Dawn asks, "Are the fully hydrogenated vegetable oils any better for you than the partially hydrogenated vegetable oils? Why does hydrogenation make oil bad?"
Fully hydrogenated oils are probably better for you than partially hydrogenated oils, but I wouldn't say they're good for your health -- maybe less dangerous is a better way to put it -- because they'e not trans fats.
Trans fats raise your LDL cholesterol levels (the bad kind) and lower your HDL cholesterol levels (the good kind) at the same time. Regularly eating trans fats raises your risk of developing heart disease and stroke. It’s also linked to developing type 2 diabetes.
But the fact that fully hydrogenated oils aren't trans fats is the only health difference that I can find. Both forms are high in calories (all fat has 9 calories per gram) and if you find them anywhere, it's probably going to be in heavily processed foods that aren't good for you anyway.
What's Hydrogenation Again?
Hydrogenation is the process by which hydrogen is forced into heated vegetable oil by using a catalyst such as nickel. Forcing hydrogen into the oil changes the chemical structure from a liquid into a more solid shape. Fully hydrogenating the oils makes them solid, similar to the saturated fats found in meat. Partial hydrogenation leaves the oil with softer buttery consistency.
Food manufacturers may use partially hydrogenated oil in processed foods, baked goods and stick margarine. It lasts longer than regular oil and gives pastries their texture.
Partially hydrogenated oils have fallen out of favor due to the trans fats that are created by the hydrogenation process. The sad part is that partially hydrogenated vegetable oils were originally thought to be a healthier alternative to saturated fats, some of which are associated with cardiovascular disease risk. But it turns out that trans fats are even worse than saturated forms.
They're bad enough that trans-fats are required to be listed on Nutrition Facts labels and their use has been banned in some places.
Why Fully Hydrogenated May Be Better
Fully hydrogenated oils are much more like stearic acid, which is a less harmful form of saturated fat. Stearic acid doesn't raise LDL cholesterol levels and it's fairly stable for kitchen use.
The problem is that fully hydrogenated oils are solid and waxy so they're difficult to use. They can be blended with polyunsaturated oils like soy and sunflower oils through a process called interesterification to improve the texture and soften it up a bit. The problem is that research isn't clear on how these interesterified fats will impact cholesterol levels and cardiovascular disease risk.
So I'm hesitant to say that consuming fully hydrogenated oils are somehow good for you -- at least until we know more about interesterified fats. It's probably best to avoid all types of hydrogenated oils to be safe.
American Heart Association. "Frequently Asked Questions About "Bad" Fats." Accessed July 5, 2014. http://www.heart.org/HEARTORG/General/Frequently-Asked-Questions-About-Bad-Fats_UCM_306349_Article.jsp.
American Heart Association. "Trans Fats." Accessed July 5, 2014. http://www.heart.org/HEARTORG/GettingHealthy/FatsAndOils/Fats101/Trans-Fats_UCM_301120_Article.jsp.
Harvard School of Public Health. "Shining the Spotlight On Trans-Fats." Accessed July 5, 2014. http://www.hsph.harvard.edu/nutritionsource/nutrition-news/transfats/.
Hayes KC1, Pronczuk A. "Replacing trans fat: the argument for palm oil with a cautionary note on interesterification." J Am Coll Nutr. 2010 Jun;29(3 Suppl):253S-284S.