Food is either your biggest ally right now or your worst enemy, and how you approach it will decide where it stands. That's why you must be prepared with knowledge when you head to the grocery store. Read the labels. 'fat free' means the item contains less than 0. 5 grams of fat; 'low-fat' foods have three grams of fat or fewer; and 50 percent less fat than its full-calorie counterpart defines a 'light' product.