Obesity is a chronic disease that affects millions of people worldwide and has become a major health problem.

It has spread to almost every corner of the planet to the point that "[obesity] has grown to epidemic proportions," says the World Health Organization (WHO).

More than 5 million people die every year because of the diseases and harmful effects associated with obesity.

By 2035, more than half of the world's population, more than 4 billion people, could be obese or overweight, according to the World Obesity Atlas 2023.

What is obesity?

Obesity is a complex disease that can occur at any age and affects adults and children alike.

The World Health Organizations (WHO) defines obesity as "abnormal or excessive fat accumulation that presents a risk to health."

As of 2016, 1.9 billion or 39% of adults worldwide, were overweight and 650 million or 13% were living with obesity. The proportion of adults with obesity has nearly tripled since 1980, according to the WHO's Global Health Observatory. Children are not exempt — around 38 million children under 5 are overweight or obese, and more than 340 million children aged 5 to 19.

In essence, obesity occurs when a person eats an excess of calories that the body converts into fat. This imbalance between the calories you take in and the calories you burn, — for example through exercise— can be caused by many reasons, including social, lifestyle, psychological, biological, and genetic factors.

Obesity: Not an issue of self-control

Contrary to a still-widespread belief, obesity is not the result of a lack of willpower.

"We know that appetite and satiety are inherited and that up to 70% of your weight is genetically determined. There is every reason to treat obesity as a chronic, relapsing disease," John Wass, professor of endocrinology at Oxford University, told The Lancet.

The factors responsible for obesity can be divided into two categories: Internal factors, such as a person's biological and genetic make-up, and external factors, such as environmental and social conditions, — like your income, access to healthcare or access to safe outdoor areas where you can exercise.

Genetics, lack of exercise, bad eating habits, psychological issues, health problems, social and economic conditions, drugs, pollutants, and more can all contribute to the development of obesity.

Medical conditions like Cushing's disease and certain medications, including steroids and some antidepressants, can also cause weight gain or obesity.

What does obesity do to your body?

Symptoms related to excess fat and body weight include difficulty doing physical activities, pain in different parts of the body and even osteoarthritis, a degenerative joint disease. Obesity can also make it difficult to breathe at night, a condition called sleep apnea.

Obesity can cause type 2 diabetes, increase blood pressure, cholesterol and blood sugar, and lead to cardiovascular disease, like strokes and heart attacks due to the increasing buildup of fatty material in the arteries.

Obesity has also been associated with certain types of cancers like endometrial, esophageal, liver, kidney, and colon, among others, and it can severely affect a person's mental health, causing anxiety and low self-esteem, as well as increase the risk of depression.

How do you measure obesity?

Obesity is often determined by measuring a person's body mass index (BMI) and diagnosed when a person's BMI is above 30. A value above 25 indicates overweight, according to the WHO.

The BMI is calculated by dividing the body weight in kilograms by the square of height in meters. You can calculate your BMI with the UK's National Health Service BMI calculator.

But the BMI is a contentious measure. It doesn't take into account age, sex and which percentage of body weight is made up by muscle mass as compared to fat.

For instance, a weightlifter with a lot of muscle mass can have a high BMI above 30 and not have excess fat.

There are other tools that can more accurately measure the amount of fat, but they are not widely available.

"The best way of measuring [body fat] is using magnetic resonance imaging [MRI]," Alexander Miras, professor of endocrinology at Ulster University in the UK, told DW.

But MRIs are expensive and are mostly used to determine body fat content for research purposes.

An excess of abdominal fat is linked to higher health risks than fat in the rest of the body. Because of that, medical guidelines recommend looking at waist circumference, and not just BMI, when determining someone's health.

How is obesity treated and prevented?

At the individual level, the best way to prevent obesity is to eat a balanced diet, with less fats and sugars and more fruit, vegetables grains and nuts. The other component is staying active: The WHO recommends about 20 minutes of exercise a day.

But individual efforts can only fully work in conjunction with support on the policy and public health level.

"If anything, there has been far too much emphasis on blame and personal responsibility, including from doctors," Francesco Rubino, chair of metabolic and bariatric surgery at King's College London, told The Lancet.

Since obesity is a chronic disease, doctors should provide a long-term treatment plan that can include lifestyle modifications like changing eating habits, switching to a more balanced diet and changing how people move, like promoting physical activity. Regular follow-ups are important.

Weight loss doesn't have to be extreme at first. Studies have shown that even a 10% decrease in body weight can significantly reduce the risk of obesity-related risk factors, and that it can be achieved in 6 months with well-designed treatment programs. After that, more intense weight loss programs need to be discussed with your doctor.

What drugs and surgeries are available?

If diet and exercise don't work, there are few medications approved in the US to treat obesity that can help in addition to diet and exercise. But they come with potential side effects.

Semaglutide, sold under the brand names Ozempic, Wegovy or Rybelsus, has been recently approved for weight loss treatment in the US and by the National Institue for Health and Care Excellence (NICE) in the UK, and might lead to more than a 10% weight loss in addition to diet and exercise. The drug has to be self-injected once a week, and can have side effects like nausea, vomiting, diarrhea and constipation.

Semaglutide reduces appetite by imitating the GLP-1 hormone that is released after we eat.

For people with severe obesity, usually a BMI higher than 40, bariatric surgery such as a gastric bypass, may be an effective treatment for weight reduction and health improvement if dieting and exercise have failed. Since every case is different, it's important to discuss benefits, risks and implications of the procedure with a doctor.