Obesity: causes, symptoms, types, health risks and prevention

Obesity is more than simply being overweight. It is a complex, long-term health condition involving excess body fat that can affect almost every major system in the body. It is influenced by eating patterns, physical activity, genetics, sleep, hormones, medicines, environment, stress and socioeconomic factors.

According to the World Health Organization (WHO), more than 1 billion people worldwide were living with obesity in 2022. Among adults, obesity has more than doubled since 1990, while obesity among children and adolescents aged 5 to 19 has increased four-fold over the same period.

This makes understanding obesity important—not just for weight management, but for preventing conditions such as type 2 diabetes, high blood pressure, heart disease, stroke, fatty liver disease, sleep apnoea, osteoarthritis and certain cancers.

What is the definition of obesity?

Obesity is generally defined as an abnormal or excessive accumulation of body fat that may impair health.

For adults, body mass index (BMI) has traditionally been used as a screening measure:

• BMI below 18.5: underweight
• BMI 18.5–24.9: healthy weight
• BMI 25–29.9: overweight
• BMI 30 or above: obesity
• BMI 30–34.9: class 1 obesity
• BMI 35–39.9: class 2 obesity
• BMI 40 or above: class 3 obesity

BMI is calculated by dividing weight in kilograms by height in metres squared.

However, BMI is only a screening tool. It does not directly measure body fat or show where fat is stored. Someone with a high amount of muscle, for example, may have a high BMI without having excessive body fat.

Waist circumference and other measures of body composition can provide additional information about health risk.

Is obesity a disease?

Yes. Obesity is recognised as a chronic disease by major medical organisations, including the WHO.

It is also important to understand that obesity is not simply a matter of willpower. Genetics, appetite regulation, hormones, medications, sleep, mental health, food availability, physical environment and social factors can all influence body weight.

The WHO describes obesity as a chronic, relapsing disease resulting from complex interactions between biological, behavioural and environmental factors.

The newer approach to obesity goes further than simply looking at the number on the weighing scale.

What is the new definition of obesity in 2026?

A major development in the understanding of obesity came from the 2025 Lancet Diabetes & Endocrinology Commission, which proposed a framework distinguishing between clinical obesity and preclinical obesity.

Under this framework:

Clinical obesity refers to excess body fat accompanied by evidence that obesity is already affecting the function of organs or tissues, or substantially limiting everyday activities.

Preclinical obesity refers to excess body fat where organ and tissue function remains preserved, but the person has an increased risk of developing obesity-related disease.

This approach recognises an important limitation of BMI: BMI alone cannot tell us whether excess body fat is already causing illness.

Importantly, this is a newer clinical framework rather than a simple replacement of every existing BMI classification worldwide. BMI remains widely used for screening and population-level assessment, while clinicians may increasingly consider waist measurements, body-fat distribution, symptoms, laboratory results and organ function.

What are the classes of obesity?

For adults, obesity is commonly divided into three classes based on BMI.

Class 1 obesity

BMI is 30 to less than 35.

This is sometimes informally called “stage 1 obesity”, although class 1 obesity is the more standard terminology.

Class 2 obesity

BMI is 35 to less than 40.

Class 2 obesity is associated with a greater likelihood of developing weight-related health problems compared with lower BMI categories.

Class 2 obesity can increase the risk of conditions such as type 2 diabetes, high blood pressure, cardiovascular disease, sleep apnoea, fatty liver disease and osteoarthritis.

Class 3 obesity

BMI is 40 or higher.

It is also known as severe obesity. At this level, the probability of obesity-related complications generally increases, although individual risk varies.

People with class 3 obesity may require a comprehensive medical assessment rather than relying solely on diet and exercise advice. Depending on individual circumstances, treatment may include nutritional counselling, behavioural interventions, medicines and, for some people, metabolic or bariatric surgery.

What is stage 4 obesity?

There is no universally accepted medical classification called “stage 4 obesity.”

The commonly used adult BMI classification has three obesity classes: class 1, class 2 and class 3.

Some websites or non-medical sources use the phrase “stage 4 obesity” to describe extremely high levels of obesity, but this is not a standard WHO or CDC obesity category.

If you come across the term, it is better to look at the actual BMI, waist circumference, body-fat distribution and health complications rather than relying on the phrase “stage 4”.

What are 5 symptoms of obesity?

Obesity itself may not always cause obvious symptoms. However, some people may experience:

1. Breathlessness, particularly during physical activity
2. Excessive sweating
3. Fatigue or reduced stamina
4. Joint pain, especially involving the knees, hips or lower back
5. Snoring or disturbed sleep, particularly when sleep apnea is present

Other possible signs include difficulty performing everyday physical activities, reduced mobility, reflux, skin-fold problems and difficulty sleeping comfortably.

Importantly, someone can have obesity without experiencing noticeable symptoms. This is why regular health checks can be useful.

What are five causes of obesity?

Obesity usually has more than one cause. Five important contributors include:

1. Energy-dense diet

Regular consumption of foods and drinks high in calories, added sugars and unhealthy fats can contribute to excess calorie intake.

2. Physical inactivity

A sedentary lifestyle reduces energy expenditure and can make maintaining a healthy weight more difficult.

3. Genetics

Genes can influence appetite, hunger, satiety, fat storage and how the body uses energy.

4. Poor sleep and chronic stress

Insufficient sleep and long-term stress can affect appetite, eating behaviour and hormones involved in weight regulation.

5. Medical conditions and medicines

Certain endocrine disorders, genetic conditions and medications can contribute to weight gain. Examples include some medicines used for psychiatric conditions, epilepsy, diabetes and other chronic diseases.

Obesity should therefore not automatically be attributed to overeating or lack of exercise.

Which foods cause obesity?

No single food automatically causes obesity. The overall dietary pattern and long-term energy balance matter much more.

However, regularly consuming large amounts of sugar-sweetened drinks, sweets, highly processed snacks, fast food, fried foods, refined carbohydrates and other calorie-dense foods can make excess calorie intake easier.

Ultra-processed foods may also be particularly easy to overconsume because they can be highly palatable, convenient and energy-dense.

A healthier eating pattern generally includes vegetables, fruits, pulses, whole grains, nuts, seeds and appropriate sources of protein while limiting excess added sugar and highly processed foods.

What diseases are caused by obesity?

Obesity is associated with a wide range of health conditions, including:

• Type 2 diabetes
• High blood pressure
• Heart disease
• Stroke
• Fatty liver disease
• Sleep apnea
• Osteoarthritis
• Gallbladder disease
• Kidney disease
• Certain cancers
• Infertility and pregnancy complications
• Metabolic syndrome
• Gout
• Breathing problems
• Some mental health conditions

Obesity does not mean that a person will definitely develop any particular disease. Rather, excess adiposity can increase the risk of developing several chronic conditions.

Which organ is most affected by obesity?

There is no single organ that is always “the most affected” by obesity.

Excess body fat can affect the heart, liver, pancreas, kidneys, brain, blood vessels, lungs and musculoskeletal system.

The liver is particularly important because excess fat accumulation can lead to metabolic dysfunction-associated steatotic liver disease (MASLD), which may progress to inflammation, fibrosis and, in some people, cirrhosis.

At the same time, the cardiovascular system is significantly affected because obesity can contribute to high blood pressure, abnormal cholesterol levels, diabetes and inflammation, all of which increase cardiovascular risk.

Who is at highest risk for obesity?

Obesity can affect people of any age and background. Risk may be higher in people who have:

• A family history of obesity
• A sedentary lifestyle
• A diet high in energy-dense foods
• Poor or insufficient sleep
• Chronic stress
• Certain medical conditions
• Certain medications
• Limited access to affordable nutritious food
• Limited opportunities for physical activity

Risk is also influenced by the environment in which people live, work and grow up.

Why is childhood obesity increasing?

Childhood obesity is increasing globally because children’s environments have changed significantly.

Common contributing factors include increased availability of calorie-dense foods, sugary drinks and highly processed foods; reduced physical activity; increased sedentary time; urbanisation; changes in transportation; food marketing; inadequate sleep and socioeconomic factors.

Children cannot always control these influences themselves. Parents, schools, communities, healthcare systems, food manufacturers and policymakers all have roles in creating healthier environments.

WHO data show that the prevalence of obesity among children and adolescents aged 5–19 increased from around 2% in 1990 to 8% in 2022.

How much of Gen Z is obese?

There is no single reliable global percentage for obesity specifically among Generation Z because Gen Z is a generational label rather than a standard epidemiological age group.

Different studies use different birth-year definitions, while health surveys usually report obesity by age groups.

For perspective, a large analysis of US National Health and Nutrition Examination Survey data from 2013–2023 found obesity in about 29.7% of young adults aged 18–25. This is a US statistic for a defined age group and should not be interpreted as the obesity rate of Gen Z worldwide.

Globally, WHO reports that obesity among children and adolescents aged 5–19 reached 8% in 2022.

What country is number 1 in obesity?

The answer depends on the population, year, age group and data source.

According to the World Obesity Federation’s current country ranking for adult obesity prevalence, Nauru is listed first, with an estimated adult obesity prevalence of about 72.7%.

This type of ranking should be interpreted carefully because countries differ in how data are collected, the years represented and whether estimates are crude or age-standardised.

Is 30 kg overweight or obese?

You cannot determine whether 30 kg is overweight or obese without knowing the person’s height, age and, in some cases, sex and body composition.

For adults, BMI is calculated as:

BMI = weight in kg ÷ height in metres²

For example, 30 kg would produce a BMI of 30 only if the person’s height were approximately 1 metre. For a taller person, 30 kg would not be classified as obesity.

For children, BMI is interpreted differently because age and sex-specific growth charts are used.

What does stage 1 obesity look like?

There is no specific physical appearance that defines “stage 1 obesity”, and the more medically appropriate term is class 1 obesity.

Two people with the same BMI can have very different body shapes and fat distributions. Some people may carry more fat around the abdomen, while others may store more fat around the hips, thighs or other areas.

This is another reason why body weight alone cannot provide a complete picture of health.

Can obesity be cured?

Obesity is a chronic disease, so the word “cured” can be misleading.

However, obesity can be treated and managed, and meaningful improvements in health are possible.

Treatment may involve:

• Sustainable dietary changes
• Regular physical activity
• Behavioural and psychological support when appropriate
• Improved sleep
• Management of underlying medical conditions
• Weight-management medicines for eligible patients
• Metabolic or bariatric surgery for selected patients

Even a relatively modest reduction in excess body weight can improve blood pressure, blood sugar and other health markers in many people.

The goal should not simply be a lower number on the weighing scale. A good treatment plan aims to improve health, physical function and long-term quality of life.

What are 5 ways to prevent obesity?

Five practical approaches include:

1. Build a balanced eating pattern

Prioritise vegetables, fruits, pulses, whole grains, nuts, seeds and appropriate protein sources while limiting excessive added sugar and highly processed foods.

2. Stay physically active

Regular physical activity helps maintain a healthy weight and supports cardiovascular and metabolic health.

3. Protect your sleep

Most adults need around 7–9 hours of sleep each night. Poor sleep can affect appetite, food choices and metabolic health.

4. Monitor weight and waist circumference

Regularly checking weight and waist measurements can help identify gradual changes before they become difficult to reverse.

5. Address weight gain early

Persistent weight gain deserves attention rather than being ignored until obesity becomes severe. Early lifestyle changes and appropriate medical guidance can make long-term management easier.

Which body part is hardest to lose fat?

There is no single body part that is universally the hardest to lose fat from.

Fat loss occurs throughout the body, but the pattern varies between individuals because of genetics, hormones, age and sex.

Many people find abdominal fat particularly persistent. Others may notice slower changes around the hips, thighs or lower abdomen.

It is also important to remember that spot reduction is not possible through exercises targeting one body part. Doing hundreds of abdominal exercises, for example, does not specifically remove belly fat.

Overall fat loss, combined with resistance exercise and adequate nutrition, is the usual approach.

When should you see a doctor about obesity?

Consider speaking with a healthcare professional if you have persistent weight gain, a BMI in the obesity range, increasing waist circumference, unexplained fatigue, breathlessness, loud snoring, high blood pressure, abnormal blood sugar or cholesterol, joint problems, or other concerns related to your weight.

A medical assessment can look beyond BMI and consider blood pressure, blood glucose, cholesterol, liver health, sleep, medications, lifestyle, family history and other risk factors.

This is particularly important because obesity-related problems can develop before obvious symptoms appear.

Frequently asked questions about obesity

Is obesity the same as being overweight?

No. In adults, overweight generally refers to a BMI of 25 to less than 30, while obesity begins at a BMI of 30. BMI is a screening measure and should be interpreted alongside other health indicators.

Can a thin person have obesity?

A person can have excess body fat even when their BMI is not in the obesity range. This is one reason modern approaches increasingly consider waist circumference, body-fat distribution and metabolic health.

Does obesity always cause diabetes?

No. Obesity increases the risk of type 2 diabetes, but not everyone with obesity develops diabetes.

Can children have obesity?

Yes. Childhood obesity is a major global health concern. However, BMI is interpreted using age- and sex-specific growth charts rather than adult BMI cut-offs.

Is belly fat more dangerous?

Excess abdominal or visceral fat is strongly associated with metabolic and cardiovascular risk. Waist circumference can therefore provide useful information that BMI alone may miss.

Can losing weight reduce health risks?

Yes. Weight loss can improve several obesity-related risk factors, including blood pressure, blood sugar and metabolic health. The benefits depend on the individual and the amount of weight lost.

Final thoughts

Obesity is a complex medical condition, not simply a question of appearance or willpower. It develops through an interaction of biological, behavioural, environmental and social factors and can affect almost every major organ system.

BMI remains useful for screening, but it does not tell the whole story. Waist circumference, body-fat distribution, metabolic health, symptoms and organ function can provide a more complete picture.

The good news is that obesity is treatable. Early recognition, sustainable lifestyle changes and appropriate medical care can reduce health risks and improve quality of life.

If you are concerned about your weight, waist size, blood sugar, blood pressure or other obesity-related risks, discussing your individual situation with a qualified healthcare professional is a sensible first step.

To consult a dietician at Sparsh Diagnostic Centre, call our helpline number 9830117733.

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Disclaimer:

No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.