox"
Obesity is very complex, and we're only beginning to understand the causes and impact of this relatively new condition.
Some argue that the correlational data on weight and health outcomes is weak. They contend that the multibillion-dollar fitness and weight loss industries perpetuate an "obesity myth" and hysteria around weight to suit their own needs. Underlying this argument is a belief in the "obesity paradox" — that being overweight is associated with some protective health effect, and even mild obesity confers no excess health risk.
This isn't a view widely held by the medical and public health communities. First of all, there's a big flaw with all the studies on the obesity paradox: they only look at people's weight at one point in time. This is like asking someone who has smoked for her entire life but quit last week about her health, and then classifying her as a nonsmoker without incorporating any data about her long affair with cigarettes.
Andrew Stokes, a professor who has researched obesity at Boston University, actually looked at more than 10 years of data from the Centers for Disease Control and Prevention and death records of American adults between the ages of 50 and 84, and went back in time, accounting for people's weight histories. This made it possible to break up the normal weight category into two separate groups that are usually lumped together: those who had maintained a normal weight throughout their lives, and those who were normal weight at the time of the study but had experienced weight loss.
The obesity paradox boils down to reverse causation
Stokes found that people who were always normal weight had an extremely low risk of death, but that the other normal weight group — with people who were formerly obese — had a much higher mortality rate. After redefining the normal weight category to only include the stable weight individuals, he found much stronger associations between excess weight and mortality.
"It all boils down to reverse causation," he said. "The obesity paradox has been found in a lot of different populations: in the general population, in older and sick people, people with heart failure and other conditions." But when you start picking apart the science, looking at people over time, the paradox disappears. And overall, studies have found that excess weight — especially obesity and morbid obesity — is a major risk factor for death and disease.
Obesity is not a uniquely American phenomenon
Worldwide, obesity has nearly doubled since 1980. People are now fatter everywhere, even in the poorest developing countries, where malnutrition and starvation are widespread problems. In 2008, more than 1.4 billion adults were overweight.
Of those, over 500 million men and women were obese. The trend now reaches into developing countries. Obesity is a problem in Mexico, the Middle East, and several countries in Africa.
Obesity rates are stabilizing
There is some good news on obesity: the growth in obesity rates in the US and around the world appears to have slowed in recent years. This chart shows the prevalence of overweightness and obesity between 1980 and 2013. There was a surge between 1992 and 2002, but since then growth has tapered off, especially in the developed world.
In the US, the State of Obesity report found that over the past 35 years obesity rates in adults more than doubled, but between 2009 and 2012 they stayed the same. In children, rates have more than tripled since 1980 but have also plateaued for the last several years.
Exactly why this has occurred remains a scientific mystery, but researchers think it's the impact of some combination of changes in policy, increased awareness about healthy lifestyles, and the attention that has been paid to stopping obesity in early childhood.
The most basic explanation for how we got so fat
Another obesity-related riddle: no one knows exactly why so many people got so fat in recent decades.
But the most straightforward explanation for the rising rates of overweightness and obesity is that food production increased and food became cheaper than it ever was, and so people started eating more calories, particularly from processed foods. The average American’s total calorie intake grew from 2,109 calories in 1970 to 2,568 calories in 2010. As Pew Research put it, that’s "the equivalent of an extra steak sandwich every day." And a lack of energy balance — more calories in than we expend — is the simplest of explanation for obesity.
There are many other hypotheses to explain what is driving obesity
We're also moving less. One study showed that there is a striking correlation between rising obesity and the rising popularity of driving. The implication here is that the more we use cars to get around, the less we use our bodies to do so and the fewer calories we burn off. According to the CDC, fewer than half of all adults now meet the Physical Activity Guidelines.
Relative to grains, meat, dairy, and fat, Americans eat few fruits and vegetables. On average, people in the US now eat about 250 to 300 grams of carbohydrates per day, which makes up more than half of their caloric intake. Obesity experts agree that in order to maintain a healthy weight, about half of every meal should come from plants, and yet plants make up a small proportion of the American diet.
More than half of our food dollars are now being spent on restaurant foods and processed, convenient, on-the-go meals. You’re much more likely to pack on calories when you eat out. Compared with home-cooked meals, breakfasts at sit-down restaurants typically have 261 more calories, lunches have 183 more calories, and dinners have 219 more calories. That’s about 600 extra calories per day.
Big Food also plays a role in the obesity epidemic. Added sugars — the sweeteners in processed foods like cakes and sugary beverages — make up an increasingly large part of the American diet. Women should have no more than six teaspoons of sugar per day, and men no more than nine teaspoons. Yet the Center for Science in the Public Interest reported that the average American is now consuming 23 teaspoons of added sugar each day.
The increasing role sugar plays in the diet has become a popular explanation for the obesity epidemic. But many other causes have been blamed: salt, fat, French fries, Facebook, too little sleep, too much stress, chemicals in the environment, and even viruses.
While every one of these theories is a compelling explanation for the surge in obesity rates, what’s really driving the trend is probably a mixture of genetic, environmental and behavioral factors.
There is no “best diet” for losing weight
How do we reverse obesity? Study after study has shown that there truly is no magic diet, no belly blaster or flab fighter, no weight loss wonder or thinning superfood. You can ignore the noise, the hype, the branding, the celebrity-endorsed diet books, and the Dr. Oz–backed metabolism-boosting miracles in a bottle. The one thing you need to know about dieting is rather straightforward: what works is cutting calories in a way that you like and can sustain. Fewer calories means more weight loss.
A number of high-quality studies have found that overall, popular diets work about the same for people. For example, a randomized trial involving 300 women on Atkins (very low in carbohydrates), Zone (low-carb), Ornish (very high-carb), and LEARN (low in fat, high in carbs, based on US guidelines) diets found that while women on Atkins lost a little more, weight loss with a low-carb diet is "likely to be at least as large as for any other dietary pattern."
Despite the new fads, scientists have not uncovered a single "best diet" that works for everyone, and given our individual genetic variation, they probably never will. So for now, you can read a very compelling argument for the Mediterranean diet, and another science-driven polemic that’s diametrically opposed to that, arguing that more saturated fat is the key to health.
Some of the best observational evidence on what works for weight loss comes from the National Weight Control Registry, a study that has parsed the traits, habits, and behaviors of adults who have lost at least 30 pounds and kept it off for a minimum of one year. There are currently more than 10,000 members enrolled in the study, who respond to annual questionnaires about how they've managed to keep their weight down.
The researchers behind the registry found that people who have had success losing weight share a few habits: they weigh themselves at least once a week. They exercise regularly at varying degrees of intensity, with the most common exercise being walking. They restrict their calorie intake, stay away from high-fat foods, and watch their portion sizes. They also tend to eat breakfast. But there's a ton of diversity as to what makes up their meals (meaning there was no "best" diet or fad diet that did the trick.) And they count calories.
"What makes maintaining weight loss seem 'almost impossible,'" writes obesity doctor Yoni Freedhoff, "are the goal posts society has generally set to measure success." So no to quick diets, yes to long-term lifestyle changes. They can help.
Policies might be helping to curb the obesity epidemic
Instead of leaving weight control up to individuals, proponents of obesity-fighting policies believe we need to reengineer our environments to nudge people to make healthier choices. Obesity-related policy experiments are being tried all over the world.
Voters in Berkeley, California, just introduced the first tax on sugary beverages in the nation, which will add 1 cent per ounce to drinks with added sugar. Mexico put a soda tax in place in 2013. While these taxes have sometimes changed consumption habits, they haven't yet succeeded in reducing obesity rates.
In the face of a public health problem of epidemic proportions, proponents say trying out policies that encourage healthy behavior and prevent people from ever becoming overweight is important. As with war and terrorism, however, answers may not be easy to come by.
You didn't answer my question!
This is very much a work in progress. It will continue to be updated as events unfold, new research gets published, and fresh questions emerge.
So if you have additional questions or comments or quibbles or complaints, send a note to Julia Belluz at email@example.com.