Why belly fat increases after 30 even if your diet hasn’t changed, AIIMS-trained doctor explains

Many people notice a frustrating shift after they cross 30 — the same foods suddenly lead to belly fat, workouts don’t deliver the same results, and even small “cheat meals” seem to show up on the waistline. According to Dr Saurabh Sethi, this change is not about willpower but biology.

The California-based gastroenterologist, trained at AIIMS as well as Harvard and Stanford universities, recently explained why metabolism slows with age — even when diet and activity levels remain unchanged.

Dr Sethi said that after the age of 30, the body naturally loses around 3–8% of muscle mass every decade. Since muscle is metabolically active tissue, less muscle means fewer calories burned at rest. “Even a loss of one kilogram of muscle reduces daily energy expenditure,” he noted.

The impact goes beyond calorie burn. Muscle tissue is responsible for handling nearly 70–80% of glucose disposal in the body. When muscle mass declines, glucose stays in the bloodstream longer and is more likely to be converted into fat — particularly around the abdomen.

Insulin sensitivity also drops steadily with age, declining by about 4–5% per decade. This means the same carbohydrates can trigger higher blood sugar spikes after 30, accelerating fat storage around the waist.

Hormonal shifts play a critical role as well. Growth hormone levels fall, sex hormones such as testosterone and estrogen decline, and cortisol — the stress hormone — tends to rise. “This combination strongly favours storage of deep abdominal fat,” Dr Sethi explained.

This fat, known as visceral fat, accumulates faster than the fat just under the skin. Unlike subcutaneous fat, visceral fat surrounds internal organs and is closely linked to insulin resistance, chronic inflammation, fatty liver disease and type 2 diabetes.

Dr Sethi warned that the effects are more pronounced in people who already have conditions such as fatty liver, prediabetes, diabetes or high triglyceride levels. In such cases, insulin resistance pushes fat directly into the abdomen and liver.