Why people become overweight - Harvard Health (2023)

Why people become overweight - Harvard Health (1)

Everyone knows some people who can eat ice cream, cake, and whatever else they want and still not gain weight. At the other extreme are people who seem to gain weight no matter how little they eat. Why? What are the causes of obesity? What allows one person to remain thin without effort but demands that another struggle to avoid gaining weight or regaining the pounds he or she has lost previously?

On a very simple level, your weight depends on the number of calories you consume, how many of those calories you store, and how many you burn up. But each of these factors is influenced by a combination of genes and environment. Both can affect your physiology (such as how fast you burn calories) as well as your behavior (the types of foods you choose to eat, for instance). The interplay between all these factors begins at the moment of your conception and continues throughout your life.

The calorie equation

The balance of calories stored and burned depends on your genetic makeup, your level of physical activity, and your resting energy expenditure (the number of calories your body burns while at rest). If you 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.

Genetic influences

To date, more than 400 different genes have been implicated in the causes of overweight or obesity, although only a handful appear to be major players. Genes contribute to the causes of obesity in many ways, by affecting appetite, satiety (the sense of fullness), metabolism, food cravings, body-fat distribution, and the tendency to use eating as a way to cope with stress.

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The strength of the genetic influence on weight disorders varies quite a bit from person to person. Research suggests that for some people, genes account for just 25% of the predisposition to be overweight, while for others the genetic influence is as high as 70% to 80%. Having a rough idea of how large a role genes play in your weight may be helpful in terms of treating your weight problems.

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 characteristics:

  • You have been overweight for much of your life.
  • One or both of your parents or several other blood relatives are significantly overweight. If both of 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-calorie diet for many months.

Genes are probably a lower contributor for you if you have most or all of the following characteristics:

  • 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 exercise program.
  • 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 great that 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 modest if your weight is normal and doesn't increase even when you regularly indulge in high-calorie foods and rarely exercise.

People with only a moderate genetic predisposition to be overweight have a good chance of losing weight on their own by eating fewer calories and getting more vigorous exercise more often. These people are more likely to be able to maintain this lower weight.

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What are thrifty genes?

When the prey escaped or the crops failed, how did our ancestors survive? Those who could store body fat to live off during the lean times lived, and those who couldn't, perished. This evolutionary adaptation 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 rather than a blessing. Not only is food readily available to us nearly around the clock, we don't even have to hunt or harvest it!

In contrast, people with a strong genetic predisposition to obesity may not be able to lose weight with the usual forms of diet and exercise therapy. Even if they lose weight, they are less likely to maintain the weight loss. For people with a very strong genetic predisposition, sheer willpower is ineffective in counteracting their tendency to be overweight. Typically, these people can maintain weight loss only under a doctor's guidance. They are also the most likely to require weight-loss drugs or surgery.

The prevalence of obesity among adults in the United States has been rising since the 1970s. Genes alone cannot possibly explain such a rapid rise. Although the genetic predisposition to be overweight varies widely from person to person, the rise in body mass index appears to be nearly universal, cutting across all demographic groups. These findings underscore the importance of changes in our environment that contribute to the epidemic of overweight and obesity.

Environmental causes of obesity

Genetic factors are the forces inside you that help you gain weight and stay overweight; environmental factors are the outside forces that contribute to these problems. They encompass anything in our environment that makes us more likely to eat too much or exercise too little. Taken together, experts think that environmental factors are the driving force for the causes of obesity and its dramatic rise.

Environmental influences come into play very early, even before you're born. Researchers sometimes call these in-utero exposures "fetal programming." Babies of mothers who smoked during pregnancy are more likely to become overweight than those whose mothers didn't smoke. The same is true for babies born to mothers who had diabetes. Researchers believe these conditions may somehow alter the growing baby's metabolism in ways that show up later in life.

After birth, babies who are breast-fed for more than three months are less likely to have obesity as adolescents compared with infants who are breast-fed for less than three months.

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Childhood habits often stick with people for the rest of their lives. Kids who drink sugary sodas and eat high-calorie, processed foods develop a taste for these products and continue eating them as adults, which tends to promote weight gain. Likewise, kids who watch television and play video games instead of being active may be programming themselves for a sedentary future.

Many features of modern life promote weight gain. In short, today's "obesogenic" environment encourages us to eat more and exercise less. And there's growing evidence that broader aspects of the way we live — such as how much we sleep, our stress levels, and other psychological factors — can affect weight as well.

The food factor as one of the causes of obesity

According to the Centers for Disease Control and Prevention (CDC), Americans are eating more calories on average than they did in the 1970s. Between 1971 and 2000, the average man added 168 calories to his daily fare, while the average woman added 335 calories a day. What's driving this trend? Experts say it's a combination of increased availability, bigger portions, and more high-calorie foods.

Practically everywhere we go — shopping centers, sports stadiums, movie theaters — food is readily available. You can buy snacks or meals at roadside rest stops, 24-hour convenience stores, even gyms and health clubs. Americans are spending far more on foods eaten out of the home: In 1970, we spent 27% of our food budget on away-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, a trend that has spilled over into many other foods, from cookies and popcorn to sandwiches and steaks. A typical serving of French fries from McDonald's contains three times more calories than when the franchise began. A single "super-sized" meal may contain 1,500–2,000 calories — all the calories that most people need for an entire day. And research shows that people will often eat what's in front of them, even if they're already full. Not surprisingly, we're also eating more high-calorie foods (especially salty snacks, soft drinks, and pizza), which are much more readily available than lower-calorie choices like salads and whole fruits. Fat isn't necessarily the problem; in fact, research shows that the fat content of our diet has actually gone down since the early 1980s. But many low-fat foods are very high in calories because they contain large amounts of sugar to improve their taste and palatability. In fact, many low-fat foods are actually higher in calories than foods that are not low fat.

The exercise equation

The government's current recommendations for exercise call for an hour of moderate to vigorous exercise a day. But fewer than 25% of Americans meet that goal.

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Our daily lives don't offer many opportunities for activity. Children don't exercise as much in school, often because of cutbacks in physical education classes. Many people drive to work and spend much of the day sitting at a computer terminal. Because we work long hours, we have trouble finding the time to go to the gym, play a sport, or exercise in other ways.

Instead of walking to local shops and toting shopping bags, we drive to one-stop megastores, where we park close to the entrance, wheel our purchases in a shopping cart, and drive home. The widespread use of vacuum cleaners, dishwashers, leaf blowers, and a host of other appliances takes nearly all the physical effort out of daily chores and can contribute as one of the causes of obesity.

The trouble with TV: Sedentary snacking

The average American watches about four hours of television per day, a habit that's been linked to overweight or obesity in a number of studies. Data from the National Health and Nutrition Examination Survey, a long-term study monitoring the health of American adults, revealed that people with overweight and obesity spend more time watching television and playing video games than people of normal weight. Watching television more than two hours a day also raises the risk of overweight in children, 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 other activities that burn more calories (watching TV burns only slightly more calories than sleeping, and less than other sedentary pursuits such as sewing or reading). But food advertisements also may play a 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 people to eat more calories, and particularly more calories from fat. In fact, a study that limited the amount of TV kids watched demonstrated that this practice helped them lose weight — but not because they became 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 promote weight gain. Stress is a common thread intertwining these factors. For example, these days it's commonplace to work long hours and take shorter or less frequent vacations. In many families, both parents work, which makes it harder to find time for families to shop, prepare, and eat healthy foods together. Round-the-clock TV news means we hear more frequent reports of child abductions and random violent acts. This does more than increase stress levels; it also makes parents more reluctant to allow children to ride their bikes to the park to play. Parents end up driving kids to play dates and structured activities, which means less activity for the kids and more stress for parents. Time pressures — whether for school, work, or family obligations — often lead people to eat 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 be another one of the causes of obesity. Neurological evidence indicates that the brain's biological clock — the pacemaker that controls numerous other daily rhythms in our bodies — may also help to regulate hunger and satiety signals. Ideally, these signals should keep our weight steady. They should prompt us to eat when our body fat falls below a certain level or when we need more body fat (during pregnancy, for example), and they should tell us when we feel satiated and should stop eating. Close connections between the brain's pacemaker and the appetite control center in the hypothalamus suggest that hunger and satiety are affected by temporal cues. Irregular eating patterns may disrupt the effectiveness of these cues in a way that promotes obesity.

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Similarly, research shows that the less you sleep, the more likely you are to gain weight. Lack of sufficient sleep tends to disrupt hormones that control hunger and appetite and could be another one of the causes of obesity. In a 2004 study of more than 1,000 volunteers, researchers found that people who slept less than eight hours a night had higher levels of body fat than those who slept more, and the people 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 appetite, as anyone who's ever gorged on cookies or potato chips when feeling anxious or sad can attest. Studies have demonstrated that some people eat more when affected by depression, anxiety, or other emotional disorders. In turn, overweight and obesity themselves can promote emotional disorders: If you repeatedly try to lose weight and fail, or if you succeed in losing weight only to gain it all back, the struggle can cause tremendous frustration over time, which can cause or worsen anxiety and depression. A cycle develops that leads to greater and greater obesity, associated with increasingly severe emotional difficulties.

To find weight loss solutions that can be tailored to your needs, buy the Harvard Special Health Report Lose Weight and Keep It Off.

FAQs

What is the main reason people are overweight? ›

Obesity is generally caused by eating too much and moving too little. If you consume high amounts of energy, particularly fat and sugars, but do not burn off the energy through exercise and physical activity, much of the surplus energy will be stored by the body as fat.

What causes obesity Harvard? ›

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.

Why being overweight is a health problem? ›

Obesity increases the risk of several debilitating, and deadly diseases, including diabetes, heart disease, and some cancers. It does this through a variety of pathways, some as straightforward as the mechanical stress of carrying extra pounds and some involving complex changes in hormones and metabolism.

What are 10 causes of obesity? ›

Risk factors for obesity

Potential risk factors for developing obesity and excess weight include: Family history. Unhealthy diet and regularly overeating. Lack of physical activity and sedentary lifestyle.

What are 6 foods that cause obesity? ›

Limit these foods and drinks:
  • Sugar-sweetened beverages (soda, fruit drinks, sports drinks)
  • Fruit juice (no more than a small amount per day)
  • Refined grains(white bread, white rice, white pasta) and sweets.
  • Potatoes (baked or fried)
  • Red meat (beef, pork, lamb) and processed meats (salami, ham, bacon, sausage)

What conditions cause weight gain? ›

Conditions That Can Cause Weight Gain
  • Hypothyroidism. 1/13. ...
  • Depression. 2/13. ...
  • Insomnia. 3/13. ...
  • Menopause. 4/13. ...
  • Cushing's Disease. 5/13. ...
  • Polycystic Ovarian Syndrome. 6/13. ...
  • Congestive Heart Failure. 7/13. ...
  • Sleep Apnea. 8/13.
6 Nov 2020

What are the two main factors that change your body weight? ›

Factors that affect weight
  • genetics.
  • cultural background.
  • medical conditions and disability.
  • mental ill-health – which can affect appetite and motivation to shop and cook healthy food or be active.
  • some medicines – which can increase appetite or slow the metabolism.
  • eating disorders.
  • drugs, tobacco or alcohol.
29 Jul 2021

Can overweight people be healthy? ›

While being overweight is a precursor to obesity and, like obesity, can increase the risk of diabetes, heart attack and stroke, it's also possible to be overweight and still healthy, especially if you're free from chronic diseases like hypertension or diabetes.

Who is most affected by obesity? ›

Obesity affects some groups more than others

Non-Hispanic Black adults (49.9%) had the highest age-adjusted prevalence of obesity, followed by Hispanic adults (45.6%), non-Hispanic White adults (41.4%) and non-Hispanic Asian adults (16.1%).

What are the risk factors of obesity? ›

Overweight and obesity are caused by many factors including behaviors like eating patterns, lack of sleep or physical activity, and some medicines, as well as genetics and family history.

What are solutions to obesity? ›

Common treatments for overweight and obesity include losing weight through healthy eating, being more physically active, and making other changes to your usual habits. Weight-management programs may help some people lose weight or keep from regaining lost weight.

How can we reduce obesity? ›

Choosing healthier foods (whole grains, fruits and vegetables, healthy fats and protein sources) and beverages. Limiting unhealthy foods (refined grains and sweets, potatoes, red meat, processed meat) and beverages (sugary drinks) Increasing physical activity. Limiting television time, screen time, and other “sit time”

Why I am gaining weight so fast? ›

Poor sleep, sedentary activities, and eating too many processed or sugary foods are just some of the habits that may increase your risk of weight gain. Yet, a few simple steps — such as mindful eating, exercise, and focusing on whole foods — can help you reach your weight loss goals and improve your overall health.

Does rice make you gain weight? ›

In short, rice may lead to weight gain if it is eaten with a less nutritious diet, but it can help contribute to weight management if eaten as part of a well-balanced diet.

Does stress cause weight gain? ›

Stress could be one of the culprits. It plays a role in weight gain. While it can make you have less of an appetite at first, long-term "chronic" stress actually boosts your hunger.

Why I gain weight even I eat less? ›

There are some medical conditions that can drive weight gain and make it much harder to lose weight. These include hypothyroidism, polycystic ovarian syndrome (PCOS), and sleep apnea. Certain medications can also make weight loss harder — or even cause weight gain.

Why do I gain weight when I eat less? ›

This phenomenon is called “starvation mode,” and while your body isn't actually starving, it will naturally hold onto whatever calories it receives in an effort to help you maintain your energy balance.

What is the most important factor in losing weight? ›

An energy deficit is the most important factor in weight loss. A low-calorie diet with a low fat or carbohydrate content has been recommended; however, in some cases, a very-low-calorie diet is required for a short period.

What affects weight loss the most? ›

Physical activity and diet are often talked about hand-in-hand when people want to lose weight. The extent to which we're active throughout the day – and the types of activities we do – have a direct impact on the amount of calories our bodies burn.

How does sleep affect weight loss? ›

Overall, individuals who increased their sleep duration were able to reduce their caloric intake by an average of 270 kcal per day – which would translate to roughly 12 kg, or 26 lbs., of weight loss over three years if the effects were maintained over a long term.

Is obesity genetic or environmental? ›

Obesity is simply defined as having too much body fat for your particular weight or height. An average figure from the relevant research estimates that obesity is about 40% genetic and 60% due to environmental factors.

What is obesity What are its causes and risk factors? ›

Overweight and obesity are caused by many factors including behaviors like eating patterns, lack of sleep or physical activity, and some medicines, as well as genetics and family history.

How do genetics cause obesity? ›

The most commonly implicated gene is MC4R, which encodes the melanocortin 4 receptor. Changes in MC4R that diminish its function are found in a small fraction (<5%) of obese people in various ethnic groups. Affected children feel extremely hungry and become obese because of consistent overeating (hyperphagia).

How can we reduce obesity? ›

Choosing healthier foods (whole grains, fruits and vegetables, healthy fats and protein sources) and beverages. Limiting unhealthy foods (refined grains and sweets, potatoes, red meat, processed meat) and beverages (sugary drinks) Increasing physical activity. Limiting television time, screen time, and other “sit time”

Can obesity be cured? ›

Reducing calories and practicing healthier eating habits are vital to overcoming obesity. Although you may lose weight quickly at first, steady weight loss over the long term is considered the safest way to lose weight and the best way to keep it off permanently.

Is obesity a choice? ›

When it comes to obesity, multiple factors are at play, many of which are beyond your control, including genetics, childhood habits, medical conditions, and hormones. Though becoming overweight or developing obesity may not be a choice and shedding excess weight may be difficult, you can lose weight if you choose to.

What is the fat gene called? ›

Fat mass and obesity-associated protein also known as alpha-ketoglutarate-dependent dioxygenase FTO is an enzyme that in humans is encoded by the FTO gene located on chromosome 16.

Who is most at risk for obesity? ›

People ages 60 and older are more likely to be obese than younger adults, according to the most recent data from the National Health and Nutrition Examination Survey. And the problem also affects children. Approximately 20%, of U.S. children and adolescents ages 2 to 19 are obese.

Who suffers from obesity? ›

Obesity affects some groups more than others

Non-Hispanic Black adults (49.9%) had the highest age-adjusted prevalence of obesity, followed by Hispanic adults (45.6%), non-Hispanic White adults (41.4%) and non-Hispanic Asian adults (16.1%).

Why do I gain weight so easily? ›

Poor sleep, sedentary activities, and eating too many processed or sugary foods are just some of the habits that may increase your risk of weight gain. Yet, a few simple steps — such as mindful eating, exercise, and focusing on whole foods — can help you reach your weight loss goals and improve your overall health.

Why do I gain weight when I eat less? ›

This phenomenon is called “starvation mode,” and while your body isn't actually starving, it will naturally hold onto whatever calories it receives in an effort to help you maintain your energy balance.

Why can't some people lose weight? ›

If you have a slow metabolism, your genes may be to blame. Or you may not have enough lean muscle mass. People with lean, muscular bodies burn more calories than people with a higher percentage of body fat.

Are banana good for weight loss? ›

Bananas are good for weight loss because they have fiber, which slows digestion and keeps you full. Research has found that boosting fiber intake can reduce the risk of weight gain by up to 30%. Eat up to one banana a day as part of a healthy diet for weight loss.

How can I lose weight naturally? ›

12 tips to help you lose weight
  1. Do not skip breakfast. Skipping breakfast will not help you lose weight. ...
  2. Eat regular meals. ...
  3. Eat plenty of fruit and veg. ...
  4. Get more active. ...
  5. Drink plenty of water. ...
  6. Eat high fibre foods. ...
  7. Read food labels. ...
  8. Use a smaller plate.

Where is obesity most common? ›

Global Obesity Levels
Global RankCountry% of Adult Population That Is Obese
1Nauru61.0%
2Cook Islands55.9%
3Palau55.3%
4Marshall Islands52.9%
125 more rows
27 Mar 2020

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