A study of the protective effects of virgin olive oil against infections has been carried out by the Microbiological and Immunological Research Group at the University of Jaen (UJA) in Spain. The objective of the research was to investigate how oil fats affect neutrophil activity. Neutrophils are white blood cells which belong to the immune system and help defend the organism against infectious diseases. Gerardo Álvarez, the group's leader, points out that fat makes up at least five per cent of the calories in an average diet, and often much more. For this reason, we should know which types are better for our health.

The experiment in question demonstrates that the fats in olive oil are capable of stimulating the cells in the immune system, helping to form an effective defence against diseases. This is not what happens, for example, with fish oils, which provoke over-stimulation of these defence cells. This has a negative effect on the cells and often kills them. In the experiment, mice were infected with with the bacteria listeria monocytogenes and later fed with one of three types of oil: fish oil, sunflower oil or olive oil. The mice were fed this diet for a period of one month, a considerable amount of time given that their average lifespan is around two years.

The results obtained from the experiment on mice after the test period were as follows: the mice fed on fish oil died after a short time, while the mice fed on sunflower oil died in the medium term. However, the mice fed on olive oil survived the bacterial infection. Álvarez de Cienfuegos added that the UJA group has also investigated how these cells act in people with immunosuppression (when the immune system is not working normally). Typically, people with immunosuppression are of advanced age, have diabetes or are receiving chemotherapy or similar treatment. The fats in extra virgin olive oil stimulate the immune system's defences without damaging them. Therefore, they are the healthiest fats that we can include in our diet.