That is a really great question Heather. I never really gave it a lot of thought. I don't use the slow cooker near as often as the pressure cooker though. I know there are some things that are better for you when they have been slow cooked, like tomatoes. Maybe someone in the know will answer. I am curious now.
Broadly speaking it depends on the nutrient.
Most of the common vitamins and minerals don't break down under the level of heat we use for cooking, however, a water soluble vitamin such as vitamin C will leach out. That is why boiling your veggies will lead to lower nutrition.
In general, Swampy doesn't worry about this, because humans are eating considerably more food by volume than they need, and as long as your diet is good quality and in balance any lower nutrition from cooking the food is made up for in the additional amount.
Thanks, guys! Yeah, you're right, swampcritter...most of us do eat more than we need ;-) I guess I'll just continue using the crock pot then and not worry about the nutrients being lost. Thanks again! I really appreciate it!