If you are overweight and have a lot of body fat, you may be obese. Obesity is a serious disease that affects 31% of the adult American population¹. This common illness can lead to serious health problems including heart disease, stroke, cancer, depression, and type 2 diabetes. It is the second-leading cause of preventable death in the US².
While the rate of obesity is exceptionally high, it is a preventable condition. By learning as much as you can about the disease, you can avoid excess weight gain and health issues. Let's take a closer look at what causes obesity.
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The most common way to identify obesity is to measure your body mass index (BMI).
To work out your BMI, you need to know two measurements:
Your weight in pounds
Your height in inches
The formula to calculate your BMI is:
Your weight (in pounds)/height (in inches squared) x 703
To interpret your BMI, you can refer to this table:
Under 18.5: underweight
5 – 24.9: healthy weight
0 – 29.9: overweight
30 and above: obese
It's important to remember that BMI is calculated differently³ for children and teenagers.
Having a high BMI is not enough to diagnose obesity, but it suggests that you need to speak to your doctor.
Your doctor will diagnose obesity by reviewing several other factors, including your medical history and your waist circumference. They will conduct a general physical exam and check for any other medical problems. The information gathered can help your doctor to suggest an effective course of treatment.
Obesity usually occurs when the amount of calories you consume through food and drink is higher than the calories your body burns. This is known as a calorie surplus.
Your body is constantly burning calories, but some activities burn more than others. For example, you burn more calories when you are walking than when you are sitting on the sofa. When your body is taking in more calories than it burns, it stores excess fat. Over time, this may lead to weight gain and health problems.
So why might you be consuming more calories than you are burning? And what other factors lead to obesity?
An imbalanced diet that is high in fat and sugar is the leading cause of obesity. This kind of unhealthy diet usually includes:
Processed and fast foods
Consuming these foods in limited amounts doesn’t usually lead to obesity, but excessive consumption over time is likely to result in weight gain and health problems.
Eating out frequently or ordering takeout food increases your risk of obesity. Restaurant food tends to be high in fat and sugar, and you might eat a larger portion than you need.
While the quality of food is important, so is the quantity. People tend to overeat when they:
Eat while distracted (for example, when working or watching TV)
Use food to make them feel better
Drink high-calorie beverages (liquid calories don't create the same feeling of fullness as solid calories do)
Even if you only eat healthy foods, overeating can still lead to a high calorie intake and obesity.
Lack of exercise
Your body is burning calories constantly, but the rate changes depending on your activity levels. Without exercise, you are likely to burn between 250 and 500 calories per day. Since most people consume around 2,000 calories each day, this is hardly enough to maintain a healthy weight and will result in a calorie surplus.
Having a sedentary lifestyle is a common cause of obesity. Technology has made it possible to work, study, shop, and even date without leaving the house. According to the World Health Organization⁴, between 60% and 85% of people lead a sedentary lifestyle.
To maintain a healthy weight, you need to do moderate-intensity aerobic exercise for at least 150 minutes per week⁵. There’s no need to do all this exercise at once — you can break it down into shorter sessions on a daily basis.
Moderate-intensity aerobic exercise might include dancing, cycling, jogging, or even just a brisk walk. However, if you are obese and you want to lose weight, you would need to be even more active than this.
Some people with obesity have genes that make it more likely that they will become overweight. One such gene is the fat mass and obesity-associated gene (FTO)⁶, which is carried by about 43% of the population⁷.
FTO comes in two forms — high obesity risk and low obesity risk. A carrier inherits two copies of FTO, one from each parent. If you inherit two high-risk variants, the risk of obesity is high.
In rare cases, obesity can be caused by changes in the MC4R gene, which causes the person to feel extremely hungry and eat too much food.
A single gene is rarely responsible for obesity. One study⁸ showed that more than 52 genes are associated with the condition, but their effect is rather small.
If a person is more at risk of obesity because of their genes, it doesn't mean that they can't avoid it. Healthy lifestyle choices can counteract gene-related obesity risks.
Several medical conditions can result in excess weight and obesity.
Hypothyroidism — an underactive thyroid interferes with your metabolism
Cushing's syndrome — causes high cortisol production that increases appetite
Polycystic ovary syndrome (PCOS) — affects the way ovaries work and leads to weight gain
Certain medications that treat epilepsy, diabetes, and mental illnesses can contribute to weight gain.
If you are leading a healthy lifestyle but gaining weight anyway, you should consult your doctor. They can diagnose any underlying medical conditions that are causing your weight gain. If they find that your medication is causing weight gain, they may be able to prescribe an alternative.
Lifestyle and environmental factors
Other causes of obesity include:
Lack of sleep
Obesity can also become more of a problem with age. As you get older, your metabolism changes. Older people don't need to consume as many calories as younger people do. Meanwhile, they tend to become less active.
Even if you are more likely to become obese because of your genetics, good lifestyle choices can help you to maintain a healthy weight. These healthy lifestyle strategies include:
Eating a healthy diet
Keeping active and exercising regularly
Scheduling regular check-ups with your doctor
Monitoring your weight
Getting enough sleep
Practicing mindfulness and relaxation techniques to counter stress
It is possible to lose weight if you are obese and your doctor will be able to advise you on how to do this. However, you should always try to prevent obesity from occurring in the first place by making sensible choices.
Obesity is a common disease that leads to serious health problems. Despite being preventable and treatable, the condition continues to affect billions of people worldwide.
Understanding what causes obesity can help you to make sensible lifestyle choices and avoid common risk factors. Your main goal should be to maintain a healthy calorie intake that provides your body with the energy it needs to keep active. Consuming too many calories and living a sedentary lifestyle will significantly increase your risk of becoming obese.
If you believe you already have a healthy lifestyle but you are still gaining weight, it’s time to speak to your doctor as there may be other factors to consider.
BMI percentile calculator for child and teen | Centers for Disease Control and Prevention
Sedentary lifestyle: A global public health problem | World Health Organization
How much physical activity do adults need? | Centers for Disease Control and Prevention