Mediterranean diet has been related with reduced morbidity and better well-being. The aim of this study was to assess whether the adherence to the Mediterranean diet were associated with mental and physical health related to quality of life.
This analysis included 11 015 participants with 4 years of follow-up in the SUN Project (a multipurpose cohort study based on university graduates from Spain). A validated 136-item food frequency questionnaire was used to assess the adherence to the Mediterranean diet at baseline, according to a nine-point score, presented in four categories (low, low-moderate, moderate-high and high). Health-related quality of life (HRQL) was measured after 4 years of follow-up with the Spanish version of the SF-36 Health Survey. Generalized Linear Models were fitted to assess adjusted mean scores, the regression coefficients (β) and their 95% confidence intervals (95% CIs) for the SF-36 domains according to categories of adherence to Mediterranean diet.
Multivariate-adjusted models revealed a significant direct association between adherence to Mediterranean diet and all the physical and most mental health domains (vitality, social functioning and role emotional). Vitality (β=0.50, 95% CI=0.32–0.68) and general health (β=0.45, 95% CI=0.26–0.62) showed the highest coefficients. Mean values for physical functioning, role physical, bodily pain, general health and vitality domains were significantly better with increasing adherence to the Mediterranean diet. Those having improved their initial high diet scores have better scores in physical functioning, general health and vitality.
Adherence to the Mediterranean diet seems to be a factor importantly associated with a better HRQL.
The Mediterranean diet is a dietary pattern characterized by the use of certain food items with beneficial effects on health. The relationship between adherence to a Mediterranean diet and reduced mortality or lower incidence of major chronic disease has been widely studied across multiple epidemiological analytical studies (Fung et al., 2009; La Vecchia, 2009; Martínez-González et al., 2009, 2011; Sánchez-Villegas et al., 2009; Trichopoulou et al., 2009). In a recent meta-analysis of prospective studies conducted by Sofi et al. (2008), a significant relationship was observed between a greater adherence to the Mediterranean diet and reduced risk of major chronic diseases mortality.
Some clinical trials have observed an improvement in health status when interventions aimed to upgrade adherence to the Mediterranean diet were implemented (Toobert et al., 2003; Babio et al., 2009). In addition, some degree of protection attributed to the Mediterranean diet with respect to cognitive impairment, dementia (Scarmeas et al., 2009) or major depression incidence (Sánchez-Villegas et al., 2009) has been reported.
Thus, the Mediterranean diet can be considered as a model of healthy eating for both primary and secondary prevention that could be promoted to reduce the burden of disease among developed and aged societies.
Population ageing has fostered the general concern for obtaining a better health-related quality of life (HRQL). Few studies have analysed the influence of the diet on the quality of life of healthy populations (Plaisted et al., 1999; Hislop et al., 2006). Specifically in Mediterranean populations, only a cross-sectional study (n=8195) conducted in a Spanish population showed that the adherence to the Mediterranean diet was associated with higher scoring for self-perceived health (Muñóz et al., 2009).
From a public health perspective, it is necessary to identify the factors that could increase the quality of life and health status of the population. The aim of this study was to assess if the adherence to a Mediterranean diet was associated to self-perceived HRQL in a Mediterranean cohort.
Materials and methods
The subjects of this study belong to the SUN Project (Seguimiento Universidad de Navarra). This Project is an ongoing, multipurpose, dynamic cohort of university graduates conducted in Spain. As a dynamic cohort, the recruitment of the participants is permanently open. The study methods and the cohort profile have been published in detail elsewhere (Seguí-Gómez et al., 2006).
Up to May 2010, 12 493 participants had answered both the baseline questionnaire (Q_0) and the 4-year of follow-up questionnaire (Q_4). Of these, we further excluded 1182 participants who reported values for total energy intake out of predefined limits (<800 or >4000 kcal/day in men and <500 or >3500 kcal/day in women) (Willett, 1998) and 296 participants without data or with incorrect data regarding quality of life. Finally, 11 015 participants were included in this study. Of them, 4639 were men and 6376 were women.
The study was approved by the Human Research Ethical Committee at the University of Navarra. Voluntary completion of the first questionnaire was considered to imply informed consent.
Dietary intake was assessed through a validated 136-item semi-quantitative food frequency questionnaire, included in Q_0. Recently, the validity and reproducibility of this questionnaire has been re-evaluated (de la Fuente-Arrillaga et al., 2010; Fernández-Ballart et al., 2010). It showed reasonably good validity for assessing food items characteristics of the Mediterranean diet (intraclass correlation coefficients for these components vs four 3-day food records ranged from 0.40 to 0.84) (Fernández-Ballart et al., 2010).
Each item in the questionnaire included a typical portion size. Daily food consumption was estimated multiplying the portion size by the consumption frequency, for each food item (nine options ranging from never or almost never to six or more times a day). A trained dietician updated the nutrient databank using the latest available information included in food composition tables for Spain (Mataix, 2003; Moreiras et al., 2005).
Adherence to the Mediterranean diet was appraised according to the score created by Trichopoulou et al. (2003). This score includes nine components: vegetables, legumes, fruits and nuts, cereals, fish, meat and meat products, dairy products, moderate alcohol intake, and the ratio of monounsaturated fatty acids to saturated fatty acids. We assigned values of 0 or 1 to each component by using the sex-specific medians in the studied population as cutoffs.
Thus, for five components that are in line with the traditional Mediterranean diet (vegetables, legumes, fruits and nuts, cereals, and fish), subjects whose consumption was at or above the sex-specific median were assigned a value of 1, while those whose consumption was below the median were assigned a value of 0. For components that are against the concept of the traditional Mediterranean diet (meats or meat products and dairy products), participants whose consumption was below the sex-specific median were assigned a value of 1 and participants whose consumption was at or above the median were assigned a value of 0. For alcohol intake, a value of 1 was assigned to men who consumed 10–50 g/day and women who consumed 5–25 g/day. Finally, the ratio of monounsaturated fatty acids to saturated fatty acids was used to assess the quality of fat intake, giving a value of 0 to participants whose monounsaturated fatty acids/saturated fatty acids ratio was below the sex-specific median and a value of 1 to participants whose ratio was at or above the median. If participants had all of the characteristics of the Mediterranean diet, their score was the highest possible (9 points), reflecting maximum adherence. If they had none of the characteristics, their score was the minimum possible (0 points), reflecting no adherence at all. Adherence to the Mediterranean diet score was analysed as a continuous variable and also categorized into four categories with each category of the pattern representing an increment of 2–3 points in the score: low (score 0–2), low-moderate (score 3–4), moderate-high (score 5–6) and high (score 7–9).
Additionally, the 2-year (Q_2) and the 4-year (Q_4) follow-up questionnaire included several items regarding healthy (consumption in fruits and/or vegetables, fish, and olive oil) and detrimental changes (meat and dairy products) in dietary habits. These items were scored as is mentioned in Table 1.
Summing up these five items, it was built a score ranging from −10 to +10. It was considered that adherence to Mediterranean diet had increased for values 1 and had decreased or maintenance for values 1.
Finally, the joint exposure to both baseline adherence to the Mediterranean diet (four categories) and its changes during follow-up (increase vs decrease or maintenance) was also analysed.
HRQL was assessed after 4-year follow-up (Q_4) with the validated Spanish version of the SF-36 Health Survey. The SF-36 is a general health scale widely used and thoroughly validated (Ware and Gandek, 1998). This instrument has been translated into a number of languages, including Spanish (Alonso et al., 1995, 1998). This questionnaire contains 36 items, which measure eight multi-item parameters of health status: physical functioning, role limitations because of physical health problems (role-physical), bodily pain (tolerance), general health perceptions, vitality, social functioning, role limitations because of emotional problems (role emotional) and mental health. The first four domains deal with physical aspects, and the next four reflect psychological features. For each parameter, scores are coded, summed and transformed to a scale from 0 (the worst possible condition) to 100 (the best possible condition). For example, for bodily pain a score of 100 means a complete tolerance to pain.
Other covariates assessment
The baseline assessment (Q_0) also requested information about sociodemographic factors, lifestyle, health-related habits and medical history (Seguí-Gómez et al., 2006). Body mass index (BMI) was calculated as weight (in kilograms) divided by the square of height (in metres).
Physical activity was assessed through a validated physical activity questionnaire with data about 17 activities (Martínez-González et al., 2005). Leisure-time activities were computed by assigning an activity metabolic equivalent score to each activity, multiplied by the time spent in each activity and summing up all activities (Ainsworth et al., 2000). Self-reported information about physical activity and BMI has been specifically validated in subsamples of this cohort (Bes-Rastrollo et al., 2005; Martínez-González et al., 2005).
Multiple linear regression models were used to assess the relationship between the adherence to the Mediterranean diet and each dimension of the SF-36 Health Survey estimating the regression coefficients (b) and their 95% confidence intervals (95% CIs). Potential confounders included as covariates in the model were: age (years, continuous), sex, marital status (married/other), BMI at baseline and after 2 years of follow-up (underweight, normal, overweight and obese), total energy intake (kcal/day, continuous), physical activity during leisure-time (metabolic equivalents h/week, continuous), smoking (never, past and current smokers), prevalence of cardiovascular disease, diabetes, dislypidaemia and hypertension. Additional adjustment for employment status did not change the associations.
Moreover, statistical differences in multivariate-adjusted mean scores between categories of adherence of the Mediterranean diet were determined by analysis of covariance. To ascertain if these means were statistically significantly different by pairs, a post hoc procedure (Benjamini–Hochberg correction) was used.
Finally, multiple linear regression models were also used to assess the relationship between the joint exposure to both baseline adherence to the Mediterranean diet and its changes during follow-up and each dimension of the SF-36 Health Survey.
The SPSS software package for Windows version 17.0 (SPSS Inc., Chicago, IL, USA) was used for statistical analyses.
The main characteristics of participants according to categories of adherence to the Mediterranean diet are shown in Table 2. Older participants, men and married subjects were more likely to be in the highest level of adherence. Participants with the highest Mediterranean diet score reported the highest values for total energy intake, BMI and leisure-time physical activity. Self-reported prevalence of chronic diseases such as diabetes, hypertension, dyslipidaemia and coronary heart disease was also higher in this category. The percentage of current smokers was lower in the upper category of adherence than in the low to moderate category.
Table 3 shows the estimated multivariate-adjusted means (and their 95% CI) for physical and mental dimensions of the SF-36 according to baseline categories of Mediterranean diet score. Mean values for physical functioning, role physical, bodily pain, general health and vitality domains were significantly better in participants with higher Mediterranean diet score. After applying the Benjamini–Hochberg post-test correction for multiple comparisons, statistically significant differences were found for the physical functioning, general health and vitality domains between the highest category of Mediterranean diet score and each of the lower categories.
Table 4 shows the association between the baseline Mediterranean diet score and the SF-36 domains. The multivariate-adjusted models revealed a significant direct linear association between the Mediterranean diet score and all the physical and three of the mental health domains (vitality, social functioning and role emotional). Thus, an increment in one-point in the Mediterranean diet score was related, for example, to 0.50 (95% CI=0.32–0.68) point increment in vitality and 0.45 (95% CI=0.26–0.62) point increment in general health.
Table 5 shows the results of the association between the joint exposure to both baseline Mediterranean diet score (four categories) and its changes during follow-up and the SF-36 scores. Those participants with the highest baseline Mediterranean diet score and who increased their adherence during the follow-up showed significant increments in the scores of physical functioning (β=2.13; 95% CI=1.15–3.11), general health (β=2.84; 95% CI=1.16–4.51) and vitality (β=3.38; 95% CI=1.68–5.07) when were compared with those participants with the lowest baseline adherence and that had decreased or maintained this adherence after 4 years of follow-up.
This study showed a direct linear association between the adherence to a Mediterranean diet and some aspects of self-perceived physical and mental quality of life. This association was stronger for physical than for mental health.
For almost all the quality of life domains, the magnitude of the differences between the lowest and highest adherence to the Mediterranean diet categories were about 2–3 points. There is a debate on how to define meaningful differences on the SF-36 scores in a clinical setting. Even absolute differences from 3 to 10 points have been suggested (Samsa et al., 1999), we argue that the differences we have found are likely to have clinical significance for healthy people, like those included in the SUN cohort, mainly young and normal BMI professionals.
The Mediterranean diet is rich in nutrients such as vitamins, minerals, antioxidants, fibre, omega-3 fatty acids (from fish) and monounsaturated fatty acids (from olive oil), whose beneficial effects on health have been widely demonstrated (Serra-Majem et al., 2009). In fact, multiple studies have associated Mediterranean diet components with better HRQL (Silvers and Scott, 2002; Steptoe et al., 2004; Crowe et al., 2007; Liu et al., 2007; Myint et al., 2007; Mikolajczyk et al., 2009; Schiepers et al., 2010). Some of these studies have analysed the role of fish consumption (or omega-3 fatty acids intake) and quality of life with inconclusive results.
Whereas Silvers and Scott (2002) reported a positive relationship between fish consumption and self-reported mental health using the SF-36 questionnaire, they did not find a significant relationship with physical health. Contrarily, Schiepers et al. (2010) showed a direct association between fish consumption and the physical component summary score of the SF-36 but not with the mental score. The consumption of fish could be related to better HRQL because of its content in beneficial nutrients for health such as omega-3 fatty acids, vitamins and antioxidants. However, Schiepers et al. (2010) failed to find an association between omega-3 fatty acids and HRQL. Nevertheless, other investigators such as Crowe et al. (2007) observed higher score in the physical component of the SF-36 among those subjects with the highest plasma level of eicosapentaenoic acid. Fish consumption might also reflect a healthy life style or a favourable nutritional status.
An inverse relationship between fruits and vegetables consumption and perceived stress and depressive symptoms has been demonstrated in general population (Liu et al., 2007; Mikolajczyk et al., 2009). Moreover, Myint et al. (2007) found a direct significant association between fruit and vegetable consumption and self-reported physical functional health and a less consistent relationship with mental functional health in a study composed by 16 792 subjects between 40 and 79 years in the United Kingdom. In a randomized clinical trial aimed to assess the impact of brief behavioural dietary counselling and nutritional education counselling on HRQL measured through SF-36 (Steptoe et al., 2004), it was observed a positive association between baseline levels of fruit and vegetables consumption and physical and mental health status. Furthermore, the increase in fruit and vegetable consumption was associated with a change in physical health status at 8 weeks. Such an improved association was not found for mental status.
Beyond the physical benefits reported for olive oil (López-Miranda et al., 2010), recently, the protective role olive oil on mental disorders has been suggested by both cross-sectional (Kyrozis et al., 2009) as well as by prospective studies (Sánchez-Villegas et al., 2011).
However, the analysis of a single component does not contemplate the interaction with other nutrients (Sofi et al., 2008). For this reason, in the last decades nutritional epidemiologists have increased their interest to study the relation between the adherence to dietary patterns, more than to specific food items or nutrient. However, few studies have analysed the association between adherence to dietary patterns and HRQL (Plaisted et al., 1999; Hislop et al., 2006). To our knowledge, only a study conducted by Muñóz et al. (2009) has analysed the adherence to the Mediterranean diet. They found, using a cross-sectional design in a Spanish population and for both sexes, a significant direct association between the adherence to the Mediterranean diet and mental and physical health status.
Several biological and physiological mechanisms could explain the beneficial effect of the Mediterranean diet on physical health. The adherence to the traditional Mediterranean diet has been associated with a reduction in low-grade inflammatory status, better endothelial function, an improved profile of coagulation markers (Chrysohoou et al., 2004; López-Garcia et al., 2004), and less insulin resistance (MacKeown et al., 2004). All these factors are thought to lead not only to a lower risk of chronic diseases, but also to a better metabolic control of already established diseases. Indeed, a greater adherence to the Mediterranean diet has been inversely associated with the incidence of cardiovascular disease, obesity or metabolic syndrome among the participants of the SUN Project (Sánchez-Villegas et al., 2006; Tortosa et al., 2007; Martínez-González et al., 2008, 2011; Núñez-Córdoba et al., 2009).
The adherence to a Mediterranean diet has also been associated with a better mental health status. Numerous studies have showed a lower risk of neurodegenerative disease, mental disorders and protective effects against cognitive decline in those subjects with a greater adherence to this dietary pattern (Engelhart et al., 2002; Murakami et al., 2008; Akbaraly et al., 2009; Sánchez-Villegas et al., 2009; Trichopoulou et al., 2009).
Underlying mechanisms include, on one hand, omega-3 fatty acids effects on neuronal membranes, enhancing synaptic membrane fluidity and serotonin transport (Gómez-Pinilla, 2008). On the other hand, B-vitamins and folate are involved in serotonin and other monoamine neurotransmitters synthesis and other methylation reactions, for example, homocysteine catabolism (Bottiglieri, 1996).
Another factor that can be taken into account to explain our findings is the educational or social level. It is generally accepted that socioeconomic status influences dietary habits as well as human health. Low socioeconomic status is associated with poorer physical and general health and higher mortality (Guallar-Castillón et al., 2005; Kunst et al., 2005; Laaksonen et al., 2007). Moreover, some of the components of the Mediterranean diet such as fruits, vegetables, fish and olive oil are more frequently consumed by groups belonging to elevated socioeconomic status (Shohaimi et al., 2004; Panagiotakos et al., 2007).
Considering that our cohort is integrate by university graduates, and assuming that their social and educational levels are quite homogeneous, these factors are unlikely alternative explanatory variables for our results.
Our study has some limitations. Diet was ascertained both at baseline and after 2 and 4 years of follow-up, meanwhile HRQL only after 4-year of follow-up. Therefore, the beneficial effect of Mediterranean diet on HRQL could be in part because of differences in baseline physical and mental health status. Our regression analysis is aimed to adjust for a wide array of baseline characteristics, thus reducing the baseline heterogeneity between participants who more closely followed the Mediterranean diet and those who did not.
Other possible limitation of the study is that both dietary exposures and SF-36 scores are self-reported measures, which are susceptible to similar reporting bias. It is possible that the association reflects a tendency to report both the exposure and the outcomes as more favourable than they really are. We have to admit this possibility although the participants of our cohort are highly educated and motivated and about 50% of them are health professionals themselves, so it is unlikely that they may have misreported their dietary intake or self-perceived health status. Similarly, the validity of the semi-quantitative food frequency questionnaire and the Spanish version of the SF-36 Health Survey have been demonstrated.
In conclusion, self-perceived mental and physical quality of life was directly associated with adherence to the Mediterranean diet. This association was stronger for the physical domains. Our data support carrying out new studies, with a longitudinal design, in order to confirm the influence of the Mediterranean diet on HRQL. These findings could serve as basis for the planning of educational programs and dietetic interventions.
We are indebted to the participants of the SUN Study for their continued cooperation and participation. We thank to other members of the SUN Group: A Alonso, I Ara, FJ Basterra-Gortari, S Benito, M Bes-Rastrollo, JJ Beunza, M Delgado-Rodriguez, T Dierssen, J Doreste, F Guillen-Grima, J Krafka, J Llorca, C Lopez del Burgo, A Marti, JA Martinez, JM Nuñez-Cordoba, AM Pimenta, D Sanchez, M Segui-Gomez, M Serrano-Martínez and Z Vazquez. The Spanish Government Instituto de Salud Carlos III, Fondo de Investigaciones Sanitarias, projects PI042241, PI040233, PI050976, PI070240, PI0801943, and RD 06/0045 and the Navarra Regional Government project PI41/2005 have supported the SUN Project.
About this article
Nutrition Journal (2014)