What are the health risks of obesity?

Having obesity does not mean you will develop these conditions. Risk depends on many things, including genes, where body fat is stored, age and access to care. Your doctor can help you understand your own risk.

How does obesity affect health?

Obesity affects health in two main ways: fat tissue releases hormones and inflammatory signals that disturb the metabolism, and extra weight puts physical strain on joints, airways and the heart. According to the latest Global Burden of Disease estimates, a higher-than-optimal BMI causes about 3.7 million deaths worldwide each year, mainly through cardiovascular disease, diabetes and cancers.

The table gives an overview. The sections below explain each group of conditions.

How is obesity linked to type 2 diabetes?

Obesity is the strongest risk factor for type 2 diabetes, because excess fat, especially around the organs, makes the body less sensitive to insulin. In the United States, nearly 9 in 10 people with type 2 diabetes have overweight or obesity.

Weight loss can prevent diabetes and sometimes put it into remission. Losing 5 to 7 percent of body weight can prevent or delay type 2 diabetes in people at high risk. In the DiRECT trial, almost half (46 percent) of people with recent type 2 diabetes who followed a structured low-calorie diet programme were in remission after one year; among those who lost 15 kg or more, 86 percent were.

What about blood pressure, heart disease and stroke?

Obesity raises blood pressure, cholesterol and blood sugar, which together damage blood vessels and increase the risk of heart attack and stroke. It also changes the structure of the heart, which can lead to heart failure and an irregular heartbeat (atrial fibrillation).

This risk can be lowered. Regular physical activity such as walking, good food, not smoking and treating high blood pressure and cholesterol all protect the heart and blood vessels. Your doctor can tell you which steps matter most for you. See lifestyle treatment.

Which cancers are linked to obesity?

The International Agency for Research on Cancer (IARC), part of the WHO, has concluded that there is sufficient evidence linking excess body fat to 13 cancers:

• oesophagus (adenocarcinoma) and the upper part of the stomach (gastric cardia)
• colon and rectum
• liver, gallbladder and pancreas
• breast (after menopause), womb (endometrium) and ovary
• kidney (renal cell)
• thyroid
• meningioma (a usually benign brain tumour)
• multiple myeloma (a blood cancer)

The IARC noted that animal and mechanistic studies suggest intentional weight loss may lower cancer risk, but that this still needs to be confirmed in people. Taking part in regular cancer screening remains important at every body size.

What is MASLD (fatty liver disease)?

MASLD, or metabolic dysfunction-associated steatotic liver disease, is fat build-up in the liver linked to metabolic risk factors such as obesity and type 2 diabetes. It used to be called NAFLD, until a new name was agreed internationally. In some people it becomes inflamed (MASH) and can lead to scarring, cirrhosis or liver cancer.

It usually causes no symptoms, so it is often found through blood tests or scans. Losing even 3 to 5 percent of body weight can reduce liver fat, and larger weight loss can improve inflammation.

How does obesity affect sleep and joints?

Extra tissue around the neck can narrow the airway during sleep, causing obstructive sleep apnoea: pauses in breathing, loud snoring and daytime tiredness. Untreated, it raises the risk of high blood pressure, heart disease and diabetes. Signs worth mentioning to your doctor include waking unrefreshed, falling asleep during the day and a partner noticing breathing pauses.

Obesity is also a leading risk factor for osteoarthritis of the knees, hips and ankles, through extra pressure on the joints and inflammation. Losing 5 to 10 percent of body weight can reduce pain and improve walking and daily function.

What about fertility, hormones and kidneys?

Obesity can disturb hormones in all sexes. In women, it is linked to irregular periods, problems with ovulation and polycystic ovary syndrome (PCOS), and can make fertility treatment less effective. In men, it is linked to lower testosterone and lower sperm quality.

Losing around 5 percent of body weight may improve the chance of regular cycles, ovulation and pregnancy. See our page on obesity and pregnancy.

Obesity raises the risk of chronic kidney disease, mainly through diabetes and high blood pressure, but also directly. It can also cause urinary incontinence, which often improves with weight loss.

How does obesity affect mental health?

Obesity and mental health affect each other in both directions. People living with obesity have a higher risk of depression, anxiety, low self-esteem and eating disorders, and depression and some psychiatric medicines can in turn lead to weight gain. Weight stigma, at school, at work or in healthcare, is a major part of this burden. Read more on sleep, stress and mental health and on psychological support.

What are the benefits of losing 5 to 15 percent of body weight?

Losing 5 to 15 percent of body weight, and keeping it off, improves many obesity-related conditions, with larger benefits at the higher end. For a person weighing 100 kg, 5 percent is 5 kg.

Health gains also come from better sleep, more movement and a healthier diet, even when the scale moves little. Losing weight very quickly can increase the risk of gallstones, so it is best done with professional support. Our page on treatment explains the options.