• Hydrogenation is the process of passing hydrogen bubbles through an oil by heating it. If an oil is completely hydrogenated, then it becomes a solid fat, like lard. However, it is easier to hydrogenate an oil than it is to buy solid fats. This is why hydrogenated oils are present in many processed foods.
  
  

Efficiency

• Hydrogenated oil does not naturally occur. This means that your body cannot process it as efficiently as it can process animal fat or fat that comes from nuts or oil. It has not developed the proper means to do so as it did with natural fats millions of years ago. Because it cannot process hydrogenated oil as efficiently, your body need more of it than it would natural fat. This means that you need to eat more hydrogenated oil to get full, and subsequently gain weight faster.
  
  

Disease

• According to Treelight.com, there is also a correlation between hydrogenated oils and a number of diseases. One particularly attention-grabbing example is multiple sclerosis, but there is also a correlation between hydrogenated oils and milder conditions like arthritis.
  
  Hydrogenated oils also tend to come in foods that are generally unhealthy. This means that eating a lot of them has the same results as an unhealthy lifestyle. According to the Harvard Gazette, this can lead to heart disease, diabetes and a higher risk of cancer.
  
  

Deep Frying Risks

• Finally, there is the presence of hydrogenated oils in deep fried foods. Obviously, deep frying is already unhealthy because of the sheer amount of fat. If you deep fry in something natural like butter or lard it hydrogenates a little bit because of the heating involved.
  
  If you deep-fry something in partially hydrogenated oil (as almost every fast food establishment does), the oil becomes more hydrogenated in addition to any natural fats becoming hydrogenated. This means that something your body already had a hard time efficiently processing becomes even harder to process.