Macro partitioning

High-fat diets promote obesity in humans primarily by facilitating fat storage with minimal energy cost (0-2%) compared to carbohydrate conversion to fat, which is energetically expensive (25% heat loss), making high-fat diets more efficient for weight gain.

Understand that your body handles different foods differently. Converting carbohydrates into body fat is metabolically expensive and inefficient, losing 25% of the energy as heat. In contrast, storing dietary fat as body fat is highly efficient, costing almost no energy. This metabolic difference makes high-fat diets particularly conducive to weight gain when energy intake is excessive.