[QUOTE=GingerSue;2352253]how to determine which foods can be substitutes for other foods,
and the amounts,
the number or Oreo cookies that can be substituted for a fresh pear
(is there a list of food substitutes, I found some material about carbohydrate, protein, fat groups, with a short list of the kinds of foods in each category, but not specific items or amounts)?
A good reference is the Calorie King Calorie, Fat, and Carbohydrate Counter. It doesn't make specific conversions, but it does give you the necessary data to do it. There are also several online sources.
In your Oreo/Pear example, 1.5 regular Oreo cookies have about the same number of calories as a pear. However, the pear has twice the amount of carbs as the 1.5 Oreos. The pear has no fat and has lots of fiber. Oreo about 3.5 g of fat and very little fiber. Because the pear has more volume, fiber, and water content, it will be more filling, but will probably raise BG higher than the Oreos would.
The pear gives you more nutritional bang for your buck, but sometimes you just gotta have an Oreo!
Also, pears come in different sizes and varieties, so this example can vary. Above is based on about 1 cup of fresh pear slices. Asian pears are not as sweet as Bartlett pears, so 1 Asian pear equals 1 regular Oreo.