Multiple sclerosis and cancers in Croatia--a possible protective role of the "Mediterranean diet"

Multiple sclerosis (MS) is an autoimmune disease triggered by a combination of genetic and environmental risk factors which are however individually insufficient to provoke the disease. Previous investigations studied the coexistence of cancer in MS patients, and only a few relations between the geographic distribution of MS and that of cancer. The aim of this research was to find an environmental link between the aetiology of MS and cancers in Croatia. Incidence and prevalence of MS in Croatia were compared with the incidence of the most frequent cancer sites: stomach cancer, cancer of the colon and the rectum, pancreatic cancer, lung cancer, cancer of the kidneys and brain cancer. Data for MS were collected from seven population-based epidemiologic studies which used Poser's diagnostic criteria and reported the number of cases and the magnitude of the studied population. Data for cancers were drawn from the Croatian National Cancer Registry. The analysis was done for single municipalities, grouped in their belonging regions or counties, and separately for the continental and the coastal area. For each rate a 95% confidence interval was calculated. The differences between rates were tested with the chi-square test as well. In addition, MS incidence or prevalence were correlated with the corresponding cancer incidence data. Pearson's correlation coefficients were used to measure the correlation between both diseases. Calculations were done with the statistical package Statistica V 7.1. and the Smith's Statistical Package freeware In the continental area of Croatia the mean annual incidence (per 100,000 inhabitants) of MS was nearly two folds higher than in the coastal area: 2.1 vs. 1.3 (p = 0.0029). The difference was lower when expressed by prevalence: 46.5 vs. 36.7 (p = 0.0601). Among the malignant neoplasms, in the continental area significantly higher incidence rates were found for stomach (32.9 vs. 20.8; p = 3.14E-14) and lung cancer (55.8 vs. 46.4; p = 1.21E-05), whilst colon cancer alone (20.4 vs. 15.7; p = 9.44E-05) or colorectal cancer (38.3 vs. 31.6; p = 8.18E-05) had a significantly higher incidence in the coastal area. The geographic distribution of MS expressed by incidence was significantly correlated with pancreatic (r = 0.62024, df=23, p = 0.00094) and lung cancer (r = 0.46380, df=23, p = 0.01953). This research adds further malignant neoplasms, possibly exposure-related, to the list of diseases with geographic distribution like MS. The similarity of MS distribution with the named malignancies is unlikely to be incidental. MS in Gorski Kotar and Slavonia seems to be associated with a diet rich in meat and fat. A diet rich in fat and meat and poor in vegetables is a risk factor for stomach, colorectum, pancreatic as well as lung cancers. Some authors have documented a possible protective role of the "Mediterranean diet" for the named cancers. Olive oil is the main source of fat in the "Mediterranean diet". Oleocanthal, aphenolic compound of the extra-virgin olive oil was found to inhibit the cyclooxigenase enzymes which are involved in demyelination and tumorigenesis. We hypothesize that the "Mediterranean diet", olive oil and particularly oleocanthal, to have a protective role in MS too.