Healthy weight and BMI: overweight, causes, risks and prevention

Weight is a sensitive subject, and it is not only about willpower. This review explains what BMI is, how overweight and obesity are defined, what causes them, the health risks, and practical steps that support a healthy weight. It uses information from the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC).

1. Introduction

Body mass index (BMI) is a measure of weight for height: your weight in kilograms divided by your height in metres squared. WHO defines overweight in adults as a BMI of 25 or higher and obesity as a BMI of 30 or higher, and says waist circumference can also help identify excess weight ^[1]. For children and teenagers aged 5 to 19, WHO compares BMI with growth references for their age, and for children under 5 it uses weight for height ^[1]. The CDC describes the BMI calculator as a screening tool for weight, not a diagnosis ^[2].

The scale of the issue is large. WHO reports that in 2024 about 1.1 billion people worldwide, roughly 1 in 7, were living with obesity, and 2.6 billion adults (45%) were overweight. Overweight in children and adolescents aged 5 to 19 rose from 8% in 1990 to 20% in 2024 ^[1].

2. Causes and risk factors

WHO describes obesity as a chronic disease that arises from the interaction of environmental, psychosocial and biological factors ^[1]:

• Environments that limit access to affordable healthy food and to safe places for physical activity.
• Market forces and weak legal and regulatory frameworks.
• Biology, including genetic susceptibility.

The CDC says that several other factors may also affect weight gain, and that both healthy eating and physical activity matter for weight ^{[2, 3]}. WHO also notes that many low- and middle-income countries now face undernutrition alongside rising overweight, and that children there are exposed to cheap, energy-dense, nutrient-poor foods which, combined with low activity, drive childhood obesity ^[1].

3. Health risks

WHO states that a higher than optimal BMI caused an estimated 3.7 million deaths in 2023, mainly from heart disease, diabetes and cancers, and that obesity contributes to type 2 diabetes, heart disease, liver and kidney disease and several cancers ^[1]. The CDC notes that people with obesity have a higher risk of many serious diseases and conditions than people at a healthy weight ^[2].

In children and adolescents, WHO says excess weight is linked to stigma, bullying, lower quality of life and poorer school performance, and often continues into adulthood ^[1]. WHO reports that in 2024 about 35 million children under 5 were overweight, and that 170 million children and adolescents aged 5 to 19 were living with obesity ^[1]. It projects that without action the global cost of overweight and obesity will reach US$3 trillion a year by 2030 ^[1].

4. Healthy eating for weight

The CDC lists these eating tips ^{[2, 4]}:

• Eat a variety of healthy foods as part of your regular pattern, including vegetables, fruit, whole grains, protein foods and dairy without added sugars.
• Limit added sugars. Sugary drinks are the leading source of added sugars in the United States, and the CDC notes one 12 ounce can of regular soda contains about 10 teaspoons of added sugar.
• Fruit and vegetables can help you feel full without adding many calories.
• Choose canned fruit packed in water or its own juice, and frozen or canned vegetables without added salt.
• Try beans in place of some meat, and bake or grill fish and chicken instead of frying.
• Avoid fad diets, which promise fast results but limit nutrition and tend to fail in the long run.

WHO's prevention advice includes limiting sugar-sweetened drinks and eating a diet rich in fruit, vegetables, legumes, whole grains and nuts ^[1].

5. Activity, sleep and everyday habits

The CDC suggests working up to 150 minutes a week of moderate activity, such as dancing or yard work, to help maintain weight (for example 30 minutes a day on 5 days a week, or about 22 minutes a day), and notes that some people need more, and that losing weight and keeping it off usually needs a high amount of activity unless eating also changes ^[3]. Walking is often an easy way to add activity ^[2]. The CDC lists healthy growth and weight as depending on healthy eating, physical activity, optimal sleep and stress reduction ^[2]. WHO also includes regular physical activity, adequate sleep and avoiding tobacco and harmful alcohol use in its individual prevention measures ^[1].

6. Special groups

WHO advises reaching a healthy weight before pregnancy and breastfeeding exclusively for the first 6 months, then continuing to age 2 or beyond ^[1]. The CDC says parents and caregivers can support healthy growth and a healthy weight in children ^[2]. Because children's BMI is judged against growth references for their age ^[1], ask a health professional to interpret it rather than using adult numbers.

7. Medical care

WHO says practitioners should routinely assess weight, screen for complications, and offer person-centred, long-term care. Support can include behavioural help, nutrition counselling, psychological support, medication and, where appropriate, metabolic surgery ^[1]. WHO says care plans should be individualised, long-term and include regular follow-up ^[1]. Any medicine or surgery is a decision for you and your doctor.

8. When to see a doctor

See a doctor if your BMI is in the overweight or obese range, if your waist is increasing, if you have other risk factors such as high blood pressure, diabetes or a family history of heart disease, if your weight changes without a clear reason, or if you are worried about a child's weight or growth. Do not start extreme diets or unregulated products. Tell your doctor about every treatment you use.

In an emergency, call your local emergency number (112 in India, 911 in the United States).

9. Conclusion

BMI is a simple screening tool, and overweight and obesity arise from many factors, not only personal choices. Everyday steps such as eating a varied diet, limiting added sugar, being active, sleeping enough and avoiding tobacco support a healthy weight. A doctor can assess your own risks and plan care with you.