"I've been reading your blog for a while, and more and more I've been buying into the nutrient dense food paradigm. I was wondering, perhaps you would never deep-fry a sweet potato; but if you did, which oil would you use? I think I've heard that peanut oil is a high-heat oil, but how bad is it for you to eat?"I don't do much deep-frying, but it has nothing to do with health and trying to avoid oil and/or fat. Mostly, I don't want to deal with the mess and disposal of the cooking oil.
However, when I do deep-fry—maybe once a year, and usually French fries or calamari—I'll employ very high-quality (organic, plus mechanically, not solvent, extracted) peanut or canola oil. These oils are relatively tasteless but have high smoke points, which is needed when deep-frying.
Better—as Rodale News suggests—are lard and duck fat, but, to be honest, I haven't been as vigilant as I should be about getting them because I deep-fry so infrequently. Actually, that's probably a good reason to buy lard or duck fat!
As I've discussed before, try to avoid the popular commercial brands of vegetable oils, which have been treated with solvents and come from genetically-engineered and/or pesticide-laden plants. In that light, Planters and Crisco peanut oil, I would argue, are very, very bad for you. Spectrum organic peanut oil would be a better choice.