Most trials in which people lose weight report improvements in mood. If you look better you’ll probably feel better about yourself. But, in this trial (Arch Intern Med. 2009;169:1873-1880) the positive benefit of losing weight didn’t produce the same long-term improvement in mood for those eating a low carb, high protein diet compared to a low fat, high carbohydrate diet. This essential finding was also reported in trials giving people a high protein breakfast versus a high carbohydrate breakfast. Logically, one might think that high protein means more amino acids, and more amino acids such as tryptophan would mean more serotonin. But the reverse is true. Brain levels of serotonin, and mood, go up after eating carbohydrate not protein, even when there’s no tryptophan in the food [R. Wurtman and J. Wurtman, ‘Carbohydrates and depression’, Scientific American, Vol 260(1), 1989, pp. 68–75] While this seems counter-intuitive there’s a simple reason for it. Tryptophan, the protein constituent from which we make mood-boosting serotonin, is carried from the blood into the brain by insulin, and insulin is only released if you eat carbohydrate. That’s one of the reasons why I advocate a Low GL Diet that includes carbohydrate in each meal, albeit slow-releasing carbohydrates. That way you get the weight loss, plus the mood boost, without the high blood sugar levels found in traditional low-fat high calorie diets.