Effects of diet composition during positive energy balance
It is during periods of positive energy balance that differences in carbohydrate and fat have the greatest impact upon body weight regulation. This is because of differences in the efficiency of metabolic pathways involved in disposing of excess carbohydrate vs. fat. One study (107) demonstrated that while the majority of excess energy is stored regardless of its composition, a greater proportion of excess energy is stored when the excess is from fat as compared to when the excess is from carbohydrate. This is a clear example of a situation where fat intake leads to more body energy storage than the same amount of energy from carbohydrate.
Total energy expenditure increases more with carbohydrate overfeeding than with fat overfeeding. This is because carbohydrate oxidation increases to a greater extent than fat oxidation decreases during carbohydrate overfeeding. The difference between carbohydrate and fat in the proportion of excess energy stored is greatest during the first week of overfeeding. This suggests that the more sustained the overfeeding, the less the difference between carbohydrate and fat overfeeding. If obesity develops due to brief, periodic episodes of overeating, differences between fat and carbohydrate are likely to be more important than if obesity develops from sustained positive energy balance.