We often use the words “overweight” or “obese” to describe when the body has accumulated too much fat. Fat is a normal part of the body, providing insulation and protecting some internal organs, as well as making some important hormones. Normal healthy people have about one quarter to one sixth of their body weight as fat (women typically have proportionally more than men), but the more they have above that, the more health problems it can lead to.

A BMI between 25 and 30 kg/m² is called “overweight”, and a BMI above 30 kg/m² is considered “obese”. The likelihood of health problems related to weight increases below as well as above the healthy range, but there is no obvious point where a “low risk” turns into a “high risk” – it is a gradual increase in risk as weight increases. People with excess body fat tend to have higher levels of fats in the blood (cholesterol and triglycerides) and higher blood pressure, which makes heart disease more likely. Excess body fat makes the body less responsive to the hormone insulin, and so makes diabetes more likely, and at the same time increases some hormones that make cells more likely to divide and grow, which makes some cancers more likely. Because BMI can’t distinguish between weight from fat and weight from muscle, in some specific cases, the BMI may not be a very accurate indicator of overall body fat in particular for people with a lot of muscle, like athletes, or people with less than normal amounts of muscle, like older people.

But whatever your BMI, the bigger your waist (waist circumference) is, the higher the likelihood that you will develop an obesity-related health problem.