Few things captivate the public more than a new diet.  From Atkins to Ornish to the Mediterranean diet, each new theory attracts attention and true-believer adherents and generates lots of book sales and interviews on daytime TV.  People passionately argue about whether a diet low in carbohydrates or low in fat is best for weight loss.  But until now no large trial has ever been done to answer the question. This week’s New England Journal of Medicine published the largest study that di...