THE BASICS: What is the Problem With Added Oils?

The use of oils in or on our food has become very confusing.  First we are told to cut down on oils and fats to keep us healthy and prevent weight gain.  More recently headlines declare that fats and oils are in fact good for us and we should eat all we want of them.  Is there any way to find out the truth?  Of course there is.  In spite of the erratic messages being sent out in our news broadcasts, science keeps plugging away in the background, and we need only look at the huge volume of well-performed studies on this subject to find the truth in the evidence.

To start, let’s skip to the end of the story.  You can get all the healthy fats you need from whole foods such as nuts and nut butters, seeds and avocados.  When it comes to added oil, none is best, or at least as little as possible.  Even the much revered olive oil is problematic.  I’ll try to explain why.


Eating oils (including olive oil) inhibits the dilation of your blood vessels causing restricted blood flow

Ingestion of vegetable oils harms the endothelium, that important single layer of cells that lines your arteries.  (See “The Basics: What Does Eating Plants Do For Your Blood Vessels?”)  Blood vessels stiffen for several hours after eating a high fat meal which means that they will not easily relax and dilate. This results in compromised blood flow through the arteries.  If one fatty meal follows another, the blood vessels can remain inflexible for long periods of time.  A host of studies have illuminated the effect of different fats on endothelial function.  For instance, subjects in one study ate five meals with differing sources of fat – olive oil, canola oil, salmon, olive oil plus antioxidant vitamins (vitamins C and E) and olive oil plus antioxidant food (balsamic vinegar and salad).  Three hours after eating each meal blood flow was measured using a technique called FMD (brachial artery flow mediated vasodilation).  The only meal that restricted blood flow was the one using olive oil alone as the fat source and it caused a 31% reduction in FMD (1).  Another study showed that the addition of walnuts to a fatty meal significantly improved artery function while the addition of olive oil had no effect.  In this study both walnuts and olive oil reduced oxidation and inflammation (2).  In 2007 a study compared the effect of ingestion of olive oil, soybean oil or palm oil.  All three oils reduced endothelial function (FMD decrease of about 32%).  This effect happened whether the oil was fresh or reused (3).  Another study illustrated that olive oil caused only marginally less damage to arteries than the considerable endothelial damage caused by a hamburger and fries or a sausage and egg breakfast sandwich at one of our ubiquitous fast food take-outs (4).

Extra virgin olive oil may perform somewhat better.  It retains some of the antioxidants found in whole olives and doesn’t appear to increase inflammation as much as regular olive oil does.  As a result ingestion of extra virgin olive oil has shown a slight increase in endothelial function compared to butter, which considerably impairs endothelial function for as much as 6 hours after a meal. (13)  However, analysis of studies looking at total olive oil consumption and its relationship to major cardiovascular events, and that control for healthy dietary habits such as vegetable intake, reveal that neither regular olive oil nor extra virgin olive oil results in significant reductions in heart attacks or stroke (14).

Some studies have shown that simply eating a Mediterranean diet rich in olive oil can improve endothelial function (24).  This effect is thought to be due to the abundant antioxidants that are found in the fruits and vegetables that are a part of this diet.  Even the simple addition of vegetables to a fatty meal can partially restore arterial function (25).  The beneficial components of a Mediterranean diet may stem from its antioxidant-rich foods and any subsequent health improvements may be in spite of, rather than because of, the olive oil (1).


Eating oils (including olive oil) contributes to the formation of artery-clogging atherosclerotic plaques  

Atherosclerosis is the build-up of fatty deposits in the walls of arteries.  Though this can happen anywhere in the body, it is of particular concern when it occurs in the coronary arteries, those vessels that supply the heart with oxygen.  Over years these plaques increase in size, narrowing the path through which the blood flows.  When a plaque ruptures, a heart attack or stroke can be the devastating result.

One study showed that men eating diets lower in fat formed eight times fewer atherosclerotic lesions than men eating the typical Western diet (which contains about 33% of calories from fat).  In the same study only three types of fat significantly increased the likelihood of new atherosclerotic lesions.  These fats were lauric acid (a saturated fat found in coconut and palm kernel oils), oleic acid (found in olive oil, chicken, pork and baked goods), and linoleic acid (found in vegetable oils and animal meats) (5).  The PREDIMED study followed thousands of people at high risk of heart disease for several years.  The subjects were put into three random groups.  All groups were eating a basic Mediterranean type diet.  Group one added extra virgin olive oil; group two added nuts; group three were told to cut down on fat.  (This third group did not cut down on fat, consequently their role became that of a control group with no dietary changes).  At the end of the study the “control” group showed significant worsening of carotid artery thickening and plaque.  The added olive oil group showed no significant changes.  However, the added nuts group showed significant reversal in artery thickening and a complete stop in plaque progression. (6)

On an additional favourable note, studies on diets very low in total fat and cholesterol have shown that atherosclerosis can not only be prevented but atherosclerotic plaques will shrink and atherosclerosis can be reversed (18,22,23).


Added plant oils (isolated from their whole plant sources) promote cancer growth more than does saturated fat

Research suggests that pure polyunsaturated fatty acids (such as those found in oils extracted from plants and fish) are more susceptible to oxidation than saturated fat.  Oxidation produces “reactive oxygen species” known to increase cancer.  Research on humans confirms the link between oils and cancer (8,9,19,20,21).  In large populations of people, worldwide studies show that the lower the intake of all fats, the less the risk of many cancers including breast, colon and prostate cancers.  (10,11,12)


Oils are very high in fat and not helpful in losing weight or maintaining a healthy weight

Using olive oil as an example, every tablespoon contains 13.5 grams of fat and 120 calories.  The people of Greece today eat the modern Mediterranean diet and obtain 55% of their daily fat from olive oil.  They have the highest obesity rates in Europe (7).  In addition, study of a free-living Greek population found no associations between total olive oil intake and all-cause mortality (15).


Oils are processed foods, virtually devoid of most nutrients (although they do contain Vitamins E & K)

All plant matter contains oil as a part of its whole food package and a plant-based diet is more than capable of supplying all the healthy fats you need for good health and vigour.  Oil found within plants is bundled up with fiber, antioxidants, phytonutrients, vitamins and minerals; ingredients which provide protection against the direct impairment in endothelial function and inflammation that pure oils can cause (1,16). In the natural plant form oil is released slowly to the body in contrast to the overwhelming flood of fat and calories that comes from eating extracted oils.  The bonus is that the accompanying health-giving ingredients that all plants offer will be absorbed right along with the oil.


There are only two essential fatty acids and these can be readily obtained from plants in the diet

The body can synthesize most of the fats it needs from the diet. Two oil types are called “essential” fatty acids because they cannot be synthesized in the body and must be obtained from food.  These are  alpha-linolenic acid (ALA) and linoleic acid (LA) and they are used to build specialized fats called omega-3 and omega-6 fatty acids (17).   However, the need for both of these fatty acid types is very small and our body can easily obtain much more than the required amount from plants in the diet.  More detail on the topic of omega-3 and omega-6 will be covered in a separate article.


There is absolutely no reason to use oils for cooking

A small amount of water, vegetable broth, apple juice or wine will keep food from sticking to pans.  For healthy baked items, oil can be replaced with applesauce, ground flax, prune puree and other substitutes.


The take home message

Added oils are harmful and unnecessary.  Let the whole plant foods you eat provide you with all the healthy fats that you need for a healthy, vigorous life.  You will reap the benefits.



1   Vogel, R.A., Corretti, M.C., Plotnick, G.D.  The postprandial effect of components of the Mediterranean diet on endothelial function.  J Am Coll Cardiol. 2000 Nov 1; 36(5):1455-60.

2  Cortés, B., Núñez, I., Cofán, M., Gilabertm R. et al.  Acute effects of high-fat meals enriched with walnuts or olive oil on postprandial endothelial function.  J Am Coll Cardiol. 2006 Oct 17; 48(8):1666-71.

3   Rueda-Clausen, C.F., Silva, F.A., Lindarte, M.A., Villa-Roel, C. et al. Olive, soybean and palm oils intake have a similar acute detrimental effect over the endothelial function in healthy young subjects.  Nutr Metab Cardiovasc Dis. 2007 Jan; 17(1):50-7.

4  Vogel, R.A. Brachial artery ultrasound: a noninvasive tool in the assessment of triglyceride-rich lipoproteins. Clin Cardiol. 1999 Jun;22(6 Suppl):II34-9.

5  Blankenhorn, D.H., Johnson, R.L., Mack, W.J. et al.  The influence of diet on the appearance of new lesions in human coronary arteries.  JAMA. 1990 Mar 23-30;263(12):1646-52.

6   Sala-Vila, A., Romero-Mamani, E.S., Gilabert, R. et al.  Changes in ultrasound-assessed carotid intima-media thickness and plaque with a Mediterranean diet: a substudy of the PREDIMED trial. Arterioscler Thromb Vasc Biol. 2014 Feb; 34(2):439-45.

7 Calle-Pascual, A.L., Saavedra, A., Benedi, A., Martin-Alvarez, P.J. et al. Changes in nutritional pattern, insulin sensitivity and glucose tolerance during weight loss in obese patients from a Mediterranean area. Endocrinologia y Nutricion, Hospital Universitario San Carlos, Madrid, Spain. Horm Metab Res. 1995 Nov; 27(11):499-502.

8  Griffini, P., Fehres, O., Klieverik, L., Vogels, I.M. et al.  Dietary omega-3 polyunsaturated fatty acids promote colon carcinoma metastasis in rat liver. Cancer Res. 1998 Aug 1; 58(15):3312-9.

9 Coulombe, J., Pelletier, G., Tremblay, P.,  Mercier, G.,  Oth, D.  Influence of lipid diets on the number of metastases and ganglioside content of H59 variant tumors.  Clin Exp Metastasis. 1997 Jul; 15(4):410-7.

10   Carroll, K.K. Experimental evidence of dietary factors and hormone-dependent cancers. Cancer Res. 1975 Nov; 35(11 Pt. 2):3374-83.

11  Weisburger, J.H., Worldwide prevention of cancer and other chronic diseases based on knowledge of mechanisms. Mutat Res. 1998 Jun 18;402(1-2):331-7.

12 Rao, G.N.   Influence of diet on tumors of hormonal tissues.  Prog Clin Biol Res. 1996; 394:41-56.

13   Tentolouris, N., Arapostathi, C., Perrea, D., Kyriaki, D., Revenas, C., Katsilambros, N.. Differential effects of two isoenergetic meals rich in saturated or monounsaturated fat on endothelial function in subjects with type 2 diabetes. Diabetes Care. 2008 Dec; 31(12):2276-8.

14    Buckland, G., Travier, N., Barricarte, A., Ardanaz, E. et al. Olive oil intake and CHD in the European Prospective Investigation into Cancer and Nutrition Spanish cohort. Br J Nutr. 2012 Dec 14; 108(11):2075-82.

15  Trichopoulou, A., Costacou, T., Bamia, C., Trichopoulos, D. Adherence to a Mediterranean diet and survival in a Greek population. N Engl J Med. 2003; 348:2599–2608.

16  Rankin, J., Peairs, A.  Low Carbohydrate, High Fat Diet Increases C-Reactive Protein during Weight Loss.   Journal of the American College of Nutrition April 2007; 26(2):163-169.

17   Groff, J.L., Gropper, S.S., Hunt, S.M. Advanced Nutrition and Human Metabolism. New York: West Publishing Company; 1995.

18  Haskell, W.L., Alderman, E.L., Fair, J.M. et al.  Effects of intensive multiple risk factor reduction on coronary atherosclerosis and clinical cardiac events in men and women with coronary artery disease. The Stanford Coronary Risk Intervention Project (SCRIP).  Circulation. 1994; 89:975-990

19  Cai, F., Dupertuis, Y.M., Pichard, D. Role of polyunsaturated fatty acids and lipid peroxidation on colorectal cancer risk and treatments. Curr Opin Clin Nutr Metab Care. 2012 Mar; 15(2):99-106.

20  Azrad, M., Turgeon, C., Demark-Wahnefried, W. Current evidence linking polyunsaturated fatty acids with cancer risk and progression. Front Oncol. 2013 Sep 4; 3:224.

21 Bartsch, H., Nair, J.,  Owen, R.W.   Dietary polyunsaturated fatty acids and cancers of the breast and colorectum: emerging evidence for their role as risk modifiers.  Carcinogenesis (1999) 20 (12): 2209-2218

22  Ornish, D., Brown, S.E., Scherwitz, L.W., Billings, J.H. et al.  Can lifestyle changes reverse coronary heart disease? The Lifestyle Heart Trial.   Lancet. 1990 Jul 21; 336(8708):129-133.

23  Esselstyn, C.B., Ellis, S.G., Medendorp, S.V., Crowe, T.D.  A strategy to arrest and reverse coronary artery disease: a 5-year longitudinal study of a single physician’s practice.      J Fam Pract. 1995 Dec; 41(6):560-568.

24  Esposito, K., Marfella, R., Ciotola, M. et al.  Effect of a Mediterranean-style diet on endothelial dysfunction and markers of vascular inflammation in the metabolic syndrome: a randomized trial. JAMA. 2004 Sep 22;292(12):1440-6.

25  Esposito, K., Nappo, F., Giugliano, F., Giugliano, G., Marfella, R., Giugliano, D.   Effect of dietary antioxidants on postprandial endothelial dysfunction induced by a high-fat meal in healthy subjects.  Am J Clin Nutr. 2003 Jan;77(1):139-43.



Promoting a healthy adventurous lifestyle powered by plants and the strength of scientific evidence.

My name is Debra Harley (BScPhm) and I welcome you to my retirement project, this website. Over the course of a life many lessons are learned, altering deeply-rooted ideas and creating new passions.

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