The short answer is Yes! They will age you faster than you can say Jack Robinson and are a major contributor to various diseases. Here is why:

Dietary fats are divided into three main categories: monounsaturated fatty acids (MUFAs), polyunsaturated (PUFAs) and saturated fatty acids (SFAs).

Most oils contain a mixture of the various types of fatty acids but have one that is dominant, and they are therefore categorised under that type. For example, olive oil is mainly made up of MUFAs but also contains some PUFAs, thus it is categorised as a monounsaturated oil.

Vegetable oils have been touted as healthy and are often used for frying but should never be heated because most of them are extremely unstable and will oxidise when heated. 

The omega-6 to omega-3 ratio

Vegetable oils are mostly made up of polyunsaturated fatty acids (PUFAs).

PUFAs are further divided into omega-6 and omega-3. The body needs a healthy omega-6 to omega-3 ratio, ideally around 2:1 or 1:1. A ratio higher than 4:1 is likely to result in some kind of health problem. Most vegetable oils contain high levels of omega-6, whereas omega-3s are found in fish oil and flaxseed oil.

The sad truth is that the Western diet (processed foods) and the promotion of vegetable oils as a healthy substitute for saturated fats has created a ratio of 15 omega-6 to 1 omega-3 in many people. This high amount of omega-6 is promoting various diseases like cancer, cardiovascular disease, autoimmune and inflammatory diseases. To improve our health we will need to reduce omega 6 and increase our intake of omega 3 (fish+flaxseed oil).

Why do high levels of omega-6 cause disease?

Omega-6 causes pro-inflammatory actions, and omega-3 creates an anti-inflammatory response in the body. We need a bit of both to be healthy because omega-3 helps fight inflammation, and omega-6 is needed to create an inflammatory response that can fight infection and injury. When the omega-6 to omega-3 ratio is out of whack and omega-6 levels are overly excessive, it will contribute to unnecessary inflammation and disease.

Vegetable oils cause aging and inflammation

Vegetable oils are chemically unstable oils that easily oxidiee if exposed to light, heat or oxygen. 

Because vegetable oils have been extracted using chemicals at high heat, the vegetable oils that you buy at the supermarket are already rancid or partly oxidised, especially if they come in a clear bottle, in which case they have been exposed to both heat, oxygen and light.

We all know we want ANTI-oxidants, not oxidants. Vegetable oils are oxidants to your body, meaning they create free radicals that cause aging, inflammation and eventually disease.

Studies are now pointing towards vegetable oils as the villain in heart disease because they cause oxidation and inflammation. It is the oxidation of LDL cholesterol (especially the small, dense LDL) that is the culprit, not LDL itself. Antioxidants and healthy fats are protective against small, dense LDL oxidation.

Vegetable oils are toxic

Vegetable oils contain harmful chemicals and are highly processed. The chemical hexane is used to extract the oils during processing, and if you buy a non-organic variety of vegetable oils they are likely to contain pesticides and be genetically modified.

If you have to use vegetable oils, make sure they are organic and cold-pressed or expeller-pressed, and only use small amounts to keep your omega-6 to omega-3 ratio in check.

In addition to the above, many vegetable oils contain trans fatty acids or trans fats (TFA). TFAs have been deemed as unsafe by the FDA in the US and by FSANZ (Food Standards AU/NZ)

“In June 2015 the US Food and Drug Administration (US FDA) stated that partially hydrogenated oils (the primary source of manufactured TFAs) are no longer “generally recognised as safe”.

“There is strong evidence that TFAs increase the amount of ‘bad’ low-density lipoprotein (LDL) cholesterol in our blood, a major risk factor for coronary heart disease. Also, TFAs may decrease the levels of ‘good’ high-density lipoprotein (HDL) cholesterol in blood.” (Link) 

Unfortunately, In Australia, it's not compulsory for the amount of TFAs to be included on a food product's nutrition panel, therefore it can be hard to know if a food contains TFAs.

The foods that tend to be high in trans fats are microwave popcorn, commercial cakes and biscuits, crackers, croissants, fast foods, deep-fried foods and vegetable oils 

Vegetable oils to avoid:

• Safflower oil
• sunflower oil
• canola oil (modified rapeseeds)
• corn oil
• cottonseed oil
• peanut oil
• soybean oil
• rice bran oil
• grapeseed oil (can be ok in small amounts if cold pressed and organic)

 The Oils that are good for you

Use coconut oil and ghee for high-temperature frying. For lower temperature frying and baking you can also use olive oil and avocado oil. I use butter for baking and on vegetables post-cooking. The below oils I use sparingly on salads or post-cooking to add flavour:

Sesame oil is unstable (high PUFA content) but can make or break an Asian dish, so add it after cooking to avoid its oxidation from the high heat, and use it only occasionally due to its relatively high omega-6 content. Sesame oil is high in antioxidants, which may help reduce oxidation.

Walnut oil is very lush and it works a treat in salads and desserts — but use sparingly as it is high in omega-6. Use it only in cold dishes.

Both oils should be kept in a cool and dark place, and I would recommend discarding it after 5-6 weeks to avoid ingesting the oil in an oxidised state. Make sure these oils are cold or expeller pressed and packaged in dark bottles to protect them from being oxidised by the light and

References:

Simopoulos, A., P. (2006). Evolutionary aspects of diet, the omega-6/omega-3 ratio and genetic variation: nutritional implications for chronic diseases.

Biomedicine & Pharmacotherapy, 60(9). 502-507

Simopoulos, A. P. (2002). The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomedicine & Pharmacotherapy, 56(8), 365-379

Ramsden, C., E.  Zamora, D., Leelarthaepin, B., Majchrzak-Hong, S. F., Faurot, K, R, Suchindran, C. M., . . . Hibbeln, J. R. (2013). Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis

BMJ, 346:e8707

doi: http://dx.doi.org/10.1136/bmj.e8707

 Chowdhury, R., Warnakula, S., Kunutsor, S., Crowe, F., Ward, H. A., & Johnson L. (2014). Association of Dietary, Circulating, and Supplement Fatty Acids With Coronary Risk: A Systematic Review and Meta-analysis. Ann Intern Med. 2014, 160, 398-406.

FSANZ: https://www.foodstandards.gov.au/consumer/nutrition/transfat/Pages/default.aspx