BMI for Asian Populations: Why It Can Mislead and What to Use

Body mass index (BMI) is the most widely used screening number in health, and also one of the most misunderstood. The standard World Health Organization cut-offs were largely developed from studies of European populations. A large body of research since then has shown that people of Asian descent tend to carry more body fat, and more fat around the abdomen, at the same BMI, and they develop type 2 diabetes and heart disease at lower BMI levels. If you are of South, East or Southeast Asian background, the standard categories may understate your risk.

The Standard BMI Categories

BMI is weight in kilograms divided by height in metres squared. The WHO classifies a BMI below 18.5 as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight and 30 or above as obese. For a person who is 170 cm tall and weighs 70 kg, BMI is 70 ÷ 1.70² = 24.2, which falls into the normal range.

Why Asian Populations Are Different

In 2004, a WHO expert consultation reviewed the evidence and concluded that the proportion of Asian people with a high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs below the standard cut-off of 25. The consultation did not replace the international categories, but it recommended additional public health action points at BMI 23.0, 27.5, 32.5 and 37.5, and encouraged countries to decide their own thresholds. People of Asian descent tend to have a higher percentage of body fat for a given BMI, and a greater share of that fat stored as visceral fat around the organs, which is more closely linked to insulin resistance.

What Individual Countries Use

Several countries have adopted their own cut-offs. Japan defines obesity at a BMI of 25 or above. China classifies a BMI of 24 or more as overweight and 28 or more as obese. Singapore's health authorities treat a BMI of 23 to 27.4 as moderate risk and 27.5 or more as high risk. Consensus guidelines for India suggest overweight from 23 and obesity from 25. In the United States, the American Diabetes Association recommends testing Asian Americans for diabetes from a BMI of 23, compared with 25 for the general population. In our example, the same 170 cm, 70 kg person falls in the 23 to 27.4 range, which would be considered increased risk.

Other Populations, Other Caveats

BMI has limitations for everyone. It cannot tell muscle from fat, so very muscular people can register as overweight, and older adults who have lost muscle can look normal while having high body fat. Some groups, such as people of Polynesian descent, tend to have more lean mass at a given BMI, and international guidance on appropriate cut-offs for them is still developing. BMI is a screening tool, never a diagnosis.

Better Measures to Add

Waist circumference is simple and informative. The International Diabetes Federation's definition of central obesity uses a waist of 90 cm or more for South Asian and Chinese men, and 80 cm or more for women of these groups. Measure at the midpoint between the bottom of your lowest rib and the top of your hip bone, after breathing out normally.

The waist-to-height ratio is another useful check: divide your waist by your height. A ratio of 0.5 or above is associated with increased health risk across most populations, and the advice to keep your waist to less than half your height is easy to remember. For an 88 cm waist and a 170 cm height, the ratio is 0.52, which is above the threshold even though the BMI looked normal.

Body fat percentage, measured by a reliable method, and clinical markers such as blood glucose or HbA1c, blood pressure and cholesterol give a more complete picture than any single number.

Practical Steps

• Calculate your BMI, then compare it with both the standard WHO categories and the Asian action points.
• Measure your waist and calculate your waist-to-height ratio.
• If your BMI is 23 or higher, or your waist is above the thresholds, ask your doctor about screening for blood sugar and cholesterol, especially if diabetes runs in your family.
• Focus on habits that reduce abdominal fat: regular physical activity, adequate protein, more fibre-rich foods and fewer sugary drinks and refined carbohydrates.

What to do with this

Check your BMI against both the standard categories and the Asian action points, measure your waist, and bring the numbers to your next check-up. A BMI of 23 or above together with a larger waist is a reason to ask about blood sugar testing, not a diagnosis.