Olives and olive oil – how good are they nutritionally?

"Eat to Live, Not Live to Eat"
John H Weisburger, 2000

The olive tree (Olea europaea) has been grown from ancient times for its fruits. It’s a very hardy species that thrives in a Mediterranean climate and consequently was extensively incorporated into the national cuisines of southern European countries. In more recent times it has gained a valued reputation as one of the important components of the healthy Mediterranean Diet (MD) that has been demonstrably effective in reducing the incidence of chronic diseases so prevalent throughout developed countries and increasingly elsewhere. Most fruit production worldwide (about 90%) is used to make olive oil for garnishing and cooking. High consumption is the norm in southern European countries. For example, in Greece, adults consume an average of 80g/d and in Spain, as a result of high olive oil intake, fats may represent over 40% of dietary calories. The following commentary is focussed only on the nutritional aspects of plant-based oils; their comparative organoleptic properties are quite subjective and strongly dependent on historical and national traditions and will not be considered here.

Although established as a revered and proven component of the MD, are there other plant-based oils that are equally beneficial for long term well-being or maybe even better? Does the attention given to distinctions between various virgin oils and other cheaper refined oils have any nutritional basis? Or is it more a case of olive oil users supporting what is a long-cherished culinary tradition in the Mediterranean basin based on availability in earlier times when globalised sourcing of foods was less feasible and there was little scientific knowledge comparing the relative merits of others?

Healthy diets recommended by world authorities should have 20-35% of total daily calories contributed by fats. It’s common knowledge that not all fats are equal. Trans fats should be avoided altogether or at least kept below 1%, and saturated fats which predominate in animal-based foods should be restricted to less than 7% of energy intake. The value of plant-based fats is that they have low saturated fats (apart from palm and coconut) and consist mainly of mono- and poly-unsaturated fatty acids (MUFA and PUFA respectively). Only two fat types are essential for humans – the polyunsaturated omega-6 and omega-3 oils (n-6 and n-3 resp) as we can synthesize all the rest from other nutrients. The problem in Western diets is to ensure sufficient n-3 intake to avoid deficiency symptoms and also contribute to long-term well-being, with 1-3 g/d commonly recommended (of which 10% can be the long chain n-3 marine oil types) as n-6 consumption is usually far in excess of needs; the preferable limit for these is less than 10% energy intake. Total fat intake in a healthy diet should be more than the 3g/d contributed by essential fats to enable, for example, absorption of fat-soluble vitamins and other phytonutrients from the gut food matrix. For many years it was thought that the main contribution of olive oil to the MD was the high concentration of MUFA (73%), principally as oleic acid. But more recently it has been found that non-fatty acid phytochemicals (polyphenols/antioxidants) also have an important role, maybe even the dominant one. This is a nutritional basis for distinguishing between virgin and more refined olive oils as the former have much higher antioxidant levels than the latter.

Oils with high levels of PUFA including the n-6 and n-3 fatty acids are much more prone to degradation/oxidation (rancidity) than MUFA. Given the main use of olive oil is in producing cooked foods, it has the relative advantage with high MUFA of increased thermal stability but the disadvantage of low PUFA, meaning it is low in essential oils (9.8% n-6 and 0.8% n-3). Canola oil is an alternative to olive oil (19% n-6 and 9.1% n-3). Although it only became available in1979 following successful Canadian research and consequently has not benefited from the many centuries of use that olive oil has enjoyed, it currently far exceeds it in world production and consumption. Epidemiological and clinical trial evidence for its positive effects in humans versus onset of chronic diseases is as impressive as olive oil. At a more basic level it has roughly equivalent MUFA content (63% vs 73%) conferring cooking stability, but more importantly a much better content and balance of the n-3 essential fatty acids. The following paper (Exp Biol Med (2008) 233:674–688) makes the case for lowering the ratio of omega-6/omega-3 oils in Western diets.

Several sources of information suggest that human beings evolved on a diet with a ratio of omega-6 to omega-3 essential fatty acids (EFA) of about 1 whereas in Western diets the ratio is 15/1– 16.7/1. Western diets are deficient in omega-3 fatty acids and have excessive amounts of omega-6 fatty acids compared with the diet on which human beings evolved and their genetic patterns were established. Excessive amounts of omega-6 polyunsaturated fatty acids (PUFA) and a very high omega-6/omega-3 ratio, as is found in today’s Western diets, promote the pathogenesis of many diseases, including cardiovascular disease, cancer, and inflammatory and autoimmune diseases, whereas increased levels of omega-3 PUFA (a lower omega-6/omega-3 ratio) exert suppressive effects. In the secondary prevention of cardiovascular disease, a ratio of 4/1 was associated with a 70% decrease in total mortality. A ratio of 2.5/1 reduced rectal cell proliferation in patients with colorectal cancer, whereas a ratio of 4/1 with the same amount of omega-3 PUFA had no effect. The lower omega-6/omega-3 ratio in women with breast cancer was associated with decreased risk. A ratio of 2–3/1 suppressed inflammation in patients with rheumatoid arthritis, and a ratio of 5/1 had beneficial effect on patients with asthma, whereas a ratio of 10/1 had adverse consequences. These studies indicate that the optimal ratio may vary with the disease under consideration. This is consistent with the fact that chronic diseases are multi-genic and multi-factorial. Therefore, it is quite possible that the therapeutic dose of omega-3 fatty acids will depend on the degree of severity of disease resulting from the genetic predisposition. A lower ratio of omega-6/omega-3 fatty acids is more desirable in reducing the risk of many of the chronic diseases of high prevalence in Western societies, as well as in the developing countries.

The n-6/n-3 ratio for olive oil is about 12.2/1 and for normal canola it is 2.1/1 with enhanced n-3 forms having an even lower ratio. With comparable impressive evidence for their health conferring properties over animal-based fats, it may thus be possible to achieve further improvements in the MD by substituting canola oil for olive oil. Additional considerations are that canola has approximately half the saturated fat content as olive oil (7.4 vs 14%) and it’s cheaper than the various speciality virgin olive oils. But realistically, traditional customs are very resistant to change. So if individuals choose to keep olive oil as a part of the MD it is important to realise that the whole fruit, de-bittered by the Spanish brining technique, has the same oil with more healthy fibre, lower calories with 50% contained water, and for black olives, about 20 times (!) the antioxidant level as extra virgin olive oil. Such a large difference in nutrient content between different parts of a food (eg after peeling, de-seeding, juicing or oil expression) is unusual but some differences are the norm and should always be considered when deciding on the nutritional merit of a given processing technique. Two obvious factors contributing to such a disparate outcome in this instance are that water soluble nutrients will not be included in separated oil, and other phytonutrients that are extensively membrane-bound will also be left behind with oil expression. So committed MD olive oil consumers might expect enhanced health outcomes by substituting or supplementing part of their oil intake with table olives. If olive oil was to be the sole source of n-6/n-3 fatty acids in the diet, then for men, they would have to consume about 210g /d to meet recommended n-3 levels versus 16g/d for canola which also provides a superior n-6/n-3 ratio.

Both olive and canola oils are valuable foods, but when used at elevated temperatures for frying, they are nevertheless still subject to finite chemical degradation to a number of unhealthy substances that occur more extensively above 150°C. This is why food outlets such as fish and chip shops have to totally replace their oil periodically when deterioration has progressed beyond certain levels. Production of these substances is minimal when the oil is used only once for cooking a given meal, provided the temperature/time is not excessive. However with the higher temperatures/rapid cooking used in popular Asian stir frying techniques (oil taken to the smoke point and beyond) more dangerous carcinogens are formed, some volatile. As a result, a several-fold higher incidence of lung cancer is seen in cooks frequently using this technique, especially where there may be poor or ineffective ventilation. Also, with higher temperature/cooking frying time combinations, the degradation of valuable phytochemicals can be profound. This may be made even worse when water-based boiling follows the frying.