BMI Explained: What It Is and 5 Common Claims

BMI, or Body Mass Index, is often used as a simple tool to determine whether someone falls within a healthy weight range based on their height and weight. However, many people have questions about its accuracy and limitations. In this blog post, we’ll explain exactly what BMI is, how it’s calculated, and explore 5 common claims surrounding it. From its usefulness in assessing health to concerns about its reliability, we’ll dive into the facts to help you better understand this widely-used health metric.

Claim: Obesity rates have increased dramatically worldwide in recent decades.

Question: What is the percentage increase in global obesity rates from 1975 to 2022?

The global obesity rates have indeed increased significantly from 1975 to 2022. Here are the specific statistics:

• In 1975, the global prevalence of obesity was approximately 3% for men and 6.6% for women. By 2022, these rates had increased to 14.0% for men and 18.5% for women.
• The worldwide prevalence of obesity has more than tripled between 1975 and 2022. For example, obesity rates in adults nearly tripled among women (from 6.6% to 18.5%) and quadrupled in men (from 3% to 14.0%).

To quantify the percentage increase:

• For men: From 3% to 14.0%, which is an increase of 366.7% ((14.0 - 3) / 3 * 100).
• For women: From 6.6% to 18.5%, which is an increase of 180.3% ((18.5 - 6.6) / 6.6 * 100).

Data Source:

World Obesity Federation: Prevalence of Obesity

World Health Organization: Obesity and overweight

Claim: High BMI is associated with increased health risks, including cardiovascular diseases and certain cancers.

Question: What is the relative risk of developing cardiovascular disease or specific cancers for individuals with obesity compared to those with normal BMI?

Individuals with obesity have a significantly higher risk of developing various health conditions, including cardiovascular diseases and certain cancers.

• Cardiovascular Disease: Obesity is associated with a significantly increased risk of cardiovascular diseases. For example, a study indicated that obesity is linked with a nearly 50% increased risk of cardiovascular disease compared to those with normal BMI.
• Cancers: Obesity is linked to an increased risk of several types of cancer, including breast, colon, kidney, pancreatic, and esophageal cancer. The relative risk can vary by cancer type but is generally increased. For instance, obesity is associated with a 20-30% increased risk of postmenopausal breast cancer and a 30-70% increased risk of colon cancer compared to individuals with normal BMI.

Data Source:

A 2022 update on the epidemiology of obesity and a call to action

Claim: Even small weight reductions can have significant health benefits.

Question: What percentage improvement in health outcomes is associated with a 5-10% reduction in body weight for overweight individuals?

A 5-10% reduction in body weight can lead to significant improvements in health outcomes for overweight individuals.

• Health Outcomes: Studies have shown that a 5-10% weight loss can result in improvements in blood pressure, cholesterol levels, and blood glucose control. For example, a 5-10% weight loss can reduce the risk of developing type 2 diabetes by 58%, reduce blood pressure by 5 mmHg, and decrease LDL cholesterol by 10 mg/dL.
• Percentage Improvement: While the exact percentage improvement can vary depending on the specific health outcome, a general estimate is that a 5-10% weight loss can lead to a 20-30% reduction in the risk of developing chronic diseases such as diabetes and cardiovascular disease.

Data Source:

A 2022 update on the epidemiology of obesity and a call to action

Claim: BMI varies across different ethnic groups, with some populations having increased health risks at lower BMI levels.

Question: What are the specific BMI thresholds for increased health risks in different ethnic populations, particularly Asian groups?

BMI thresholds for increased health risks do vary across different ethnic groups, particularly in Asian populations.

• Asian Populations: For Asian populations, the World Health Organization recommends lower BMI cut-offs for defining overweight and obesity due to the higher risk of health problems at lower BMI levels. Specifically, for Asian adults, a BMI of 23-24.9 kg/m² is considered overweight, and a BMI of 25 kg/m² or higher is considered obese. This is compared to the general cut-offs of 25-29.9 kg/m² for overweight and 30 kg/m² or higher for obesity in other populations.
• Other Ethnic Groups: Similar adjustments are recommended for other ethnic groups. For example, in some Pacific Island populations, the risk of health problems may be higher at even lower BMI levels.

Data Source:

World Health Organization: Obesity and overweight

Global Trends in Obesity - NCBI

Claim: BMI doesn't distinguish between muscle and fat, which can lead to misclassifications.

Question: What percentage of individuals classified as overweight or obese by BMI actually have healthy body fat percentages?

BMI does not distinguish between muscle and fat, which can lead to misclassifications, particularly in individuals with high muscle mass.

• Misclassification: Studies have shown that BMI can misclassify athletes or individuals with high muscle mass as overweight or obese even when they have healthy body fat percentages. However, specific percentages of misclassification vary and are not universally quantified. It is generally acknowledged that BMI is a crude measure and should be used in conjunction with other measures such as waist circumference and body fat percentage to get a more accurate assessment of health risks.

Data Source:

A 2022 update on the epidemiology of obesity and a call to action

World Health Organization: Obesity and overweight