In a summary of the research literature, replacing 10-15% of dietary fat with carbohydrate results in a modest weight loss of 2-9 lbs. over the short term (6 months). However, over the long term (1 or more years) this weight is regained and there is no association between percent of dietary fat and body weight (Reference 1).
Why is this the case? For two reasons actually:
1) Dietary fat promotes satiety. It helps you feel full. Most people get really hungry a few hours after eating a low fat meal, particularly if it is high in carbohydrate and low in protein.
2) Dietary fat helps to stabilize blood sugar. Swings in blood sugar common with low fat, high carbohydrate eating patterns result in a reactive hypoglycemia that drives down blood sugar and increases insulin levels. For most people, this will result in increased hunger and overeating.
Incidentally, these hormonal changes in blood sugar and insulin levels may also promote fat storage independent of caloric intake.
The most convincing evidence that low fat diets do not result in long term weight loss has been indirectly conducted right here in
References1) Willett WC. Dietary fat plays a major role in obesity: no. Obesity Reviews 2002; 3:59-68.