Here's one example (there are many). On the right, we have Jif (reduced fat by a whopping 25%). On the left, we've got "full fat" peanut butter.
Let's look at the Nutrition Facts ...
Full Fat vs. Low Fat
Calories: 200 vs. 190 – 5% Less
Sodium: 85 vs.285 – 300% More
Sugar: 2 vs. 4 – 100% More
Upon reading the label, we realize that calories are about the same. However, when you take the fat out, you also take the flavor out. To put the flavor back in the product, voilà ... you add sugar and salt!
Moral of the story: Ignore all claims of reduced or low fat! Because, they don't significantly reduce the calorie count and they add ingredients we don't need ... sugar and salt.