People with a higher income and higher levels of education had a greater adherence to MD-like eating patterns, as measured by three different parameters: two a priori Mediterranean scores (the traditional one introduced by Trichopoulou and a more recent IMI), and the a posteriori dietary patterns derived from the PCA. Evidence on the health benefits of the MD is based on several studies and meta-analyses.1
However, adherence to this healthy eating pattern is rapidly disappearing in the countries of Southern Europe where it originated and persisted for centuries, including the areas of Northern Africa in which there is an increasing prevalence of metabolic disorders and consequent CVD mainly owing to the changing lifestyle and/or habits.22
SES has been included among the factors related to chronic disease onset, and disparities in dietary habits by social class have been advocated to explain at least in part the higher CVD risk factors profile observed among low SES groups.23
Our results agree with the conclusions reported in the review by Darmon and Drewnowski10
that higher-quality diets are mainly consumed by better educated and more affluent people, while lower socioeconomic groups tend to have lower-quality diets thus exposing themselves to a higher risk of developing diet-related diseases. Similar conclusions were reached by other investigations too 24
suggesting that low socioeconomic groups end up having poorer diets. These findings are supported, at least in part, by the fact that following a Mediterranean dietary style could represent a matter of money.9
Indeed researchers in Spain showed that an MD is definitely more expensive to follow than Western dietary patterns: this may represent a strong economic obstacle when counselling people about the opportunity to follow a healthy diet because cost may become a prohibitive factor.9
Aggarwal et al25
demonstrated that the well-known socioeconomic disparities in diet quality is mediated by the food cost confirming that lower SES groups tend to consume more energy-dense and nutrient-poor diets. However, the economic advantages of a Mediterranean way of eating in terms of cost-effectiveness should be highlighted as shown in patients with a previous CVD that could represent an exceptional return on investment.26
Subjects with a lower income had a greater prevalence of obesity too. The association between obesity and socioeconomic factors has been previously observed 27
suggesting that the latter plays an important role in the risk of obesity and overweight not only in adults but also in children.28
However, our data show that the strong association observed between a lower income or education with obesity was not mediated by the diet quality. Indeed, an additional analysis combining the impacts of education or income on diet quality and then on obesity in a unique statistical model, showed that the association remained unchanged when diet quality was included. According to these results, the changes in obesity rates observed in different income and education categories appear not necessarily mediated by diet quality. However, the epidemiological evidence supporting a causal link between MDs and body weight is contrasting.29
It is quite clear that accumulating proofs on the benefits of Mediterranean-like diets is an insufficient prevention strategy as conditions allowing people to stick to healthier dietary habits should also be clearly identified.
This study contributes to provide further evidence for the assumption that dietary habits are strongly influenced by socioeconomic factors, in particular by income which appears to play an important role in determining people's food choices.30
As far as education is concerned, previous studies found a relationship between higher levels of education and healthy diets.31
In our research, education was found to be independently associated to MD and did not modify the association between income levels and a healthy dietary pattern as shown in the stratified analysis by education levels.
The promotion of healthy lifestyles and diets to prevent weight gain and related diseases has taken the top position in the priority list of the public health experts all over the world since obesity has become a life-threatening epidemic. So far the traditional MD has proven to be an effective ‘remedy’ to the spreading of major chronic diseases, obesity and mortality. Our study highlights the strong linkage among low income, poor adherence to MD and obesity prevalence.
Limitations of this study
A major limitation of the present study is that people self-reported their own income which is a quite sensitive issue. Indeed, we recorded a high percentage (30.7%) of non-respondent subjects who refused to declare or did not know their family income. Yet, such a large non-respondent group is very common in this type of investigations, especially among women and the elderly.32
However, there was no difference between the entire Moli-sani population and the subsample analysed here as far as dietary habits and socioeconomic variables were concerned.
Another inherent limit is represented by the cross-sectional nature of our study.
In addition, caution is needed in extending the results presented here to larger contexts since data were collected in a region located between Central and Southern Italy, Mediterranean by tradition and culture.13
Yet, the main characteristics of our population sample are comparable to those of the Italian Cardiovascular Epidemiological Observatory,33
a large survey including random samples of the general population recruited all over Italy; therefore our sample can be considered a representative at least of the whole Italian population.
Strengths of this study
Our very large population sample is composed of subjects coming from quite a homogeneous environment with no marked differences in terms of socioeconomic disparities, different from metropolitan areas, where previous studies found huge gaps among social classes and related health statuses at relatively small distances from the city centre.34
Bearing this in mind, the differences we observed in the adherence to MD according to income indicate that also in an environment homogeneous both for genetics and lifestyles, income and education can still play a role in influencing dietary choices. Furthermore, the diet quality showed a continued improvement across a relatively small range of economic strata. Our ‘poorest’ are represented by people earning less than 10 000 Euro/net per year, while the ‘richest’ group is composed of subjects with more than 40 000 Euro/net/year. Such differences among income classes are quite restrained and recall what has already been said for the pretty homogeneous environment where our sample comes from. We are not dealing with real huge socioeconomic and income differences. Despite this homogeneity, we did observe notable changes in diet quality among different groups.
The differences observed across the income strata would likely become even more evident in an MD importing countries where getting typical Mediterranean products is more difficult and expensive.
In addition, apparently for the first time this topic was addressed by using two a priori Mediterranean scores (the traditional one introduced by Trichopoulou and a novel IMI), and the a posteriori dietary patterns derived from the PCA. This leads to overcoming the limitations each of these approaches may present. Indeed, the ‘a priori’ scores only reflect some aspects of diet and do not account for correlations between score components. Instead, the ‘a posteriori’ approaches have the weakness of low reproducibility, different populations having different non-predefined dietary patterns. Therefore, the use of an index based on the foods actually available to Italians and traditional Italian cooking styles should improve the ability of the index to classify the Italian cohort.