Fat versus sugar in weight loss.

I watched the Horizon programme on it. Of course, the sugar defines all carbohydrates not just refined sugars. So basically identical twin doctors (male!) decided to try, for one month only, to try opposit diets. One ate only fat (including animal protein) but no carbs at all. The other twin only ate carbs. It was interesting to watch and I think they decided that the best diet out there is a little bit everything! However, the frequently reminded their viewers that their "experiment" couldnt be taken seriously as age, sex and ethnicity would have to be taken into account and would need proper research to find a difinitive answer.