**4. Discussion**

The present pooled analysis of two large-scale population-based studies conducted in Greece assessed the impact of olive oil consumption, as a dietary fat in the preparation and cooking of foods, on successful aging. The main study findings reveal that, following adjustment for potential confounding factors including age, sex, and smoking, participants in the "Exclusive culinary use of olive oil" category, defined as the sole consumption of olive oil for food preparation and cooking, were significantly associated with successful aging. The observed beneficial effects were most prominent among individuals aged older than 70 years old. It is noteworthy that combined use of olive oil with other dietary fats during cooking did not exhibit a notable impact on successful aging. Therefore, the exclusive use of olive oil in the preparation and cooking of foods may enhance healthy aging, particularly among the elderly.

One of the potential pathways for explicating the association between exclusive olive oil intake and successful aging can be described via the free radical theory of aging. According to this theory, free radical formation and accumulation can cause damage to biological tissues, leading to the accelerated aging phenotype. However, olive oil is able to reduce free radical production at the mitochondrial level compared to other oils such as the ones rich in polyunsaturated fatty acids [21]. Finally, monounsaturated fatty acids, such as those of olive oil, are positively correlated with longevity, as well as diminishment of age-related morbidities (e.g., cognitive deficits) [21,22]. Within this context, the findings of this study further support the free radical theory of aging (from the epidemiological perspective), exhibiting that higher olive oil intake is associated with successful aging. In addition, similar findings were reported in the previous study of centenarians in Sicily, where participants were most likely to report, among other health promoting factors, high intake of olive oil [23]. Another potential pathway promoting the present findings could be the fact that due to olive oil's qualities, its habitual use enhances palatability and facilitates a high intake of vegetables [24]. It is found that fruits and vegetables and their antioxidant compounds seem to have beneficial effects in the healthy aging metric [25].

Moreover, it should be noted that olive oil phenols, such as tyrosol, hydroxytyrosol, and oleocanthal, and other important bioactive compounds could be partially responsible for the positive association between olive oil consumption and successful aging. Bioactive compounds of olive oil, e.g., hydroxytyrosol, oleuropein, and tyrosol, have antioxidant and antimicrobial effects [26]. Nutritional attributes vary depending on the different types and sub-types of olive oil. Olive oil's antioxidant content exhibits a vital role for most of its biological activities, while oleic acid, squalene, and terpenoids have antitumor effects [27]. Based on the review of Giovannelli, it is suggested that the

activity of olive oil phenolic compounds has beneficial effects on the aging process, while animal studies support that olive oil phenols can prevent age-related mental and physical dysfunctions [28]. Actually, extra virgin olive oil, due to biophenolic compounds, has been associated with many beneficial effects on diminishing the risk of several diseases, including cardiovascular disease, several cancers, and age-related illness [29].

In developed countries, the continued prolongation of the average human lifespan, albeit with increased chronic disease morbidity rates, incurs continuously increasing healthcare associated costs and emerging public health challenges [30]. Primary public health prevention strategies, such as the promotion of healthy dietary patterns for successful aging, may ultimately deter the onset of chronic disease and healthcare associated costs. As a result, such strategies encouraging the exclusive use of olive oil as a preparatory (unheated) and cooking oil may help improve the sustainability of populations and public health systems alike. However, since it is not ye<sup>t</sup> fully clarified which particular types of olive oils may render the greatest health impacts, further studies are necessary for elucidating whether olive oils with the highest levels of phenolic compound content (namely extra-virgin olive oil) could offer the most health benefits [31].

In addition to the benefits of olive oil as an unheated product, it is also worth mentioning that there is a continuous debate on which type of oil or fat in general is best to cook with. It is well known that the higher the level of unsaturation of the fatty acids, the lower the heat stability of the oil; thus, polyunsaturated are better than saturated fats at room temperature, but when they are heated, their structures change and harmful chemicals can appear [32]. Oils rich in polyunsaturated fatty acids, like corn or sunflower oil, generated very high levels of oxidative products (e.g., aldehydes), whereas dietary fats rich in saturated fatty acids (e.g., butter) or monounsaturated fatty acids (e.g., olive oil) produce fewer aldehydes or other potentially dangerous byproducts [33]. According to the USDA (United States Department of Agriculture) extra virgin olive oil—the most popular culinary type of olive oil—contains 73.330 g of monounsaturated fatty acids, 13.330 g of saturated fatty acids, and 6.670 g of polyunsaturated fatty acids per 100 mL [34], and it may be considered as the best oil for consumption [35].
