What are overweight and obesity?

Overweight and obesity aren't defined by appearance, but by precise measurements, standardized internationally. The first is body mass index (BMI), the second is waist circumference. Knowing them lets you place your own body size without relying on looks alone, your own or others', and helps explain why doctors now describe obesity as a chronic disease, not a lack of willpower.

A simple definition, a major public health issue

BMI is calculated by dividing weight, in kilograms, by the square of height, in meters. A person who is 1.70 m (5 ft 7 in) tall and weighs 80 kg (176 lb) has a BMI of 27.7. In adults, the World Health Organization (WHO) uses the following classification:

• under 18.5: underweight
• 18.5 to 24.9: so-called normal weight
• 25 to 29.9: overweight
• 30 to 34.9: class 1 obesity, described as moderate
• 35 to 39.9: class 2 obesity, described as severe
• 40 and above: class 3 obesity, described as extreme

These thresholds were set because the risk of associated diseases, such as type 2 diabetes or cardiovascular disease, rises progressively from a BMI of 25 onward. They don't draw a line between “healthy” and “sick” people: they are statistical reference points, valid at a population level.

The WHO defines obesity as a complex chronic disease, characterized by an excess of body fat that can harm health. This choice of words matters. It means obesity results from interactions between genetics, biology, environment, and living conditions, and that it calls for long-term medical support, not a simple individual choice.

Marked differences between countries

In France, according to the national OFÉO 2024 study conducted for the French Obesity Federation (Ligue contre l'obésité), 18.1% of adults have obesity, or nearly 10 million people. Nearly half of adults (48.8%) are overweight or living with obesity.

In the United States, prevalence is markedly higher. According to CDC data collected between August 2021 and August 2023, 40.3% of adults have obesity, including 9.4% with severe obesity. That's more than double the French rate.

Worldwide, the WHO estimates that in 2022, 2.5 billion adults were overweight, or 43%, including 890 million living with obesity, or 16%. One person in eight worldwide lives with obesity. The share of adults affected has more than doubled since 1990. The phenomenon is no longer confined to wealthy countries: it is now growing fastest in low- and middle-income countries.

A rapid rise, including in France

The obesity rate in France has doubled over a quarter century, from 8.5% in 1997 to more than 18% in 2024. This rise particularly affects young adults aged 18 to 25, among whom the increase is sharpest. So this is not a phenomenon limited to older generations.

Social and regional inequalities are stark. Obesity is more common among people with lower incomes and in certain regions, notably Hauts-de-France and Grand Est. These gaps are a reminder that body size depends heavily on the environment someone lives in: the price and availability of food, time available to cook, and the ability to walk or be physically active.

Why do people gain weight?

The idea that it would be enough to “eat less and move more” poorly describes reality. Weight is regulated by the brain, which adjusts appetite and energy expenditure based on hormonal signals. Many factors shift this balance.

• Genetics. Twin studies show that heredity explains a significant share of the differences in body size between people. It doesn't determine a fixed fate, but it makes some people more sensitive to an environment that promotes weight gain.
• Food and its environment. The constant availability of ultra-processed foods, high in sugar and fat, low in satiety, and cheap, plays a major role. Sugary drinks and portion sizes matter too.
• Sedentary behavior. Time spent sitting, at work as well as in front of screens, reduces energy expenditure.
• Sleep and stress. Too little sleep increases the hormones that stimulate appetite. Chronic stress promotes abdominal fat storage.
• Certain medications and illnesses. Corticosteroids, some antidepressants, antipsychotics, and diabetes medications can cause weight gain. More rarely, a hormonal condition is the cause.
• Life stages. Quitting smoking, pregnancy, menopause, or a change in work schedule are moments when weight often shifts.

Understanding these mechanisms helps move past guilt. No one chooses their genetics, or the environment they grew up in.

The limits of BMI

BMI is a simple, useful tool, but an imperfect one.

It doesn't distinguish fat from muscle. A very muscular athlete can have a BMI above 25, or even 30, with no excess fat. Conversely, an older person who has lost muscle mass can have a normal BMI and too much body fat.

It says nothing about how fat is distributed, even though that matters a great deal for health.

It isn't interpreted the same way for everyone. In people of Asian descent, diabetes risk rises at lower BMI levels, and the WHO proposes lower alert thresholds for them, at 23 and 27.5. In children and teenagers, growth charts by age and sex are used instead of these thresholds. During pregnancy, BMI doesn't apply.

This is why an international expert commission, published in January 2025 in The Lancet Diabetes & Endocrinology, proposed no longer relying on BMI alone. It recommends confirming excess fat with another measurement, such as waist circumference, and distinguishing “preclinical” obesity, with no impact on organ function, from “clinical” obesity, which already impairs it.

Why waist circumference complements BMI

Not all fat has the same effect. Abdominal fat, known as visceral fat, which surrounds the organs in the belly, is associated with higher metabolic and cardiovascular risk than the same amount of fat located under the skin of the hips or thighs. Two people with the same BMI can therefore have very different risks.

Waist circumference is the simplest reflection of this. It is measured standing up, with a tape measure placed halfway between the lowest rib and the top of the hip bone, at the end of a normal exhale, without pulling in the stomach.

For people of European descent, the usual reference points are:

• a waist circumference of at least 94 cm (37 in) in men and 80 cm (31.5 in) in women corresponds to increased risk;
• from 102 cm (40 in) in men and 88 cm (34.5 in) in women, the risk is substantially increased.

The French National Authority for Health (HAS) recommends measuring waist circumference alongside BMI, particularly when BMI falls between 25 and 35.

A reference point, not a judgment

A high BMI is neither a definitive diagnosis nor a judgment of the person. It's information that invites a look at other things: blood pressure, blood sugar, cholesterol, sleep, shortness of breath, joint pain. Many people who are overweight are in good health, and a modest weight loss, of 5 to 10%, is often enough to markedly improve these markers when they are disrupted.

Talking to your primary care doctor about it allows this assessment to happen before a complication develops, and helps find support that is appropriate and respectful.

To calculate your own BMI and waist circumference, take the “Am I overweight or obese?” test.