Body weight is an essential part of a person’s self-image, and when pounds accumulate, so often does distress. Our culture is, to put it mildly, preoccupied with weight. Large weight gain is almost always noticed, if not discussed, and can lead to stigmatization and even discrimination.
And yet, surveys show that almost half of people who are overweight or even obese don’t realize it, and most parents of obese children describe their kids as being “about the right weight.”
Obesity is one of the biggest health risks we face. It increases the risk of many chronic disorders, notably heart and liver disease, hypertension, type 2 diabetes, some types of cancer, gallstones, sleep apnea, osteoarthritis, reflux disease, and respiratory problems—and it can seriously undermine quality of life.
The adverse health effects of obesity have blunted some of the gains in longevity resulting from medical advances and declining smoking rates. It’s estimated that obesity is associated with anywhere from 10 to 25 percent of all deaths and shortens life expectancy by four to seven years.
Obesity rates in the U.S. have been rising for a century and have more than doubled in just the past 35 years, affecting every ethnic, demographic, and age group. More than one-third of American adults (80 million) are now obese—that is, very overweight—and another third are slightly or moderately overweight, according to government statistics. Even worse, the rate of severe obesity (usually more than 100 pounds overweight) has more than tripled during this period. What’s more, 17 percent of young Americans (ages 2 to 19) are now obese, and as many are overweight. These statistics are based on the body mass index (BMI), a formula that relates weight to height.
Obesity rates have also been climbing in Canada and other countries, though few come close to or exceed our rate (the most obese: Kuwait and Saudi Arabia).
In light of these trends, in 2013 the American Medical Association started classifying obesity as a disease, a decision that still generates debate. Also that year, the American Heart Association, the American College of Cardiology, and the Obesity Society announced in joint guidelines that obesity should be treated as a chronic disease.
Why have we been gaining so much weight?
On the most basic level, people gain weight when there’s an energy imbalance—they consume more calories than they burn. But it’s overly simplistic to blame the obesity epidemic solely on people eating too much because of lack of willpower and on sedentary lifestyles. If there ever was a multifactorial condition, obesity is it—a complex of interacting genetic, metabolic, behavioral, hormonal, psychological, cultural, environmental, and socioeconomic factors, some of which are easier to alter than others.
Here's a summary of key factors involved in the obesity epidemic:
Since the 1970s, American men have increased their daily calorie intake by an average of 210 calories, and women by about 270 calories, according to an analysis in the American Journal of Clinical Nutrition in 2013. Calorie intake appears to have peaked about a decade ago, dropping slightly since then. Most of the extra calories have come from high-carbohydrate (that is, sugary or starchy) foods and beverages. Liquid calories are especially bad for weight control since they do not reduce appetite as much as solid foods.
“Toxic” food environment
One reason for our increased calorie intake: We are surrounded by inexpensive, energy-dense food, usually sold and served in oversized portions. It isn’t simply that food companies profit by selling us more food, but also that people have gotten used to the abundance of cheap food (per ounce, if not per serving) and expect to find it everywhere. This calorie glut is made possible largely by government subsidies for wheat, soy, and especially corn—key ingredients in “junk” food and in feed for cattle and pigs. Other contributors to our higher calorie intake include increased and cheaper output from factory farming, improvements in food palatability (thanks to added sugar, fat, and sodium and manipulating texture and other food qualities), and marketing by the food industry.
As a result, Americans now spend a smaller share of their income on food than any society in history or anywhere else in the world (a good thing), yet we get more “empty” calories for it (not good), according to a paper about the economics of obesity, published in 2014 in CA: A Cancer Journal of Clinicians. This has fueled overconsumption of high-calorie fare such as fast food, corn-fed beef and pork, packaged snacks, ready-to-eat meals, and soft drinks.
Americans eat too much sugar, which has been a major player in the obesity epidemic. Sugars found naturally in foods such as fruit (mostly fructose) and milk (lactose) are not the problem. The culprit is the sugar liberally added to so many foods—not only candies and cookies, but also staples like pasta sauces, ketchup, canned baked beans, and breakfast cereals. On average, we consume about 90 grams (22 teaspoons) of added sugar a day—providing 350 calories—more than one-third from soft drinks.
The fructose in that added sugar may be a particular problem. Say fructose and most people think high-fructose corn syrup (HFCS)—the No. 1 sweetener, added to so many soft drinks and processed foods. HFCS is slightly more than half fructose, but plain old table sugar (sucrose) is also half fructose, while honey averages about 40 percent fructose. Research suggests that high intake of fructose can have adverse effects on blood cholesterol and triglycerides, worsen blood sugar control, promote abdominal fat gain, and pose other health risks. It may also have less effect on satiety than other sugars. In any case, regardless of how all this added sugar is metabolized, the main problem is that we’re consuming too much of it.
Less home cooking
Longer average work hours and more two-worker households mean less time for home-cooked meals, especially those prepared from scratch, which tend to be more healthful and lower in calories than packaged or ready-to-eat foods. What’s more, in the past few decades Americans have been eating more meals in restaurants. Studies have consistently found that restaurant fare (whether fast food or fine cuisine) tends to be higher in calories than home-cooked meals, largely because it’s served in such oversized portions. Not surprisingly, then, research has linked frequent eating out to increased body weight.
Too much sit-down time
Americans are much less physically active, on average, than they were in the past. Major culprits include the increasingly sedentary nature of many forms of work; dramatically increased time spent sitting in cars, watching TV, and using computers; and lack of daily physical activity in most schools. Last year an analysis of U.S. data in the American Journal of Medicine concluded that the number of women who reported no leisure-time physical activity jumped from 19 to 52 percent in the past two decades; in men the numbers rose from 11 to 43 percent. Burning fewer calories means storing more of them as body fat. And, in turn, obesity and its consequences discourage many people from exercising and staying active. Still, for most obese people, physical activity by itself, without reduction in calorie intake and improvement in diet, won’t lead to significant and long-term weight loss.
What should your body look like? Look at most actors and fashion models and you may think people should have little or no body fat, a goal that is likely to set you up for failure. At the same time, if you’re like most Americans and you glance at your friends, family, and neighbors, you may think that being overweight or even obese is normal.
In fact, your social network is a good predictor of whether you will be obese or not. It’s not just that obese people hang out together, but also that thinner people who have many obese friends are much more likely to become obese themselves over the long term, according to a widely publicized study in the New England Journal of Medicinein 2007. It found that the risk increases by nearly 60 percent when a person has a friend who becomes obese and by about 40 percent when a sibling or spouse becomes obese. People of the same sex have a greater influence on each other’s weight than those of the opposite sex.
Genetics and upbringing
Obesity offers a good example of genes interacting with lifestyle and environment. It’s clear that obesity runs in families: If you have an obese parent, there’s a good chance you’ll become obese too; with two obese parents, the risk is even greater. Part of this is explained by the fact that genes play a large role in aspects of weight regulation, such as metabolic rate (the rate at which we burn calories when at rest and during activity). Certain genes may also disrupt appetite control systems in the brain—for instance, by affecting the action of leptin, ghrelin, and other hormones that signal the brain about hunger and satiety.
Scientists have found more than 1,000 genes that affect metabolism and weight-related behavior. These help explain, for instance, why some people have an easier time staying thin (despite constant exposure to calorie-dense foods) while others continually struggle with weight gain, and why some people do better on a low-carb diet and others on a low-fat diet.
What you eat and how active you are matter too, of course, though genetics also influences your preferences for various foods and exercise and how your body responds metabolically.
Genetics may partly explain why one person becomes fat while another does not, but it doesn’t explain the obesity epidemic, since human genetic makeup hasn’t changed significantly during the past few generations. Here’s where epigenetics comes into play. This involves changes in gene expression (not explained by changes in DNA itself) that can occur during a lifetime in response to outside influences and then be passed along to offspring. In terms of body weight, a mother’s diet, weight, lifestyle, and environmental exposures before and during pregnancy can cause epigenetic changes that alter her child’s risk of obesity and related conditions.
Parents also pass on or nurture other weight-related tendencies by example— regarding eating patterns, attitudes about exercise, and so on. In addition, your social stratum influences your health, habits, and body weight. For example, obesity is more prevalent among poorer people than among the well-to-do.
Too little sleep
Many studies over the past few years have linked inadequate sleep to an increased risk of obesity in a number of ways. Some have found that it can undermine weight-control efforts—thus, people on diets tend to be more successful when they get enough (but not too much) sleep. Other studies find that decreased sleep can lead to poorer eating habits, greater calorie intake, lower metabolic rate, and increased abdominal fat. One proposed mechanism is that not getting enough sleep affects appetite hormones, notably ghrelin and leptin, as well as insulin, leading to increased hunger and food intake, reduced calorie burning, and increased fat storage. But the relationship between sleep and metabolism is complex, and reduced sleep can affect people differently.
Inflammation and insulin resistance
Chronic inflammation in the body can be both a cause and an effect of insulin resistance and obesity—setting up a vicious cycle. For example, inflammation can contribute to the development of insulin resistance, which in turn may promote obesity. Conversely, obesity— especially abdominal obesity—worsens insulin resistance and increases chronic inflammation, partly because fat surrounding organs in the abdominal area (visceral fat) releases pro-inflammatory compounds. In effect, inflammation, obesity, and insulin resistance reinforce one another, often resulting in type 2 diabetes. What’s more, many lifestyle factors that promote inflammation and insulin resistance, such as being sedentary and having an unhealthy diet, also promote obesity.
Microbes in the gut
Recent research has linked the microbiota—the trillions of microorganisms in our colon—to many aspects of our health, including body weight. Animal studies have found that the composition of microbial populations in the colon influences energy metabolism as well as how carbohydrates and fats are digested, thus affecting the risk of obesity. These microbes, which vary from person to person, are influenced by genetic, dietary, environmental, and other factors. For instance, some research suggests that antibiotics, especially when given to children, may alter intestinal bacteria in ways that have long-term effects on body composition and weight.
Other “obesogenic” culprits
Researchers have been exploring many additional factors that may play roles in the obesity epidemic. These include various “obesogens”—chemicals in foods, products, and the environment that, in theory at least, increase fat accumulation. Some of these are endocrine disruptors, which mimic or interfere with the function of estrogen and other human hormones. We’ve discussed two of these: BPA, found in some hard plastics, the lining of cans, and certain paper receipts; and flame retardants used in furniture, carpeting, and mattresses. Observational studies have linked BPA exposure in the womb or in infancy to an increased risk of obesity later in life.
Other proposed obesogenic factors include certain diseases, such as hypothyroidism; increased chronic stress and lack of sense of control in modern life; older maternal age; wide use of medications associated with weight gain (such as some used for depression, diabetes, hypertension, epilepsy, and contraception); and the “addictive” properties of many seductively palatable processed foods.