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(This article was first printed in the Special Health Report from Harvard Medical School “Weigh Less, Live Lo
to order, visit www.health.harvard.edu/WL.)
Everyone knows some people who can eat ice cream, cake, and whatever else they want and still not gain w
matter how little they eat. Why? What allows one person to remain thin without effort but demands that anothhas lost previously?
Special Health Reports
On a very simple level, your weight depends on the number of calories you consume, how many of those calis influenced by a combination of genes and environment. Both can affect your physiology (such as how fast
choose to eat, for instance). The interplay between all these factors begins at the moment of your conception
The calorie equation
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The balance of calories stored and burned depends on your genetic makeup, your level of physical
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activity, and your resting energy expenditure (the number of calories your body burns while at rest). Ifyou consistently burn all of the calories that you consume in the course of a day, you will maintain your
weight. If you consume more energy (calories) than you expend, you will gain weight.
Excess calories are stored throughout your body as fat. Your body stores this fat within specialized fat
cells (adipose tissue) — either by enlarging fat cells, which are always present in the body, or by
creating more of them. If you decrease your food intake and consume fewer calories than you burn up,
or if you exercise more and burn up more calories, your body will reduce some of your fat stores.
When this happens, fat cells shrink, along with your waistline. See All Titles » Genetic influences Browse By Category
To date, more than 400 different genes have been implicated in the development of overweight or obesity, alt
to obesity in many ways, by affecting appetite, satiety (the sense of fullness), metabolism, food cravings, bod
Science that studied more than 900 people showed that those who have two copies of a spe
BMI higher than 30. Researchers believe the gene variant affects the regulation of another gene involved in f
(including people with Western European ancestry, African Americans, and children), they found that about 1
In another 2006 study, published in the Proceedings of the National Academy of Sciences, researchers studipeople who were normal weight, overweight, or obese. They took fat samples from around the participants’ inactivity (known as gene expression) in the different samples. In overweight people, increased expression of tThese and related studies have helped researchers better understand how and why obesity occurs. They ma
The strength of the genetic influence on weight disorders varies quite a bit from person to person. Research
predisposition to be overweight, while for others the genetic influence is as high as 70% to 80%. Having a roin terms of treating your weight problems.
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How much of your weight depends on your genes?
Genes are probably a significant contributor to your obesity if you have most or all of the following
Medical DictionaryMedical Symptom Checker
You have been overweight for much of your life.
One or both of your parents or several other blood relatives are significantly overweight. If bothof your parents have obesity, your likelihood of developing obesity is as high as 80%. You can’t lose weight even when you increase your physical activity and stick to a low-caloriediet for many months.
Genes are probably a lower contributor for you if you have most or all of the following characteristics:
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You are strongly influenced by the availability of food.
You are moderately overweight, but you can lose weight when you follow a reasonable diet and
You regain lost weight during the holiday season, after changing your eating or exercise habits,
or at times when you experience psychological or social problems.
These circumstances suggest that you have a genetic predisposition to be heavy, but it’s not so greatthat you can’t overcome it with some effort.
At the other end of the spectrum, you can assume that your genetic predisposition to obesity is modestif your weight is normal and doesn’t increase even when you regularly indulge in high-calorie foodsand rarely exercise.
People with only a moderate genetic predisposition to be overweight have a good chance of losing weight onexercise more often. These people are more likely to be able to maintain this lower weight. What are thrifty genes?
When the prey escaped or the crops failed, how did our ancestors survive? Those who could storebody fat to live off during the lean times lived, and those who couldn’t, perished. This evolutionaryadaptation explains why most modern humans — about 85% of us — carry so-called thrifty genes,which help us conserve energy and store fat. Today, of course, these thrifty genes are a curse ratherthan a blessing. Not only is food readily available to us nearly around the clock, we don’t even have tohunt or harvest it!
In contrast, people with a strong genetic predisposition to obesity may not be able to lose weight with the usuare less likely to maintain the weight loss. For people with a very strong genetic predisposition, sheer willpowTypically, these people can maintain weight loss only under a doctor’s guidance. They are also the most likel
The prevalence of obesity among adults in the United States has been rising since the 1970s (see Figure 1). genetic predisposition to be overweight varies widely from person to person, the rise in body mass index appThese findings underscore the importance of changes in our environment that contribute to the epidemic of o
Figure 1: Trends in adult weight
Percent of adults ages 20–74* who were at a healthy weight, overweight, or obese†*Data are age-adjusted to the 2000 U.S. standard population.
†Healthy weight, body mass index (BMI) = 18.5–24; overweight, BMI = 25–29; obese, BMI ≥30.
Sources: National Health and Nutrition Examination Survey (National Center for Health Statistics); Cancer T2005). Environmental influences
Genetic factors are the forces inside you that help you gain weight and stay overweight; environmental factorencompass anything in our environment that makes us more likely to eat too much or exercise too little. Takeforce for the dramatic increase in obesity.
Environmental influences come into play very early, even before you’re born. Researchers sometimes call thsmoked during pregnancy are more likely to become overweight than those whose mothers didn’t smoke. ThResearchers believe these conditions may somehow alter the growing baby’s metabolism in ways that show
After birth, babies who are breast-fed for more than three months are less likely to have obesity as adolescenmonths.
Childhood habits often stick with people for the rest of their lives. Kids who drink sugary sodas and eat high-ccontinue eating them as adults, which tends to promote weight gain. Likewise, kids who watch television andthemselves for a sedentary future.
Many features of modern life promote weight gain. In short, today’s “obesogenic” environment encourages usbroader aspects of the way we live — such as how much we sleep, our stress levels, and other psychologica
The food factor
According to the Centers for Disease Control and Prevention (CDC), Americans are eating more calories on average man added 168 calories to his daily fare, while the average woman added 335 calories a day. Whatavailability, bigger portions, and more high-calorie foods.
Practically everywhere we go — shopping centers, sports stadiums, movie theaters — food is readily availabconvenience stores, even gyms and health clubs. Americans are spending far more on foods eaten out of theaway-from-home food; by 2006, that percentage had risen to 46%.
In the 1950s, fast-food restaurants offered one portion size. Today, portion sizes have ballooned (see Figure and popcorn to sandwiches and steaks. A typical serving of French fries from McDonald’s contains three timesized” meal may contain 1,500–2,000 calories — all the calories that most people need for an entire day. Andeven if they’re already full. Figure 2: Portion inflation
Portion sizes for foods commonly consumed outside the home have increased over the years, and many exConsider a typical fast-food meal that consists of a hamburger, French fries, and a soda. In 1955, consumemultiple portion sizes. The chart above shows how all these portions compare, adjusting for size inflation ov
Based on data from the Journal of the American Dietetic Association, February 2003, pp. 231–34.
Not surprisingly, we’re also eating more high-calorie foods (especially salty snacks, soft drinks, and pizza), wsalads and whole fruits. Fat isn’t necessarily the problem; in fact, research shows that the fat content of our dfoods are very high in calories because they contain large amounts of sugar to improve their taste and palatafoods that are not low fat.
In one year, the average American adult eats 40 pounds of white bread, 41 pounds of potatoes, 30 pounds oand washes it all down with 52 gallons of soda. True, vegetable consumption has risen by about 12% since 1(including chips, fries, and mashed potato), iceberg lettuce, and other low-nutrient choices. In all, the Department of Agriculture reports that food consumption rose by 8%, or about 140 pounds per persthan we need, and the food industry spends $30 billion a year to be sure it does not go to waste. It works: In in just three decades. The exercise equation
The government’s current recommendations for exercise call for an hour of moderate to vigorous exercise a hand, more people are exercising than in the late 1980s. According to a 2004 CDC report, the percentage ofwalking, golfing, or gardening) dropped from a high of 32% in 1989 to 25% in 2002.
Our daily lives don’t offer many opportunities for activity. Children don’t exercise as much in school, often becwork and spend much of the day sitting at a computer terminal. Because we work long hours, we have troubways.
Instead of walking to local shops and toting shopping bags, we drive to one-stop megastores, where we parkdrive home. The widespread use of vacuum cleaners, dishwashers, leaf blowers, and a host of other applian
The trouble with TV: Sedentary snacking
The average American watches about four hours of television per day, a habit that’s been linked tooverweight or obesity in a number of studies. Data from the National Health and Nutrition ExaminationSurvey, a long-term study monitoring the health of American adults, revealed that people withoverweight and obesity spend more time watching television and playing video games than people ofnormal weight. Watching television more than two hours a day also raises the risk of overweight inchildren, even in those as young as three years old.
Part of the problem may be that people are watching television instead of exercising or doing otheractivities that burn more calories (watching TV burns only slightly more calories than sleeping, andless than other sedentary pursuits such as sewing or reading). But food advertisements also may playa significant role. The average hour-long TV show features about 11 food and beverage commercials,which encourage people to eat. And studies show that eating food in front of the TV stimulates peopleto eat more calories, and particularly more calories from fat. In fact, a study that limited the amount ofTV kids watched demonstrated that this practice helped them lose weight — but not because theybecame more active when they weren’t watching TV. The difference was that the children ate more
snacks when they were watching television than when doing other activities, even sedentary ones. Stress and related issues
Obesity experts now believe that a number of different aspects of American society may conspire to promoteFor example, these days it’s commonplace to work long hours and take shorter or less frequent vacations. Infor families to shop, prepare, and eat healthy foods together. Round-the-clock TV news means we hear moredoes more than increase stress levels; it also makes parents more reluctant to allow children to ride their bikstructured activities, which means less activity for the kids and more stress for parents.Time pressures — wheat on the run and to sacrifice sleep, both of which can contribute to weight gain.
Some researchers also think that the very act of eating irregularly and on the run may contribute to obesity. Npacemaker that controls numerous other daily rhythms in our bodies — may also help to regulate hunger andThey should prompt us to eat when our body fat falls below a certain level or when we need more body fat (dsatiated and should stop eating. Close connections between the brain’s pacemaker and the appetite control affected by temporal cues. Irregular eating patterns may disrupt the effectiveness of these cues in a way that
Similarly, research shows that the less you sleep, the more likely you are to gain weight. Lack of sufficient sle2004 study of more than 1,000 volunteers, researchers found that people who slept less than eight hours a npeople who slept the fewest hours weighed the most.
Stress and lack of sleep are closely connected to psychological well-being, which can also affect diet and apfeeling anxious or sad can attest. Studies have demonstrated that some people eat more when affected by dand obesity themselves can promote emotional disorders: If you repeatedly try to lose weight and fail, or if yocause tremendous frustration over time, which can cause or worsen anxiety and depression. A cycle developincreasingly severe emotional difficulties. A widespread problem Other causes of obesity
Clearly, our responses to today’s obesity-promoting environment, in tandem with genetic influences, are the people, drug side effects, illnesses, and genetic disorders can also play a role. Drug side effects Several prescription drugs can cause weight gain as a side effect by increasing appetite or slowing metabolis variety of conditions to reduce inflammation); estrogen and progesterone (used in oral contraceptives); antico anticancer medications; and drugs such as olanzapine (Zyprexa), haloperidol (Haldol), lithium (Eskalith, Litho conditions.
Paradoxically, weight gain can also be a side effect of some drugs used to treat conditions that result from oband others); these are treatments for diabetes, which is common among people with weight disorders. Severtricyclic antidepressants such as imipramine (Tofranil) or desipramine (Norpramin, Pertofrane), monoamine o(SSRIs) such as paroxetine (Paxil), citalopram (Celexa), escitalopram (Lexapro), sertraline (Zoloft), fluvoxam
When used to treat depression, SSRIs may cause weight loss initially, but within six months of treatment theyother hand, sibutramine (Meridia), another drug that stimulates serotonin (like the SSRIs), is an appetite supppatients, SSRIs such as Prozac actually cause weight loss, making them useful as a therapy for weight disorbetween weight regulation and serotonin (a key chemical communicator in the brain and elsewhere in the bo
Illnesses that affect weight A few illnesses that are characterized by an imbalance or an abnormality in your endocrine glands can also a thyroid), polycystic ovarian syndrome, and certain unusual tumors of the pituitary gland, adrenal glands, or th are not responsible for weight gain. Most are extremely rare. Hypothyroidism, which is the most common, is thyroid hormone, while medically necessary, does not usually cause a significant weight reduction. Genetic disorders Obesity is also a symptom of some rare and complex disorders caused by genetic defects. These obesity sy additional medical problems. One such disorder is Prader-Willi syndrome, a form of obesity associated with m has been traced to abnormalities in a group of genes on chromosome 15. People with this disorder are unus disorder, Bardet-Biedl syndrome, is similar to Prader-Willi syndrome, but is caused by abnormalities in differe account for only a tiny fraction of all weight disorders. Weigh Less, Live Longer: Strategies for Successful Weight Loss
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