Obesity and BMI: thresholds 30 and 40

Obesity is a chronic metabolic disease in which there is more adipose tissue than the body needs. There is no clear division of obesity into grades 1, 2, and 3 by BMI: there is a threshold of 30, from which adults are said to have obesity, and a threshold of 40, from which obesity is called severe. Below: how BMI relates to weight and height, what it does not show, and when to see a doctor.

What obesity is and how it is determined

In adults, obesity corresponds to a body mass index (BMI) of 30 and above, and overweight to a BMI of 25 and above. Formally, these two conditions differ only by threshold; in practice, they differ by risk level: as BMI increases, so does the likelihood of type 2 diabetes, heart disease, stroke, hypertension, osteoarthritis, fatty liver disease, and some types of cancer (breast, uterine, and bowel). On average, obesity shortens life expectancy by 2–10 years. Everyday companions include shortness of breath, fatigue, joint and back pain, snoring, and low mood.

How BMI relates to weight and height

BMI is an indicator that compares a person's weight with their height; in adults it is used as a guide when assessing health risks. The calculator does not distinguish fat, muscle, and bone mass: in a person with well-developed musculature it will overestimate the result, and in an older person with loss of muscle mass it will underestimate it. A second tool is waist circumference: when BMI is below 35, the waist-to-height ratio is used, and a normal waist is less than half of height. For people of South Asian, Chinese, other Asian, Middle Eastern, African, and African-Caribbean origin, the thresholds are lower: overweight is a BMI of 23–27.4, and obesity is 27.5 and above.

BMI threshold 30: the onset of obesity

Obesity is counted from a BMI of 30; a separate "first grade" with its own range is not distinguished here. The immediate cause of obesity is energy intake from food exceeding energy expenditure. The practical meaning of the threshold of 30 is different: a loss of 5–10% of body weight already improves health indicators.

BMI 40 and above: severe obesity

A BMI of 40 and above is called severe, or morbid, obesity. In severe obesity, the doctor assesses comorbidities and risks. Ranges for grades 2 and 3 are not given here.

What weight corresponds to BMI 30 and 40 at different heights

Reference points for the thresholds of 30 and 40:

• height 1.60 m: 76.8 kg at BMI 30 and 102.4 kg at BMI 40;
• height 1.70 m: 86.7 kg at BMI 30 and 115.6 kg at BMI 40;
• height 1.80 m: 97.2 kg at BMI 30 and 129.6 kg at BMI 40;
• height 1.90 m: 108.3 kg at BMI 30 and 144.4 kg at BMI 40.

Limitations of BMI and when a doctor is needed

BMI is a guide, not a diagnosis. A medical examination includes a history of weight and weight-loss attempts, an assessment of physical activity, an examination with measurement of height, weight, and blood pressure, blood tests, and, if necessary, instrumental studies. You should see a doctor if your BMI is in the overweight or obesity range and you want to address your weight; if your waist is more than half your height; if you have prediabetes, diabetes, hypertension, joint pain, elevated cholesterol, sleep apnea, or cardiovascular disease; if close relatives have had obesity; if you have or suspect an eating disorder.

Obesity by itself is not a reason to call an ambulance; 103/112 — if signs of stroke or heart attack appear (life-threatening conditions for which the risk is higher with obesity).

This material is for reference: it explains how BMI is read and does not replace a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer