We're constantly reminded that we should lose weight because being overweight is bad for our health. We argue about which diet is best, and feel guilty when we eat. We assume that it's possible for anyone to get skinny. Look at models and movie stars. They've gotten so thin that rumors of their eating disorders are regular topics of gossip magazines.

There's no doubt that Americans have gotten fatter. In the late 70s, less than half of us were overweight or obese by the government's standards. Now two-thirds of adults and one-third of kids are. It's certainly not something we're happy about. Millions of us go on diets each year, and we spend over $1 billion annually on weight-loss programs. Most of us fail to lose weight. And 90- 95% of those who do lose gain most or all of it back within a year or two.

We blame each other and ourselves for not trying hard enough. We see being fat as a failure of self-discipline. Or we think it's the diet that was the problem, and eagerly buy the newest diet book. Some theorize that it's the environment that's at fault. They lobby to pull the soda machines out of high schools and get McDonald's to lighten up their fare. New York even considered a tax on sugary drinks. Our universities, government, and national health organizations develop new studies, foundations and initiatives to find ways to help us lose weight. We never give up despite extremely low success rates.

Is there something we're missing? This week we'll describe many perspectives on the causes of overweight and obesity. We'll look at some mixed data on their health consequences. You'll find that although the public health message is very consistent, when you dig deeper the experts don't all agree. Next week we'll look at various diets and tactics that may help you avoid gaining weight--or be healthier at any weight.

We'll refer to weight categories from the Body Mass Index (BMI). It's a ratio of height to weight. You can find your BMI at Calculate Your Body Mass Index. According to this system:

• Underweight = 18.5 or less
• Normal weight = 18.5 to 24.9
• Overweight = 25 to 29.9
• Obesity = 30 or greater

What different factors may be causing the huge increase in overweight and obesity?

The list is longer than you might think. Some are deep-seated beliefs. Many come from scientific studies. And they overlap. But eating behavior is hard to study. Even nutritionists underestimate how much they've eaten, and most studies rely on self-reports of food intake. Some studies put subjects into a hospital where they can control every morsel, but you can't do that for long. And perfect compliance to a strict diet for months or years? That's just not in the cards. Due to these factors, most studies can only show correlations (rates that rise together), not cause and effect. And so far, none of these explanations has lead to a long-term, proven, practical cure.

1. The Conventional View
   • Gluttony and lack of willpower The oldest explanation for obesity is still the most common. We see fat people as greedy and weak-willed. This puts 100% of the blame on the overweight person. It assumes that anyone can lose weight by overcoming temptation. Even overweight people who've tried every diet, supplement and gimmick believe that if they just try harder they could succeed. This belief makes it acceptable in many people's eyes to blame, ridicule and belittle obese people.
2. Biological Explanations
   • Calories in/out Until very recently all nutritionists, doctors and scientists believed that overweight was a simple matter of eating more calories than you burned. Most still do. The theory says that if you eat fewer calories or exercise more, weight loss is inevitable. If you drink water instead of a daily can of Coke (155 calories), you'll lose one pound every 23 days because you'll consume over 3,500 fewer calories (the number in one pound). Although people do lose weight when they eat fewer calories, it turns out that the equation doesn't work perfectly. Some people gain or lose more than others do on the same diets, even when scientists make other factors (like exercise, height and current weight) equal. And people may lose more weight eating 1500 calories a day than 900. Calories count, but there must be other factors at work.
   • Level of activity We've come a very long way from the hunter-gathering people we evolved from. They spent most of their waking hours on the move. We spend countless hours indoors and looking at TV, phone and computer screens. We're predisposed to "energy conservation." We'll circle a parking lot for ten minutes to get a parking space nearer the store. Even a virtuous half-hour walk or hour-long workout can't compare to the daily physical labor required to survive just a few hundred years ago. Some of us manage, but most of us can't maintain a healthy weight with an inactive lifestyle.
   • Genes When you see a fat family, it's easy to conclude that they're all fat because they eat too much together. But it's harder to explain why adopted kids have BMIs more like their birth parents' than those of the adoptive parents who fed them. One study used pairs of male identical twins to see how genes affected weight gain. Experimenters fed each man 1,000 calories more than they needed to maintain their weight. Each of them should have gained exactly 23 pounds. But their gains ranged from 9.5 to 29 pounds. And each man gained the same amount as his twin. Our genes seem to determine what our bodies do with extra calories. Other studies have found genes that are linked with the desire to eat more high-calories foods.
   • Carbohydrates and blood sugar levels Dr. Atkins was seen as a dangerous quack when he came out with his steak-and-cheeseburger low-carbohydrate diet in 1972. But his reasoning doesn't sound so crazy now. He said that carbohydrates (particularly quickly digestible white sugar, corn syrup sweeteners, white flour and foods made with them) send blood sugar and insulin levels soaring. Then these levels crashed, and you were hungry again. Eating proteins and fats doesn't trigger this cycle. There's been lots of research since then, including the development of the Glycemic Index, which rates foods by their effect on blood sugar levels. Eating carbohydrates that take longer to break down into sugar (whole grains, fruits and vegetables) is linked with lower rates of overweight, obesity and diabetes. But our modern diet contains far more refined carbs.
   • Evolutionary adaptations to famine and infectious disease Humans evolved in a world in which the challenge was finding or growing enough food. Resisting food was not an issue. So when there's plenty of food, our metabolisms (how many calories we burn to keep our bodies going independent of exercise) speed up, and we use lots of calories. We also store extra calories as fat to be burned in leaner times. When food is scarce, our metabolisms slow down to conserve calories. This worked well until our food supply exceeded our needs. Now when we go on diets our bodies think there's a famine and burn fewer calories. That makes it even harder to lose weight. Another evolutionary theory relates to tuberculosis (TB). TB has killed over a billion people through history. It's still the #2 cause of death from infection. Experts think that in recent centuries nearly everyone in Europe was infected. Some theorize that people prone to abdominal fat had better immunity. They were infected but they didn't get sick. So they were more apt to survive and have children who were also prone to abdominal fat.
   • Disorders of the body's hunger and fullness feedback system Scientists have discovered that the stomach and gut secrete hormones and chemicals that powerfully turn appetite on and off. When your stomach is empty, it releases the hormone gherin, and you get hungry. When you're full, another hormone, leptin, signals your brain that you've had enough. These hormones don't work normally in some obese people. They never feel full for long. If they lose weight it gets worse, and their bodies' signals tell them they're starving. Variations of the leptin receptor gene are linked to obesity and preference for sweets.
   • Differences in gut bacteria Several studies have analyzed the bacteria in the guts of people who are obese, normal weight or who've had gastric bypass surgery. They've found big differences that may affect how efficiently they use calories. Each of us has our own community of trillions of gut bacteria called a microbiome. Twins' gut microbiomes are similar. This could be one way that genes affect weight.It may also help explain how gastric bypass causes dramatic weight loss.
3. Environmental explanations
   • Availability, variety, and marketing of food Yale professor Dr. Kelly D. Brownell studies environmental causes of obesity. He calls our society a toxic food environment. There is cheap and fattening food everywhere we look and go. Food ads aimed at kids work; parents buy them cereals so sweet they're really candy with milk on top. Many studies have shown links between our changing food environment and rising rates of obesity.
   • Liquid calories Nutrition specialist Barry Popkin says that soft drinks are linked to diabetes and obesity like tobacco is to lung cancer. Our bodies don't register the calories in liquids. So we don't eat any less whether we have a big glass of water or Coke. Sweeteners made from corn are cheap. We've gotten so hooked on sweetened drinks that we consume more sugar from beverages than from desserts.
   • Environmental cues that affect how much we eat Scientist Brian Wansink runs a food lab at Cornell University that tests factors that influence eating. He's famous for rigging a table with soup bowls that refilled as quickly as people ate. Volunteers thought they were testing recipes. Those with normal bowls ate about 9 ounces of soup. Those with bottomless bowls just kept eating until the experiment was stopped after 20 minutes. They'd eaten an average of 15 ounces. Several had eaten more than a quart of soup. This and many other experiments show that we tend to eat what's put in front of us. We judge how much to eat by how big the portion looks. Since 4 ounces of fish looks bigger on a small plate, people are more apt to feel they've had enough if it's served that way than on a larger plate. If you "pad" a hamburger with lettuce, tomato and onions, people will say they feel as full as if they got a much higher-calorie cheeseburger the same size. A smoothie that's been whipped an extra minute to increase volume may have half the calories of a less-blended one, but will satisfy the drinker equally. The old saying could be revised to say that our stomachs are as big as our eyes.
   • Sleeping less Just 100 years ago, we slept several hours a day more than we do now. Many studies have found that people who scrimp on sleep are hungrier in the day. They crave carbohydrates, and they're more likely to have weight problems. Even babies who sleep less than 12 hours a day until they're age 2 are more apt to be overweight by age 3.
   • Social influences A 2007 study of over 12,000 people in the New England Journal of Medicine found that obesity was socially contagious over a 32-year period. If your spouse got obese over time, your chances of becoming obese were 37% greater. If your friend grew obese, your odds of following suit were up 57%. We all know we're more likely to order dessert if someone else does, especially someone close to us. We're hardwired to imitate those we are close to.

If even half of these potential causes are truly at work, for most of us the chips are stacked against losing weight. So how bad is it to be overweight or obese?

What are the health consequences of being overweight or obese?

An overwhelming number of studies have found that being overweight or obese increases your risk of diabetes, heart disease, stroke, arthritis, many forms of cancer and other diseases. The message has been hammered home in the media, via scientific studies and public health initiatives to the point that few want to hear another word about the obesity epidemic. In 2004, the Centers for Disease Control and Prevention (CDC) announced that 400,000 Americans were dying every year due to obesity.

But it may not be an epidemic. Rates of obesity have leveled out in recent years. And when serious statistical errors were uncovered, the CDC was forced to retract its claim. They reworked their numbers, and found that in the year 2000 being obese killed 112,000 people. Being underweight killed 34,000. Being overweight "was not associated with excess mortality." This better matched the results of other studies that have found a U-shaped curve in large populations. A 2009 study of over 11,000 Canadians found the same thing. Overweight (but not obese) people had a 17% lower death rate over 12 years than people of normal weight. (Underweight people died at a much higher rate.) Only the very thin and very fat have higher mortality rates.

What does this mean? Some experts think our definition of overweight is set too low and that we should stop telling otherwise healthy people with BMIs between 25 and 30 to lose weight. Others point out that not all fat is equal. Belly fat is a much bigger risk factor for diabetes and heart disease than fat on the hips and thighs, for instance. The ratio of waist to hip size is a better predictor, especially in thinner people. Half of overweight people, and 1/3 to 1/4 of obese people have normal blood sugar and blood vessels.

It might be better to use more complex tools than the BMI. But that idea sets off alarm bells for many public health experts. They fear that if the word got out that being moderately overweight isn't always unhealthy, people would eat more and get even fatter. That is a legitimate fear, although it's not certain that that would be the result.

Since few of us are able to lose weight and keep it off, it may be more productive to focus on preventing weight gain, and on other ways to improve health at any weight. Next week we'll discuss ways to do both.

Updated on 10/16/09 by J. Johnson (BA English composition, Northwestern University). Published on 11/09/09.

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