Why does weight gain concentrate around the abdomen?

Fat distribution is driven by genetics, sex hormones and age. Abdominal or visceral fat is metabolically different from fat under the skin — it is more active hormonally, drains directly to the liver, and is much more strongly associated with insulin resistance, fatty liver and cardiovascular risk. This is why waist circumference predicts risk better than weight. South Asians tend to deposit proportionally more visceral fat, which contributes to the higher risk at lower BMI. It also responds relatively well to overall weight reduction and exercise — targeted abdominal exercises do not reduce it, but general activity does.