Beginning around age 30, physiological changes take place that make weight gain increasingly more likely. Muscle mass begins slowly to decline, reducing calorie needs. Yet eating habits often go unchanged. Calories that are not burned as fuel are stored as fat.
Gender differences in weight gain are evident in the weight distribution of men and women. Typically, men accumulate fat on the sides of their waist (love handles) and in their stomach (beer belly). Fat is also laid down inside the abdominal cavity, sometimes taking up space reserved for vital organs. For many middle-age women, weight gain accompanies menopause. In their prime, with oestrogen levels high, women tend to store fat in their hips and thighs. After menopause, a slowdown in metabolism, about 4-5% per decade, and reduced physical activity contribute to a shift in fat from hips and thighs to waist and belly.
In the USA it is common to gain about 4 kg per decade in one's adult years. This is not recommended, as weight gain is associated with premature death from heart disease and cancer.