Sugar vs fat: Chris and Xand Van Tulleken, both 35, spent a month following restrictive fad diets to see if one food group is behind the obesity epidemic.Is a high-fat, low-carb diet better for you than a high-carb, low-fat one? A pair of British brothers set out to end the fat versus sugar debate. Alexander (Xand) Van Tulleken, a senior fellow at the Institute of International Humanitarian Affairs at Fordham University in New York, and Chris Van Tulleken, a physician at University College Hospital, London, each agreed to stick to a specific diet for one month. Chris followed a super-low-fat...