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Many people notice a disappointing change after they cross 30 – the same foods suddenly cause belly fat, workouts don’t yield the same results, and even small “cheat meals” start appearing on the waistline. According to Dr. Saurabh Sethi, this change is not of willpower but of biology.
The California-based gastroenterologist, trained at Ames 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 about 3-8% muscle mass every decade. Since muscle is metabolically active tissue, less muscle means fewer calories are burned at rest. “Even a kilogram of muscle loss reduces daily energy expenditure,” he said.
Its effect goes beyond calorie burning. Muscle tissue is responsible for handling approximately 70-80% of glucose disposal in the body. When muscle declines, glucose stays in the bloodstream longer and is more likely to be converted to fat – especially around the belly.
Insulin sensitivity also decreases steadily with age, declining by approximately 4–5% per decade. This means that those same carbohydrates can trigger high blood sugar spikes after 30, accelerating fat storage around the waist.
Hormonal changes also play an important role. Growth hormone levels drop, sex hormones like testosterone and estrogen decrease, and cortisol – the stress hormone – begins to increase. “This combination strongly promotes deep abdominal fat storage,” Dr. Sethi explains.
This fat, known as visceral fat, accumulates faster than subcutaneous fat. 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 cautioned 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 stomach and liver.
Early signs your metabolism is slowing down
Dr. Sethi listed subtle warning signs that many people ignore:
- Belly fat increases even if body weight remains the same
- afternoon energy is wasted
- sugar cravings
- bloating after eating carbohydrates
- accumulation of fat in upper abdomen
What really helps after 30?
Instead of extreme diets or gimmicks, Dr. Sethi emphasizes consistency:
- Daily intake of 1.2-1.6 grams of protein per kilogram of body weight
- strength training at least three times a week
- Walk daily to improve insulin sensitivity
- regular seven to eight hours of sleep
“No extremes, no shortcuts,” he said, stressing that preserving muscle and improving insulin sensitivity are the keys to controlling belly fat after 30.
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