Obesity is characterized by what condition?

Obesity is fundamentally characterized by an excessive amount of body fat. This condition results from an imbalance between calorie intake and energy expenditure, leading to an accumulation of fat that can have detrimental effects on health. The standard measure used to define obesity is body mass index (BMI); a BMI of 30 or higher is considered obese, indicating significant excess body fat.

The other options do not accurately define obesity. Excessive muscle mass, for instance, is a characteristic of muscular individuals but does not relate to body fat levels. Low body mass index indicates a lack of fat and body weight, which is the opposite of obesity, while a deficiency in calorie intake would lead to weight loss rather than weight gain. Therefore, the defining characteristic of obesity is indeed the presence of excessive body fat.