Hi Liz: You're right that shortening and margarine are unhealthy because they contain trans fats. But it is not because we cannot digest these. In fact, the bacteria in your gut probably thrive on them. The problem is that trans fats break down at high temperatures (such as the temperatures in your oven) into incredibly unhealthy constituents (think free radicals). These do things such as cause DNA breakage and mutation in your cells. Certain oils also do this if heated at high temps (such as olive oil). So for pie making, I'll probably stick to butter, lard or an oil (such as canola, sunflower or safflower) that can withstand high temps.

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