A study in this week's Archives in Internal Medicine (Nov 9, 2009, Vol 169, No 20) looked at the effect on mood and cognition of a low-fat versus a low-carb diet. Patients on both diets lost the same amount of weight, 14 kilograms. However, those on the low-fat diet had greater improvements in mood compared to those following a low-carb high-fat diet.
Why the difference in mood? Serotonin may be suppressed by a low carbohydrate diet, while it may be upregulated by a lower fat diet.
Thee bottom line: It doesn't matter what you eat to lose weight. But, what you do eat in your weight loss obviously has an impact on how you feel.