What Causes Obesity?

Why Do People Gain Weight?

Given that the prevalence of obesity has been increasing at an alarming rate (in all age groups, and despite race, smoking, and education status) (Flegal, 2016), several theories about the causes of weight gain have been developed.

Set-Point Theory

The set-point theory is one of the most accepted and well-known theories of obesity. Many studies have shown that body weight is maintained in a stable range (the “set-point”) during long periods despite the number of calories you consume (energy intake) and expend (amount of energy required for body functions). When someone decides to diet by decreasing the amount of food ingested, this creates energy restriction. When the body detects that there is a reduction in energy intake, it reduces its energy expenditures if the energy restriction persists. The body does its best to keep its usual weight. As soon as the energy–balance is restored and is detected to be adequate for the new (and lower) body weight, energy expenditure increases (Farias, 2011; Weinsier, 2001). The food you ingest, energy expenditure, and the way nutrients are stored in the body (e.g., fat and sugar deposits) determines your energy balance. To maintain a stable body weight, you should match your diet (fuel supply) with your body’s energy requirements (Lopes, 2000). Your current environment also promotes excessive consumption of energy-dense foods, high in fat and sugar. These foods are usually affordable, accessible, and available in large portions. However, long-term exposure to these high-calorie diets might contribute to a set-point deviation and lead to a new steady-state weight range (Farias, 2011; Martinez, 2000; Wright, 2012).

The Predation-Release Hypothesis

The predation-release hypothesis suggests that random gene changes have occurred and favor genes that predispose to obesity. Each gene is like a recipe that gives our bodies specific instructions; each person owns a unique set of genes. However, identical twins share the same genetic code; this has allowed investigators to determine that there is a significant genetic contribution to metabolic efficiency and regulation of body weight. Some genes regulate satiety (the sensation of fullness), food intake, and regulate energy expenditure and thermogenesis (how much the body is able to burn at rest and with activity). Function of these genes might explain why some individuals have a genetic susceptibility to gain weight in the presence of a high-calorie diet or reduced daily physical activity (Martinez, 2000; Stanford, 2016).

Thrifty-Gene Hypothesis

According to the thrifty-gene hypothesis, evolution has favored weight retention; it protects our bodies against times of food scarcity and starvation. Our bodies have adapted more to protect us against weight loss than weight gain (Farias, 2011; Stanford, 2016).

Ethnic-Shift Hypothesis

The ethnic-shift hypothesis has shown that African-American women use energy more efficiently than do Caucasian women. When comparing metabolic differences among ethnic groups, studies suggest that African-Americans may regulate weight at a lower “set-point” and be at higher risk to regain lost weight (Farias, 2011; Weinsier, 2001; Martinez, 2000; Stanford, 2016).

Sedentary-Lifestyle Hypothesis

Weight gain is usually associated with a level of energy intake that is greater than energy expenditure. This energy intake relies on appetite regulation, which determines the quantity and quality of food eaten (Bayon, 2014). Low levels of physical activity decrease energy expenditure; as a result, sedentary behaviors contribute to weight gain. Current technological advances have made our lives easier. Even though these advances have made many of us more efficient, they have predisposed us to become more sedentary and consequently, to expend fewer calories. People who exercise more have a higher energy expenditure. This gives them a better capacity to regulate their energy intake while maintaining a lower weight than individuals who engage in lower levels of physical activity (Farias, 2011; Martinez, 2000; Stanford, 2016; Wright, 2012).

Drug-Induced Weight-Gain Hypothesis

Drugs, especially psychotropic medications (e.g., lithium, atypical antipsychotics), diabetic treatments, anti-hypertensives (e.g., beta-blockers such as metoprolol), steroid hormones, contraceptives, and antihistamines are associated with weight gain (Wright, 2012). For example, use of antipsychotics has been linked with significant weight gain within the first 10 weeks of use and is associated with increased appetite and an increased risk of obesity. Tricyclic antidepressants (TCAs) such as amitriptyline are also associated with a notable weight gain. Selective serotonin re-uptake inhibitors (SSRIs) such as paroxetine are used for depression and are also associated with weight gain. Anti-diabetic medications, including thiazolidinediones, sulfonylureas, glyburide, and insulin can induce significant weight gain. The weight gained from these medications might be small but the increasing prevalence of hypertension, diabetes, and depression could magnify their impact on increasing obesity rates (Dhurandhar, 2014)

Microbiome-Induced Obesity Hypothesis

Did you know that you have bacteria inside your body? Most of them are beneficial because they help to break down nutrients and synthetize vitamins. The adult intestine contains approximately 100 trillion microbes; this is called the gut microbiota, which is established at birth and is modified through infancy and later in life. Alterations in the microbiota early in life, such as exposure to candies or sweetened beverages at a very young age, can contribute to the development of obesity in adolescence and adulthood. Recently, microbial diversity in the gut (Bacteroidetes and Firmicutes) has gained the attention of the scientific community. Individuals with obesity seem to have reduced levels of Bacteroidetes, which coincides with studies done in obese mice. The diversity of bacteria in the gut regulates how the energy you obtained from food is used and how much fat is created in your body (Bäckhed, 2009). Studies are now trying to establish the role of the gut microbiota in the regulation of weight gain. Is it even possible that “poop pills” from a lean person could become the treatment for obesity? Investigators are trying to determine whether that could become one of the next steps in obesity management (Ducharme, 2016).

The Assortative-Mating Hypothesis

There is a genetic component to a high body-fat composition, and those with a genetic predisposition toward a higher body mass index (BMI) tend to reproduce at a higher rate. If those with stronger genetic predisposition for obesity reproduce at higher rates than do others, the population will become increasingly populated with individuals with obesity because the selection for genes that predispose to obesity will be favored (Stanford, 2016; Dhurandhar, 2014).

Fetal-Programming Hypothesis/Intrauterine Factors

The intrauterine environment influences the development of the growing fetus. Moreover, the offspring of nutrient-deprived mothers prefer high-fat diets and are more sedentary. 

Pre-pregnancy maternal obesity and excessive weight gain during pregnancy are both associated with increased birth weight and a higher rate of macrosomia in the offspring, which means that the newborn is significantly larger than average, with a weight greater than 8 pounds or 4,000 grams regardless of his or her gestational age. Children born to mothers before bariatric (weight-loss) surgery are more likely to have obesity than are children born to mothers after bariatric surgery. In addition, infants born either small or large for gestational age have an increased risk of obesity later in life, but minimizing gestational weight gain in pregnant women with obesity decreases their risk of having a large-for-gestational-age infant. All of this suggests that maternal over-nutrition or under-nutrition can influence outcomes of their offspring. Maternal weight gain during pregnancy might interact with genetic factors to render offspring more susceptible to developing obesity in young adulthood (Dhurandhar, 2014; Reddon, 2016).

Biological Factors

What Are Calories and How Do They Affect My Weight?

Body weight is determined by energy intake and energy expenditures. When there is an imbalance between energy intake and expenditure, a change in body weight arises. If energy expenditure exceeds that of intake, a decrease in body weight will occur (Dokken, 2007). However, it is not a simple equation, because many factors both internal (e.g., genetic, hormonal, age-related changes) and external (e.g., stress, sleep quality, social anxiety, low income) affect body weight and regulation. (Source: Potential Contributors to Obesity Infographic)

Energy intake is measured in calories. A calorie is a unit of measurement of energy, and it is defined as the amount of heat necessary to raise the temperature of one gram of water one degree centigrade. Calories are essential because the body needs them for energy to support normal metabolic functions, growth and repair of tissues, and physical activity. Everyone requires a specific number of calories. A method to calculate one’s caloric need is through the height in centimeters (H), weight in kilograms (W), age (A), sex, and physical activity of the individual by using the Harris-Benedict (HB) Equation for resting energy expenditure (REE):

Men REE = 66.5 + 13.8(W) + 5.0(H) – 6.8(A) 

Women REE = 655.1 + 9.6(W) + 1.9(H) – 4.7(A)

Let’s use the following example: You are a woman with a weight of 176 pounds, your height is 170 centimeters, and your age is 35. These values can be entered into the equation. But first, we need to transform pounds to kilograms. Because 1 kg is 2.2 pounds, you can divide 176 pounds by 2.2 and obtain 80 kg. Let’s enter these values into the equation:

Woman REE = 655.1 + 9.6(80) + 1.9(170) – 4.7(35) 

Woman REE = 655.1 + 768 + 323 – 164.5

Woman REE = 1,582 calories

This number of calories should suffice for someone who does light physical activities regularly (like walking slowly [i.e., while shopping, walking around the office], sitting at your computer, making the bed, eating, preparing food, and washing dishes).

If you get regular moderate-to-vigorous physical activity, your REE might need to be multiplied by a factor of 1.2 to 1.5, respectively, to account for extra calories that are needed during exercise. A factor of 1.2 represents an average amount of activity, whereas 1.5 would be a very high amount of activity. Let’s assume that you are doing an average amount of activity. In this case, your caloric needs would be 1,582 calories × 1.2 = 1,898 calories per day. Some examples of moderate physical activities include sweeping the floor, walking briskly, slow dancing, vacuuming, washing windows, and shooting a basketball. On the other hand, some examples of vigorous physical activities include running (at least 5 mph), swimming, shoveling, playing soccer, jumping rope, carrying heavy loads (e.g., bricks). Although the HB equation is accurate for predicting the energy requirements within a healthy population, it is not as reliable for critically ill individuals. The REE represents the amount of energy expended by a person at rest.

Basal metabolic rate (BMR) is more precisely defined as the REE measured just after awakening in the morning. In practice, REE and BMR differ by less than 10%, so the terms can be used interchangeably. Even though the HB equation might be useful as a reference, it is important to keep in mind that muscles burn more calories at rest than does fat, so someone with low percentage of body fat and a high percentage of muscle will have a higher BMR than someone with the same weight but more fat and less muscle.

There are three major sources of energy in food, called macronutrients: carbohydrate, protein, and fat. Each of them provides a certain amount of energy: carbohydrates render 4 calories per gram, proteins give 4 calories per gram, and fat gives 9 calories per gram. Fats are much richer than other macronutrients; that is why you should watch for fat that you are ingesting. All together, they add up to the total daily calorie intake. Gaining or losing weight is not merely about counting calories; there are other factors besides calories that affect body expenditure to gain or lose weight (Prosch, 2013; Picolo, 2016; Roza, 1984; UCLA, 2005; Schakel, 2009)

What Are Fat Cells and How Do They Work?

Fat deposits in our bodies are housed in fat cells. We store fat around vital organs, such as the intestines and heart (to act as a soft padding), to act as a fuel reserve and to conserve body heat. The body makes fat cells, mostly from the fat and carbohydrates in our diet. An excess of fat cells can lead to insulin resistance, diabetes, and obesity-related metabolic abnormalities (Britannica, 2015; Tchoukalova, 2010). Insulin resistance is a term used to explain dysfunction in the normal metabolism of sugars and how they are stored. Women have higher percentages of fat when compared to men. Fat levels higher than in the normal range (18%–24% for men; 25%–31% for women) increase the risk of insulin resistance; this explains why people with diabetes are usually obese (Kahn, 2000).

Why Do Some People Carry More Excess Weight Than Others?

Why can Mary eat everything she wants and appear so slim? Why is John hungry all the time and never seems to reach satiety? Why does Edward need to run 3 miles every day to maintain his weight? Why do you seem to gain weight just by breathing? Some people carry more excess weight than others. This is the result of several factors that can affect the stability of their energy balance, as an excess of energy is often stored as body fat. Every creature on Earth has evolved through millions of years to use energy efficiently, which means to use less energy to perform specific tasks. We are designed to minimize the reduction in body weight during long periods of starvation to ensure our survival (Dokken, 2007; Dhurandhar, 2014). However, many things have changed through time that seem to facilitate obesity in some groups of people. There is little doubt that the process of modernization and economic re-structuring in both developing and developed countries has led to consequences that have affected nutritional and physical activity patterns. The rise in the prevalence of obesity in populations whose family weight history has been relatively constant provides proof that current environmental factors have an important role in its increase (Martinez, 2000). Let’s explore each of the factors that can explain why some people carry more excess weight than do others.

Environmental Factors

Our current environment promotes over-eating and less physical activity. Only half of adults 18 years and older meet the 2008 federal physical activities guidelines for participation in leisure-time aerobic and muscle-strengthening activities (CDC, 2014). Roughly one-fourth of youth aged 12 to 15 years old are engaged in moderate-to-vigorous physical activity, including activities both within and outside of school, for at least 60 minutes each day (Fakhouri, 2014). Many cities have disproportionally grown and are no longer designed for their inhabitants to walk or to use their bikes to move freely from one place to another. If you live in a city with sidewalks and biking trails, you can consider yourself lucky.

Although most people would agree that technology has made our lives easier, it promotes sedentary behaviors. More than half of the world’s population now has internet access and two-thirds of the global population now use mobile phones. Nearly 2.8 billion people around the world currently use social media at least once per month, with more than 91% of them doing so via mobile devices (Digital in 2017: Global overview, 2017). Access to the internet, videogames, series and movies on-line has also had a health impact on many populations by reducing our calorie expenditure. You can even get food through internet applications, which facilitate purchases of generous high-calorie portions. But environmental influences do not stop there; fast-food restaurants, convenience stores, vending machines at schools and offices—all of these have an adverse impact on your weight. Besides this, today’s accelerated lifestyle encourages the consumption of highly processed foods, which are generally high in sugar, fat, and sodium. All of these products are heavily advertised through media, aiming at adults and children. Both pre-packaged and non-perishable, many of them seem convenient, practical, and even financially friendly. However, a diet that is low in fruits and vegetables along with a high intake of meat and highly processed foods has been associated with visceral adiposity, which refers to fat that is stored around vital organs (such as the liver, pancreas, and kidneys) (Wright, 2012; Dokken, 2007; Dhurandhar, 2014; Burgermaster, 2017).

What about the consumption of sugar-sweetened beverages (SSBs)? Their consumption will be more likely to cause an increase in body fat mass in any healthy lean adult and weight gain among obese adults. The American Heart Association (AHA) recommends that added sugars not exceed 100 to 150 kcal/day and has identified SSBs as the primary source of added sugars and calories, with no true nutrients, in our current diets.

How about drinking artificial sweetened beverages (ASBs)? Even though they are marketed as “healthy alternatives” due to their lack of calories, they stimulate sweet taste receptors, which could increase appetite, induce a preference for sweet-tasting foods, and result in over-consumption of solid foods due to awareness of the low-calorie content of ASBs. 

Systematic reviews of observational studies indicate that intake of ASBs is linked with increased BMI in both children and adults. Absence of evidence to support the role of ASBs in the prevention of weight gain and the lack of studies on other long-term effects on health strengthen the position that ASBs should not be promoted as part of a healthy diet. More recently, published data suggests that artificial sweeteners might contribute to the development of glucose intolerance by altering the composition and functions of gut microbiota (Burgermaster, 2017; Borges, 2017). Even though water is the healthiest option, breaking habits is a challenge for many; drinking water is the best long-term decision when trying to manage weight. However, it is imperative to keep in mind that weight gain is multi- factorial, and cutting out sweetened beverages is only one step toward a healthier life.

Anthropological studies also have shown that food choices of employees are influenced by the worksite. This is not limited to the meals that are available in the cafeterias, but also takes into account the support and help employees receive from their co-workers in meeting individual health and nutrition goals. In general, having healthier foods in the work environment is well accepted and appreciated. However, because work fatigue and overtime work also have been associated with weight gain, many aspects of the work environment should be re-assessed to avoid them (Devine, 2007).

Socioeconomic background influences the quality of the food we buy. Which one is more affordable: a pizza or a salad? It has been shown that living in lower income neighborhoods is associated with a lower-quality diet; moreover, there are large racial disparities in healthy food availability that could lead to a low- quality diet and to obesity (Franco, 2009). New Yorkers with a lower socioeconomic status and from minority sub-groups consume more SSBs and have higher BMIs (Burgermaster, 2017).

In addition, there is a thermal neutral zone (i.e., the range where the standard healthy adult can maintain normal body temperature without needing to use energy above and beyond normal basal metabolic rate [25°C–30°C; 77°F– 86°F]). Metabolic rate increases by 7% to 17% for every one-degree Celsius rise in core body temperature; this rate increases with heat exposure. Further, a higher core temperature decreases food intake and results in food choices with a lower energy density. With the prevalence of air conditioning we have reduced our exposure to this range; evidence suggests that being outside the thermal neutral zone might influence our energy balance. However, further studies are required to demonstrate that chronic heat exposure changes energy balance enough to influence our body weight (Dhurandhar, 2014). It is certainly unappealing to embrace the idea of life without the comfort of air conditioners.

Genetic Factors

But, you still might be thinking, why do some people struggle so much with their weight if they seem to eat healthy and try to be active? Always keep in mind that obesity is the result of multiple factors. However, there are indeed some people whose genetic lottery plays against them and predisposes them to gain weight easier than other people. The role of inheritance has long been recognized to affect intake and expenditure of the energy balance, given that some genes are involved in food intake control, the regulation of thermogenesis, and energy expenditure; these genes might specifically be affected by dietary intake and composition, which might explain why some people easily gain weight whenever they decide to “sin” and eat a slice of pizza (high in fat and carbohydrates). Individuals genetically predisposed to obesity appear to be less efficient at oxidizing fat intake, and when the consumption of carbohydrate is high, there is an increase in the formation of new fat cells in the body, which is known as de novo lipogenesis. These genetic predisposition mutations can be classified into two groups: monogenic mutations and polygenic mutations (Martinez, 2000).

Monogenic obesity comes from a single gene mutation and can be divided into syndromic or non-syndromic. Syndromic obesity refers to obesity that occurs with a distinct set of observable features (phenotypes) that result from genetic inheritance (genotype). Intellectual disability (formerly called mental retardation), dysmorphic features, and organ-specific developmental abnormalities are part of the clinical picture, along with obesity in this type of mutations. 

Non-syndromic forms are caused by mutations or structural variations in genes involved in the decrease of the hormone that suppresses appetite (leptin) and are mainly characterized by obesity caused by excessive hunger, called hyperphagic obesity. The people who are carriers of these mutations are extremely rare and lead to an early-onset extreme obesity (Dokken, 2007; Reddon, 2016).

Polygenic mutations imply that there are DNA variations in several genes, each with relatively small effect. Hundreds of these variants with small-to- modest effect plus other factors such as the socioeconomic environment and lifestyle, diet, sex and age, as well as alcohol consumption might contribute to the development of obesity (Reddon, 2016).

Socioeconomic environment and lifestyle

The proportion of variability in BMI attributable to genetic variation is increased among people born after the establishment of a modern “obesogenic” environment. Socioeconomic research indicates that higher educational status is associated with a decreased risk of obesity, but it has been suggested that BMI in adolescence is positively correlated with the level of education of the parents. Twin studies have shown that a high level of physical activity can substantially reduce the influence of genetic factors on BMI in both young and older adults. Therefore, genetic predisposition to obesity can be blunted in part through physical activity. Lifestyle factors and socioeconomic background can significantly modify the impact of obesity- predisposing gene variants (Reddon, 2016; Wardle, 2008).


Studies in Mediterranean, Asian, Caucasian, Hispanic, and Caribbean populations, suggest that a high daily energy intake, high-fat intake, or high saturated-fat intake (e.g., high intake of fried foods) can amplify the effect of certain genes that predispose to obesity risk in children, adolescents, and adults. Increased intake of SSBs has also been shown to increase the impact of obesity genes. Fat mass and obesity–associated gene (FTO gene) and its variants, Apolipoprotein A (APOA2 and APOA5), single nucleotide polymorphisms (SNPs), are all genetic mutations that have shown the strongest associations with obesity predisposition when combined with diet (Reddon, 2016; Livingstone, 2016; Voisin, 2015).

Sex and age

Sex-specific genetic effects on BMI have been observed in adolescents as well as in adults. Females are generally more likely to develop morbid obesity than males, which suggests that there might be some differences between men and women in the genetic factors that influence variation in BMI. Heritability of obesity also varies with age (BMI changes from adolescence to young adulthood, and from young adulthood to adulthood); obesity is a heritable trait. Mobility usually decreases with age, contributing to obesity, too (Reddon, 2016, Schousboe, 2003).

Alcohol consumption

Many people enjoy an occasional beer or glass of wine. But what is the impact of alcohol on your weight? Alcohol energy (7 kcal/g) can be a contributing factor to weight gain. If someone consumes a light-to-moderate amount of alcohol, there is no obesity risk. Heavy drinking and binge drinking do have an association with excess body weight. It has been found that one month of daily beer consumption (equivalent to 12g/day of alcohol for women and 24 g/day for men) does not result in significant increases in BMI compared to those who abstain. Biceps skin fold was the only anthropometric measurement that was increased in the participants who had beer. However, alcohol can influence BMI if there is the genetic predisposition. For example, the consumption of alcohol in Asian people increased the effect of SNPs in BMI. This pattern has also been shown with other mutations in African Americans (Reddon, 2016; Gregory Traversy, 2015).

Other Factors That Contribute to Obesity Psychological Factors

There are psychological and behavioral risk factors that predispose people to obesity. In those with depression, for each additional depressive symptom reported, the possibility of obesity onset increases four times. Depressed individuals eat to provide comfort or distraction from negative emotions (“comfort food”) in an attempt to raise serotonin levels—the “happy neurotransmitter”—in the brain; thus, they consume excessive amounts of carbohydrate-rich foods in an effort to regulate serotonin levels. Also, weight- control behaviors can increase the risk for binge eating, which in turn results in weight gain (Stice, Presnell, Shaw, & Rohde, 2005).

Maternal Age

Through the past decades, childbearing has been occurring later in life. The mean age at first birth has steeply increased from 21.4 in 1970 to 26.4 years in 2015 (Martin, 2017). These changes in maternal age at the time of delivery might have affected the susceptibility of offspring to increased adiposity and obesity. Evidence suggests that positive shifts in the maternal age distribution might influence the developing fetus and the likelihood of obesity in those offspring. Older mothers are more likely to develop co-morbidities and metabolic abnormalities during pregnancy, which can influence obesity, insulin resistance, and hypertension in their offspring (Dhurandhar, 2014).

Smoking Cessation

Smoking has been associated with a lower BMI and its cessation is associated with weight gain. The mechanisms through which this happens are not clear, but it is well known that nicotine (one of the main components in cigarettes) is a thermogenic agent and that smoking decreases appetite (Dhurandhar, 2014).

Sleep Debt

Do you sleep enough? Sleep is a restorative process that plays an important role in the balance of psychological, emotional, and physical health. Increasing evidence suggests that not obtaining enough sleep might be associated with adverse health effects such as obesity. Sleep debt tends to increase food intake, energy storage, and the likelihood of diabetes and heart disease. People who are sleep deprived tend to have increased evening concentrations of the stress hormone cortisol, which has been shown to suppress appetite and increase hunger by causing the decrease of leptin (the substance that suppresses appetite) and the increase of ghrelin (the hormone that stimulates appetite). Despite the physical activity, weight gain due to sleep debt affects children, adolescents, and adults. Some studies also have shown that people who sleep less than 7 hours per night have higher BMIs and are more likely to be obese when compared to people who sleep at least 7 hours (Bayon, 2014; Karine, 2004; Gangwisch, 2005; Dhurandhar, 2014).

Therefore, if you want to shed pounds, you should try to improve your sleep quality as well. The popular weight supplement Resurge promises to help you lose weight and sleep better. It contains melatonin as its first ingredient. Research shows that your body naturally produces melatonin, a hormone that tells your brain it’s time to go to sleep. Although more research is needed, current evidence suggests that melatonin can be useful in helping people get to sleep.  Resurge supplements might allow your body to absorb enough melatonin to maximize your natural melatonin production.

However, since the supplement industry is barely regulated, you might want to read some Resurge reviews before making any purchase of the supplement.


In this module, we pointed out the main factors that lead to obesity. Obesity depends on multiple factors that go far beyond caloric intake and expenditure. Evidence suggests the existence of genes that predispose to weight gain. Also, there are race, gender, age, psychological, and socioeconomic factors that influence this outcome. Healthy habits, such as regular physical activity, consistent sleep patterns, and a balanced diet that meets metabolic requirements, can counteract those influences that are beyond your control. The human body’s nature is to keep weight in a certain range, and as a matter of survival, it is still adapted to protect individuals against weight loss compared to weight gain. Just like the little genetic mutations that add together to create a modest effect in weight gain, the commitment to small changes of unhealthy habits can sum up to a huge impact toward an improved and longer life.

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