I know that partially hydrogenated oils are unhealthy. What about "fully hydrogenated" oils?
Fully hydrogenated oils are not as bad as partially hydrogenated, though more research is needed to better understand their health effects.
The two kinds of oil are very different. Food manufacturers hydrogenate liquid vegetable oils to make them more solid and shelf stable. In partial hydrogenation, the resulting fats are semi-solid at room temperature. In full hydrogenation, the oils become completely solid.
The problem with partially hydrogenated oils is that they contain trans fat, which raises LDL ("bad") cholesterol, lowers HDL ("good") cholesterol and has other harmful effects.
In contrast, fully hydrogenated oils, in essence, become saturated fats—but they contain no trans fat. And the type of saturated fat typically produced is thought to have no significant effect on cardiovascular risk.
The story doesn’t end there, however. Fully hydrogenated oils are being used as a supposedly healthier replacement for partially hydrogenated oils. But food companies often blend fully hydrogenated oils with liquid vegetable oils and put them through a process called interesterification. This changes the structure of the oil so that it performs like a partially hydrogenated oil without the trans fat. Sounds great, but we don't yet know whether interesterified fats might have their own adverse health consequences.
Read the ingredients list. If you see "partially hydrogenated oil," that means some trans fat is present, even if the label says "0" trans fat, which is allowed if a serving contains less than 0.5 grams. Some products, such as Crisco All-Vegetable Shortening, contain both partially and fully hydrogenated oils. If the label just says "hydrogenated" oil, you don't know if it's fully or partially hydrogenated. Moreover, you can't always tell from the label if a fully hydrogenated oil has been interesterified.
All this is more reason to limit or avoid foods that contain any type of hydrogenated oil. These foods—often baked sweets and snack foods—tend not to be healthy choices anyway.
Source: Originally published in The University of California, Berkeley Wellness Letter (October 2011)