Causes of obesity

Your total daily calorie needs will depend on your basal metabolic rate, how much energy you need for physical activity, and how much energy you need in order to process food:

• Your basal metabolic rate is the amount of calories you need to keep up basic body functions such as your heartbeat and breathing. On average, it makes up about 70% of your total calorie needs. Your basal metabolic rate mainly depends on how much of your weight comes from muscle tissue. Because of this, men generally have a higher basal metabolic rate than women of a similar size do. Other factors play a role too, including your age, height, and genes.
• You also need energy for physical activities like walking, climbing stairs, doing physical labor or exercising. This makes up about 20% of your total calorie needs on average.
• The energy needed to process food (to digest, store and convert it) makes up the rest of your calorie needs – about 10% on average. But it's not clear how much of a role this plays when you do things like change your diet.

People who are physically active have an advantage because sports and other activities use up calories and build muscle tissue. Muscle tissue, in turn, increases your basal metabolic rate – in other words, the energy your body needs to keep up basic functions.

Many people think that they can lose enough weight if they only get enough exercise. That’s only partially true: Exercise does indeed use up calories, but only to a certain extent. It helps you lose weight and has other health benefits, but it’s usually not enough on its own (without changing your diet) if you want to lose a lot of weight.

We know that people are more likely to become obese if they regularly eat and drink too many calories. But the following factors can play a role, too:

• Hormonal imbalances: These include, for example, an untreated underactive thyroid (slow metabolism), polycystic ovary syndrome in women (can affect insulin levels), and low testosterone levels in men (leads to a breakdown of muscle tissue).
• Eating disorders: Some people have strong food cravings where they eat and never feel full. They are at greater risk of becoming obese. If certain diagnostic criteria are met, this is considered to be a psychological illness known as a binge-eating disorder.
• Genes: Some genes affect how hungry or full you feel. Others can indirectly influence your body weight – for example, by determining how much of the food is used up by the body.
• Medication: Certain medications, especially some that are used to treat diabetes and mental disorders, can make you put on weight. If medication causes weight gain, it is usually about 2 to 5 kilograms (4.4 to 11 pounds). According to current research, hormone-based birth control like the pill doesn’t cause any weight gain – even if many people think it does.
• Mental and physical illness: Obesity is sometimes linked to health problems like depression or chronic physical illnesses. Things like osteoarthritis can reduce physical activity due to pain in the joints. Stress and emotional problems are risk factors too.
• Quitting smoking: Quitting smoking is good for your health. People who quit smoking often gain weight, though. The amount of weight gain may vary quite a bit from person to person, but it’s about 5 kilograms (11 pounds) on average.

In addition to these known risk factors, experts are also looking into others – like a constant lack of sleep. But there's not enough research on that.

Very rarely, obesity is caused by an inherited or hormonal disorder. The condition known as Prader-Willi syndrome, for example, is a genetic disorder that causes constant hunger starting in childhood. It very often leads to obesity as a result. Cushing syndrome is a hormonal disorder in which the body produces too much cortisol. Cortisol is a natural steroid that is made in the adrenal cortex. Cushing syndrome may also develop if you take steroid tablets over a long period of time.

Your body generally tries to keep its metabolism in balance. The regulation of energy in the body is controlled by the central and autonomic nervous systems, hormones and various other processes. That's what makes us feel hungry or full, for instance.

The hormones that can trigger hunger include ghrelin, which is made in the digestive tract. One of the hormones that regulates when we feel full is leptin. Both of these hormones are made in fat tissue and are also known as adipokines (fat tissue hormones). Obesity can upset the balance of these hormones in the body, for example because leptin becomes less effective (leptin resistance).

There is also a theory that enjoying food is associated with a rewarding feeling in the brain. Interactions between hormones and nerve signals are thought to be responsible for that.

It is not exactly clear what effects different hormones and other chemical messengers have, and when they lead to weight gain. Many studies were done in animals, for example, so the results may be different in humans.

Some scientists believe that the main cause of obesity lies in the way we live nowadays, including

• our sedentary lifestyle: too much sitting (in front of screens), not enough daily physical activity, and not enough exercise to make up for that;
• our diet: food that has too many calories in it, such as fast food, fried foods, sugary foods, and sugary drinks like soft drinks or iced tea;
• societal influences: for example, the easy availability of lots of food and snacks everywhere, as well as larger portions and packages sold at snack bars, restaurants and supermarkets. Research has shown that people eat more when they're given larger servings or when they eat from larger plates. What we eat and how we eat is partly affected by factors we don’t even notice.

Many people believe that obesity is caused by eating too much and not getting enough exercise. That is often true, but not always. Various other factors play a role in weight gain, too. If you are obese and would like to lose weight, you can talk to your doctor about the possible causes and your own risk factors, and then discuss possible solutions together.