THE SCIENCE OF COOKING WITH OLIVE OIL.

 

 By Mary Flynn, PhD, RD, Associate Professor of Clinical Medicine at Brown University’s Alpert Medical School in Providence, Rhode Island. 

Source: "Truth in Olive Oil".

Abstratc: She shares some vital advice about cooking with extra virgin olive oil, based on her several decades of oil-related research and clinical trials. She also explodes some common olive oil myths, including the bizarre but persistent "don't cook with extra virgin olive oil" fallacys.

Eating and cooking with olive oil

I started to develop my plant-based olive oil diet in 2000, primarily including foods that would improve health and control weight.  I saw extra virgin olive oil as a central part of this diet for several reasons.  I knew it would improve health, but I also thought that using olive oil might increase vegetable intake – another key to good health – since it makes vegetables taste so much better.  Also, I believed that using olive oil at meals would help people to eat less, because including fat in a meal makes the meal more satisfying and delays the onset of hunger after the meal.  All fats do this, but olive oil is the best choice because it's a very healthy fat.

Extra virgin olive oil is the juice of the olive fruit.  Like most unprocessed plant products, it contains a range of health-promoting phytonutrients.  The phytonutrients in olive oil have been shown to decrease the oxidation of LDL (1) (which would lower heart disease risk) and DNA (2) (which would lower cancer risk).  Other phytonutrients in olive oil have been shown to decrease blood levels of glucose and insulin (3); decrease blood pressure (4, 5); decrease blood coagulation (6); and decrease inflammation (7).  This makes extra virgin olive oil very different from seed oils, like soybean, safflower, corn, and canola oil, which have undergone a refining process that destroys phytonutrients.  Refined olive oil, typically labeled “olive oil” in stores, also lacks phytonutrients.  (Take-away point:  buy only extra virgin olive oil!)

 

I recommend the use of at least 3 tablespoons (45 ml) of extra virgin olive oil per day, most of which you’ll use to cook vegetables.  In the studies cited previously, the health benefits of extra virgin olive oil consumption started at about 2 tablespoons a day, so using 3 tablespoons will ensure that you get these health benefits, and will likely increase your vegetable intake as well.  The use of olive oil to prepare vegetables greatly improves the taste of vegetables, particularly those that are naturally bitter, like many leafy greens, or those that contain sulfur (the brassica family – broccoli, Brussel sprouts, cabbage, cauliflower, kale).  I recommend 1 tablespoon of olive oil per cup of vegetables.  In addition, brassica vegetables contain a phytonutrient family (glucosinolate) that has been shown to be cancer protective (8), but it is water soluble (9), so boiling or steaming these vegetables means you will not get the health benefits from this phytonutrient, since it will be lost in the cooking water.  Cooking the brassica family in extra virgin olive oil therefore means that you get both the health benefits of olive oil itself and of the cancer-fighting phytonutrients in the vegetables.  Plus using olive oil will make them taste so much better than boiling or steaming!

In the 12 years that I’ve employ my diet in my work, I have found, just as I’d hoped, that when my patients and study participants use more olive oil, they consume more vegetables (10, 11), a big plus for health.  In addition, when they report using more than 3 tablespoons of olive oil a day – for example when summer vegetables are plentiful – they are still losing weight.  I would guess there is some upper limit to how much you extra virgin olive oil you can consume before you start gaining weight, but I haven’t yet found it.  In fact, I tell patients that they can never eat too much extra virgin olive oil.  Sure, it’s a fat, but the benefits of extra virgin olive oil clearly outweigh its drawbacks – olive oil will improve your health.

Olive oil and vegetables

Vegetables, like olive oil, have long been associated with improved health.  However, studies looking at the consumption of vegetables do not consistently show decreases in chronic diseases.  There are at least as many studies showing no benefit as those indicating some disease protection from frequent consumption of vegetables.  I believe this inconsistency may be attributable to the type of vegetable consumed in the studies, and to the way the vegetables are prepared.  Certain plant products are clearly more health-protective than others, due to the phytonutrients they contain.  Some of the healthiest, vegetables, which I recommend to my patients, include:  all the brassica family – broccoli, Brussel sprouts, cabbage, cauliflower, kale; any vegetable with deep color, like carrots, peppers, winter squash; and anything leafy and green – spinach, collard greens, etc.  I also recommend the use of frozen vegetables; in fact, I prefer them nutritionally over fresh as in many cases they are kept on the plant longer than what one would buy in a grocery store, so their phytonutrient is likely higher and they have the same vitamin content as store-bought fresh vegetables (12, 13).

Low cost of meals that contain olive oil

I think it’s wrong, when considering the cost of olive oil, to consider it as another vegetable oil and compare its cost to theirs.  Vegetables oils are chemically extracted from seeds and nuts and do not have health benefits remotely comparable to extra virgin olive oil, which is physically extracted from a fruit – the olive.  If properly made, extra virgin olive oil is rich in many components that research has shown to produce manifold health benefits, and to be associated with a lower rate of chronic diseases.  What’s more, many vegetable oils are high in polyunsaturated fats, which readily oxidize.  I think a case could be made that vegetable oil consumption may actually increase your risk of chronic diseases.

Apart from the considerable health benefits of extra virgin olive oil, when you work out the cost of olive oil per tablespoon, it's actually quite inexpensive.  A 17 fl. oz bottle of olive oil contains approximately 32 tablespoons, and a liter contains 64 tablespoons.  Many extra virgin olive oils cost less than 30 cents per tablespoon, so my recommended dose of 3 tablespoons of oil per meal costs 90 cents.  A health-promoting meal with plenty of olive oil therefore costs substantially less than one with meat, poultry or seafood – foods which in any case won’t improve your health. 

“Raising the Bar on Nutrition” (RTB) is a program I developed for the Rhode Island Community Food Bank, after participants in my early research into my plant-based olive oil diet observed how inexpensive my diet was to follow.  The recipes for RTB are based on my diet, but use foods commonly found in most food pantries, which are locations where low-income people can get free food.  The average cost of my RTB recipes is $1.07 per serving (December 2011 pricing in Rhode Island), and all recipes include 1 to 2 tablespoons of olive oil per serving.  RTB was tested as a research protocol using a 6-week cooking program for food pantry clients.  Participants in the program had significant decreases in food insecurity, food costs, and body weight (10).

Canola oil is not a substitute for olive oil

Speaking of vegetable oils, I’m often asked about canola.  U.S. health officials originally linked the health benefits of olive oil to its monounsaturated fat content, but since they didn’t think Americans would accept a dietary oil with pronounced flavor, like olive oil has, researchers at the University of Manitoba, Canada developed canola oil, which was naturally bred from the rapeseed plant  (“canola” stands for “Canadian oil, low acid”).  Canola oil is higher in monounsaturated fat than other seed oils, though not as high as olive oil, so the health benefits related to its lipid profile will be less than olive oil.  Additionally, canola lacks numerous components contained in extra virgin olive oil which, as we’ve mentioned above, are linked to a range of health benefits.  In fact, there are no studies showing health benefits from using canola oil.  Bottles of canola oil often say “contains omega 3 fats”, which is true, but not the form of omega 3 that is known to improve health.  Canola oil contains an 18-carbon form of omega 3, while health benefits have only been demonstrated with 20- and 22-carbon omega 3s.  Humans convert only small amounts of the 18-carbon to the 20 carbon omega 3 compound (EPA) and even less to the 22- carbon compound (DHA) (14, 15), so the health claims concerning canola oil’s omega 3s may be misleading.

Olive oil and cooking

Nearly every time I lecture on olive oil, people ask whether heat destroys the oil, and whether they can cook with it.  I don’t know who invented this misconception (seed-oil companies?), but I’d love to dispel it once and for all.  High quality extra virgin olive oil can be heated to 420°F before it reaches smoke-point (ie begins to smoke and starts to form unhealthy compounds), which is higher than nearly every other vegetable oil.  Olive oil is much more stable when heated compared to most vegetable oil (16, 17).  Cooking with olive oil below the smoke-point does not destroy most of its health benefits, or make it less healthy – under normal cooking conditions, most of the therapeutic minor components are retained (18-20).  Some studies have subjected olive oil to high temperatures (180°C/356°F) for long periods of time (from 90 minutes to over 20 hours).  These conditions do tend to decrease the content of some phytonutrients, yet even under such extreme conditions, some phytonutrients remain (21).

This said, it is true that heating olive oil can modify or impair some of the flavor.  So at least from a culinary point of view, and depending on your personal taste, it’s usually better to add high-quality olive oils to finish a dish, after cooking.

I’m also frequently asked if heating olive oil produces trans fats.  Here again, the answer is No.  Trans fats are created in an industrial process that adds hydrogen to the liquid oil (partial hydrogenation), making it semi-solid.  This is how margarine is made (or at least was made, before trans fat were recognized as a health issue).  Ordinary cooking, even at high heat, never produces trans fats.

Milder extra virgin olive oil is excellent in baking, especially since heating seems to decrease the bitter taste in some olive oils (20).  I tell people to use an extra virgin oil like California Olive Ranch, which is relatively delicate in flavor, as well as economical.  (A useful book on choosing different olive oils is The flavors of olive oil. A tasting guide and cookbook by Deborah Krasner (Simon and Schuster, New York 2002), which classifies oils according to taste - delicate and mild, fruity and fragrant, leafy and grassy, peppery - and also lists some oils by country of origin.)  The oil gives muffins, quick breads, and even cookies an excellent texture, without any noticeable olive flavor.  In fact, many of my patients tell me that my olive oil-based recipes for muffins and quick breads have a better texture than other recipes they've used which call for vegetable oil or margarine.  In short, you can use extra virgin olive oil in any recipe that calls for liquid oil.

 

Footnotes

 

1. Covas MI, Nyyssonen K, Poulsen HE, et al. The effect of polyphenols in olive oil on heart disease risk factors: a randomized trial. Ann Intern Med 2006;145:333-41.
2. Salvini S, Sera F, Caruso D, et al. Daily consumption of a high-phenol extra-virgin olive oil reduces oxidative DNA damage in postmenopausal women. Br J Nutr 2006;95:742-51.
3. Madigan C, Ryan M, Owens D, Collins P, Tomkin GH. Dietary unsaturated fatty acids in type 2 diabetes: higher levels of postprandial lipoprotein on a linoleic acid-rich sunflower oil diet compared with an oleic acid-rich olive oil diet. Diabetes Care 2000;23:1472-7.
4. Ferrara LA, Raimondi AS, d'Episcopo L, Guida L, Dello Russo A, Marotta T. Olive oil and reduced need for antihypertensive medications. Arch Intern Med 2000;160:837-42.
5. Fito M, Cladellas M, de la Torre R, et al. Antioxidant effect of virgin olive oil in patients with stable coronary heart disease: a randomized, crossover, controlled, clinical trial. Atherosclerosis 2005;181:149-58.
6. Ruano J, Lopez-Miranda J, de la Torre R, et al. Intake of phenol-rich virgin olive oil improves the postprandial prothrombotic profile in hypercholesterolemic patients. Am J Clin Nutr 2007;86:341-6.
7. Beauchamp GK, Keast RS, Morel D, et al. Phytochemistry: ibuprofen-like activity in extra-virgin olive oil. Nature 2005;437:45-6.
8. Kim MK, Park JH. Conference on "Multidisciplinary approaches to nutritional problems". Symposium on "Nutrition and health". Cruciferous vegetable intake and the risk of human cancer: epidemiological evidence. Proc Nutr Soc 2009;68:103-10.
9. Fowke JH, Longcope C, Hebert JR. Brassica vegetable consumption shifts estrogen metabolism in healthy postmenopausal women. Cancer Epidemiol Biomarkers Prev 2000;9:773-9.
10. Flynn MM, Schiff AR. A Six-week Cooking Program of Plant-based Recipes Improves Food Security, Body Weight, and Food Purchases for Food Pantry Clients. Journal of Hunger & Environmental Nutrition 2013;1.
11. Flynn MM, Reinert SE. Comparing an olive oil-enriched diet to a standard lower-fat diet for weight loss in breast cancer survivors: a pilot study. J Womens Health (Larchmt);19:1155-61.
12. Rickman JC BC, Barrett DM. Nutritional comparison of fresh, frozen, and canned fruits and vegetables II. Vitamin A and carotenoids, vitamin C, minerals and fiber. Journal of the Science of Food and Agriculture 2007;87:1185-1196.