Twin doctors in Europe decided to find out.
Chris Van Tulleken went on a super low-fat diet, while identical twin Alexander went high-protein, cutting out most carbs.
After a month, Alexander lost the most weight - 9 pounds.
However, both were miserable - low-fat left Chris hungry all the time and, without fat, feeling like he was eating cardboard.
Meantime, Alexander lost his energy but gained a bad mood on the low carbs.
They say the only way to a real diet that works is to watch your calories, cut portion sizes, and eat fewer processed foods.The doctors say it's not sexy and won't be a best-seller, but it works.