Why do empty calories cause a person to gain fat weight?

In human nutrition, the term empty calories applies to food such as solid fats or added sugars supplying food energy but little or no other nutrition. The USDA advises, "A small amount of empty calories is okay, but most people eat far more than is healthy.

The phrase is derived from nutrient density (proportion of nutrients in a food relative to its energy content), and calorie density (amount of energy relative to weight of the food). Thus empty calories are accompanied by no or few nutrients. Foods containing empty calories typically contain processed carbohydrates and ethanol (alcohol), and to some extent fats. Also known as a discretionary calorie, an empty calorie has the same energy content as any other calorie but lacks many accompanying nutrients such as vitamins, dietary minerals, antioxidants, amino acids, or dietary fiber. Although carbohydrates and fats are nutrients, they are typically ignored for this analysis, with the exception of essential fatty acids.