This viral message listing supposed health benefits of butter over margarine (also the basis of previous questions about health impacts of trans fats and whether margarine is similar to plastic) makes this claim, among many others:
Eating butter increases the absorption of many other nutrients in other foods.
Everything I can find suggesting any biological mechanism for this suggests that it is to do with fat-solubility of certain vitamins - for example, HowStuffWorks has this to say:
Dietary fat, which comes from the food you eat, is crucial to the absorption of the fat-soluble vitamins, which includes vitamins A, D, K and E
However, even if this is exactly true (which isn't clear - Howstuff works appears to source this claim from Natural News, which says it is from "a study in the Americal Journal of Clinical Nutrition" but fails to mention details like, say, the title of that study), this logic would apply equally to margarine as it does to butter. Indeed, the Natural News article says that the published study compared canola oil-based salad dressing to fat free salad dressing.
So: is there any evidence for the claim that eating butter increases nutrient uptake, in any way that doesn't also apply to (or is somehow more effective than) margarine?