In a previous post, Atkins Diet: Common Errors, I commented on butter's unusual ability to provoke insulin responses. I offer this as a possible reason why, after a period of effective weight loss on a low-carbohydrate program, inclusion of some foods, such as butter, will trigger weight gain or stall weight loss efforts.
This develops because of butter's insulin-triggering effect, doubling or tripling insulin responses (postprandial area-under-the-curve). If insulin is triggered, fat gain follows.
Here's one such study documenting this effect: Distinctive postprandial modulation of β cell function and insulin sensitivity by dietary fats: monounsaturated compared with saturated fatty acids
López et al 2008
From Lopez et al 2008. Mean (± SD) plasma glucose, insulin, triglyceride, and free fatty acid (FFA) concentrations during glucose and triglyceride tolerance test meal (GTTTM) with no fat (control), enriched in monounsaturated fatty acids (MUFAs) from refined olive oil (ROO meal), with added butter, with a mixture of vegetable and fish oils (VEFO) or with high-palmitic sunflower oil (HPSO). N = 14.
The postprandial (after-eating) area-under-the-curve is substantially greater when butter is included in the mixed composition meal. This effect is not unique to butter, but is shared by most other dairy products.
Fat, in general, does not make you fat. But butter makes you fat.