The primary cause of obesity in most people is eating more calories than are used. Genetic factors also play a major role in determining a person's tendency to become obese. Inherited genes make some people much more likely to gain weight and make it far harder for them to lose it. Environmental factors such as conditions during pregnancy, food culture, access to healthy foods such as fresh produce, education about nutrition and health, stress, and lack of sleep are also significant factors in the development of obesity. Some health conditions and medications promote weight gain.
It is important to note that while science is good at finding correlations, or apparent relationships, between factors and health conditions, correlation does not prove that the factor causes the condition. We know that behavior, genetics, and environmental factors can influence a person’s risk for developing obesity, but it is difficult to know how influential any one factor is on an individual’s weight.
Age is an important factor for weight gain in many people. Although people of any age — even children — can become obese, weight gain becomes harder to avoid as we age, our hormone levels change, and our lives become less active.
The single most important factor in obesity is consuming more calories than you burn off. Eating a high-sugar, high-fat diet, snacking, and drinking high-calorie beverages such as soft drinks and some coffee drinks keeps calorie intake high. A sedentary lifestyle — spending most of your time sitting, driving, or lying down with little physical activity or exercise — burns off very few of those calories.
Getting too little sleep has been identified as a factor in weight gain.
Stopping smoking is another behavior that can cause weight gain. A gain of four to 10 pounds within six months of quitting is common, but some people gain three times that amount. Quitting smoking lowers the risk for many serious diseases, so it is still better for your health to stop smoking.
In any individual, it can be hard to separate genetic factors that contribute to obesity from home food culture and sedentary habits learned as a child from family members. Scientists have identified more than 400 genes believed to influence weight. Inheritance helps determine your appetite, your metabolism (how quickly you burn calories), food cravings, and how much food it takes to make you feel full. About 85 percent of people have “thrifty” genes that are geared to store fat and conserve energy. While thrifty genes likely helped our ancestors survive during times of famine, they promote obesity in the modern world. Researchers think that genetic factors predispose obesity more in some people than others – in some, genes make up 25 percent of the risk, while in others inheritance may contribute 70 or 80 percent of what leads to obesity.
Sex and ethnic background also influence obesity. Women are more likely to be obese than men, according to the U.S. National Institute of Diabetes and Digestive and Kidney Diseases. In the U.S., 48 percent of non-Hispanic Black adults, 42 percent of Hispanic adults, and 36 percent of non-Hispanic white adults are considered obese.
Even in the womb, environmental factors influence later obesity. Studies found that children born to mothers who had diabetes or smoked during pregnancy are more likely to become obese as adults. Other research suggests that babies who are breastfed for less than three months are more likely to develop obesity as adolescents than babies who received more than three months of breastfeeding.
The family and culture in which a person grows up often set the stage for how they eat and pass their time as adults. If your family consumed a lot of sugary beverages and high-fat food, watched a lot of television, and was not regularly active, you are likely to develop similar habits that are hard to break later in life.
Socioeconomic factors play many roles in influencing obesity. It is difficult to eat a nutritious and balanced diet without access to a grocery store with fresh produce and other healthy choices. Fresh foods can be more expensive than processed foods and can take longer to cook. Without education about nutrition or skills in meal planning and cooking, it can be daunting to choose and prepare foods that support good health and weight loss goals. It is difficult to get exercise if there are no sidewalks or parks in your neighborhood, if you can’t afford a gym membership (or if your gym has closed temporarily due to a health scare like the COVID-19 pandemic), or if it’s dangerous to go for a walk or ride a bike where you live.
Stress can be a factor in weight gain. Hormones released in people with chronic stress have been shown to stimulate hunger and encourage the choice of high-calorie foods, which act on the brain to lessen the feeling of stress. This cycle, known as the hypothalamic-pituitary-adrenal axis, can lead to eating as a way to cope with stress.
Marketing and advertising can make it confusing to figure out which foods are actually healthy. For instance, many products advertised as low-fat contain extra sugar that makes them even higher in calories than the full-fat version. And many products promoted by trendy diets promise weight loss and other benefits they do not provide. Meanwhile, sugary soft drinks, fast food, and high-calorie snacks are always convenient and inexpensive.
Some researchers refer to the modern Western lifestyle as “obesogenic” – tending to cause obesity. Convenience foods, high-calorie soft drinks, and sedentary habits all promote weight gain. As more countries become Westernized, obesity rates around the world have risen.
Weight gain is a common symptom of several chronic health conditions. Your doctor may evaluate you for one or more of these conditions if you show other symptoms associated with one of them.
Some medications, including those taken long-term for serious chronic health conditions, can cause weight gain as a side effect.