Does reusing veggie oil in a deep fryer make hydrogenated oils?

No, frying does not cause hydrogenation to occur in cooking oil. Hydrogenation is the process of bubbling hydrogen gas up into an oil to make it more heat stable, including stable during frying. We know that hydrogenation of oils increases our risk of health problems and is desirable to avoid. But exposing oil to high heat is also problematic, because the higher the heat, the more quickly the oil will oxidize. Oxidation of oils means damage to their structure in such a way that they start to become rancid. Along with the oxidation of oils at high heat, many important nutrient compounds are also damaged, including vitamins like vitamin E.

There is one technical change that I should also mention here. Frying oil causes some of unsaturated fatty acids in the oil to polymerize (combine together), and so the ratio of saturated-to-unsaturated fat can increase due to frying. Sometimes people confuse this process with hydrogenation, since hydrogenation also involves an increase in the saturated-to-unsaturated fat ratio. But in a practical sense, what’s bad about frying is that frying damages the oil and depletes the oil of significant nutrient value.

I do not recommend reusing oil. The oil you are reusing is nutritionally depleted oil that does not bring the nutritional benefits to your body that your body is looking for in a food. The oxidatively damaged molecules in the oil have to be processed by your body, and that processing will use up nutrients and energy. Even though I do not recommend oil-based frying in the first place, if you are going to oil-fry, I definitely encourage you to start with fresh oil each time. Also, you may want to purchase conditioned and refined oils, including high-oleic versions of oils, to decrease the degree of oxidation during frying.

Olive oil is a largely monounsaturated oil. Monounsaturates undergo less heat damage than polyunsaturates, and so olive oil is more stable to heat in this context. However, olive oil, especially extra virgin, cold-pressed oil, is also rich in polyphenols that are highly health-supportive but highly sensitive to heat. Frying will destroy most of these polyphenols. For this reason, I don’t recommend frying in olive oil. I would generally recommend high-oleic versions of sunflower or safflower oil, or refined avocado oil, if a person intends to include frying in his or her meal preparation.