Olives are the original product from which olive oil is derived. Depending on how the olives are processed, various categories of olive oil are obtained, such as extra-virgin olive oil. Additionally, there are thousands of olive varieties around the world, with the most popular Greek ones being: Koroneiki, Athinolia and Manaki (for oil production) and Kalamata, Amfissa and Halkidiki (table olives). The nutritional content of olives varies depending on the variety. For example, according to the US Department of Agriculture, the famous black Kalamata olives contain, per 100 g:

Contrary to olive oil, olives have not been the object of a large number of clinical studies; however, knowing that they share most of their components with olive oil, it might well be argued that they also have a beneficial effect for the body.
One thing that olives and olive oil have in common is that they are rich in monounsaturated fatty acids. Nevertheless, there are many differences between them, such as:

1. Olive oil is 100% fat, while olives are only 11-15%.
2. Olives, which must be cured or made into paste to be edible, usually contain a lot of sodium, while olive oil essentially contains no sodium.
3. The curing process removes many of the polyphenols from olives, while these are retained to a great degree in extra-virgin olive oil.
4. Olives contain fibre, while olive oil does not.
5. Some olives are processed by natural fermentation, which means they could be a source of beneficial bacteria; olive oil cannot.

As mentioned earlier, olives are subjected to methods of preservation to become edible.
These include the following:

• • Typical fermentation characteristics: olives are preserved due to physico-chemical changes from the presence of compounds such as: salt, organic acids, spices, etc.
  • Preserving atmosphere: this involves the partial or total removal of air and its replacement with a suitable inert gas or gas mixture.
  • Vacuum: Complete removal of air.
  • Addition of preservatives: approved and compliant with current law.
  • Cooling: storage at low temperatures to prevent the growth of micro-organisms that could spoil the product.
  • Pasteurisation: olives are heat-treated, so that vegetative forms of pathogenic micro-organisms are destroyed.
  • Sterilisation: olives undergo heat treatment that destroys or deactivates any pathogenic or non-pathogenic micro-organisms and their toxins.

Olive oil is integral to the Mediterranean diet and the synergy observed when these are combined has led to the results that are widely known. Specifically, the Mediterranean diet includes daily consumption of olive oil, grains, vegetables, fruit, dairy products, nuts, weekly consumption of fish, poultry, eggs and red meat (with no visible fat, once a week), and less frequently, consumption of sugar, chocolate, sweets and soft drinks. Additionally, it has been noted that the Mediterranean diet can reduce the risk for cardiovascular diseases, decrease insulin resistance and delay the onset of type 2 diabetes and reduce the risk of developing Alzheimer’s, dementia and other mental disorders.
Attention was initially focused on the protective role of the Mediterranean diet by American physiologist Ancel Keys, who observed the low occurrence of cardiovascular diseases amongst Mediterranean populations. (5) A number of subsequent studies confirmed the protective action of the Mediterranean diet, not only in primary and secondary prevention of cardiovascular diseases but also in obesity, type 2 diabetes (2), metabolic syndrome, cancer and neurodegenerative disorders. These studies include the Seven Countries Study, PREDIMED and the Lyon Diet Heart Study. The most significant findings of these studies are:

• • Seven Countries Study: It was shown that the consumption of saturated fat is linked to cardiovascular diseases. Greece, and Crete in particular, though it had a high rate of overall dietary fat consumption, had the lowest mortality rate resulting from cardiovascular disease because most of the fat was unsaturated fat from olive oil.
  • PREDIMED: Even though the sample of people who took part did not suffer from cardiovascular problems, it appeared that consuming a Mediterranean diet led to a 30% reduction in the risk for cardiovascular complications, with an impressive 40% reduction in the risk for stroke.
  • Lyon Diet Heart Study: Consumption of the Mediterranean diet by patients who had survived a myocardial infarction led to a 50% decrease in new cardiovascular events and a drop in cancer cases and deaths in general.