Epidemiological studies have shown the influence of different factors such as smoking, obesity, or crash injuries on quality of life [4-7]. However, few longitudinal studies have analyzed the influence of diet on the quality of life of healthy populations [8-10]. Several studies have found that the adherence to a Mediterranean dietary pattern rich in fruits and vegetables, legumes, fish and olive oil, was associated with higher scoring for self-perceived health [11,12]. On the other hand, some studies have shown the detrimental effects on health of a "Western-type" dietary pattern rich in processed and red meats, refined grains and commercial baked goods [13-16]. The Western dietary pattern is the common diet in Northern Europe and the USA. In recent years, however, southern European countries, which used to eat a traditional Mediterranean diet, have also been adopting a more Western-style diet. Beyond the health effects of an overall Western dietary pattern, little is known about the health effects of the diet's specific elements, such as major fat sources, on quality of life.

There has been a dramatic change in the sources of fat intake in the general population. This change mainly consists in replacing polyunsaturated (PUFA) or monounsaturated fatty acids (MUFA) with saturated fats (SFA) and trans unsaturated fats (TFA). Usually, PUFA [17] and olive oil (OO) [18-20] have been considered as healthy lipids because they reduce the incidence of cardiovascular disease (CVD). In contrast, SFA and especially TFA are known risk factors for CVD [21]. There are some reasons to expect that CVD share some common determinants, [22] endothelial dysfunction and high levels of pro-inflammatory cytokines, with other pathologies like obesity, insulin resistance and neuropsychological disorders. All of these physiological processes may have also an influence on physical and mental quality of life in healthy population.

Table ​Table22 shows the multivariate regression coefficients (b) and their 95% CI for the mental domains of the SF-36 according to baseline SFA and TFA intake (in quintiles). The multivariate-adjusted model revealed a significant inverse association between TFA intake and most of the mental domains (vitality, social functioning and role emotional). For example: b = -2.0, 95% CI = -3.4 to -0.6 (Q5 vs. Q1), for the vitality domain. Moreover a statistically significant dose-response relationship (p for trend < 0.05) was found for each domain except for mental health. No association was found for SFA intake and the mental domains.

Multivariate regression coefficients and their 95% CI for the physical domains of the SF-36 according to baseline SFA and TFA intake (in quintiles) are shown in Table ​Table33.

For SFA intake, a significant inverse association was found for physical functioning, and general health domains. For the general health domain: (Q5 vs. Q1), b = -1.6; 95% CI = -3.1 to -0.1. General health also showed a statistically significant dose -response relationship (p for trend < 0.05). For TFA, an inverse association was found for bodily pain: (Q5 vs. Q1), b = -2.6; 95% CI = -4.4 to -0.8 with a dose-response relationship (p for trend < 0.05).

When we repeated the analysis additionally adjusting for the adherence to a Mediterranean diet score, the association remained significant for TFA intake and the mental domains and bodily pain.

These associations could mean that the physiological changes that occur when this kind of fatty acid is consumed could influence mostly the mental quality of life and therefore the self perception of "well being". So, the participants with the highest intake would perceive themselves more tire and worn out, with social and role disability due to emotional problems, and with severe limiting pain than the participants with the lowest TFA intake. Although these are perceived health measures rather than biological measures, self-related heath status has been shown to be a powerful predictor of mortality at long term [2].

On the other hand, for the mental quality of life domains, the magnitude of the differences between the lowest and highest quintile of TFA intake were about 1.5-3 points. There is a debate on how to define meaningful differences on the SF-36 scores in a clinical setting. Changes in 3-, 5-, and 10- points have been suggested as being clinically significant for clinical populations [37]. Given the characteristics of our cohort that did not include patients, but healthy and relatively young adults, the practical significance of these differences could be even higher. Although few studies have examined this issue directly, several investigators have raised the question of whether individuals with more severe impairments in HRQOL require a greater change to be considered meaningful than those with less severe impairments [38].

Fatty acids of trans configuration come from two main different sources: the major source is derived from industrially produced partially hydrogenated fat used in margarines, commercial cooking, and manufacturing processes (60% of the fats), and smaller amounts are naturally present in dairy and meat products from ruminants (6% of the fats) [39]. Based on the evidence to date, TFA intake, particularly the industrial trans-18:2 isomer, is associated with substantial risk of coronary heart disease (CHD) [40-43]. The adverse effects of TFA on CVD are thought to be mediated by increases in plasma concentrations of LDL-cholesterol, reductions in HDL-cholesterol, pro-inflammatory changes, endothelial dysfunction, and possibly by insulin resistance and displacement of essential fatty acids from membranes [44,45].

The relation of TFA intake to other disease outcomes has been examined less extensively than for CHD. However, emerging evidence suggests that TFA acid intake may influence additional non -lipid related pathways and outcomes. These include effects on systemic inflammation, endothelial dysfunction, visceral adiposity, insulin resistance, and arrhythmic risk [46]. Moreover, TFA intake has been linked to accelerated cognitive decline in older adults [47], and higher risk of Alzheimer [48], and depression [49]. The harmful effect of TFA intake in these neuropsychological disorders supports our results which suggest that TFA intake specially affects mental quality of life. A possible explanation for our finding is that TFA promote endothelial dysfunction and increase the production of pro-inflammatory cytokines that may interfere with neurotransmitter metabolism and inhibit Brain-derived neurotrophic factor (BDNF) expression among other physiological effects [50,51]. BDNF is a peptide critical for axonal growth, neuronal survival and synaptic plasticity and function. Therefore, it is likely that the consumption of foods containing TFA in their composition could increase the vulnerability to some mental or neurological disorders or act negatively on mental quality of life.

Although our results suggest a detrimental role of TFA on mental quality of life, our findings are modest. The present study was carried out among a sample in which TFA intake was very low (the median of intake was 1 gram per day). This intake was lower than the median intake for Spanish population which is 2.1 grams per day, [52] and far away from the higher consumption corresponded to the United States and Canada with values of 3-4 grams per day [53]. Therefore, the repercussion of these findings might be really important in these populations where the consumption is very high comparing to our cohort and where the main sources of TFA are artificial foods [54].

Several limitations in our study have to be addressed. Diet was ascertained at baseline and quality of life after 4-years of follow-up, therefore we acknowledge that baseline scores of quality of life were unknown. Consequently, in spite of the fact that the follow-up of participants allows a sufficient long induction period, it could still be possible to speculate that a poor-quality diet may be a result of mental health symptoms, rather than a causal factor.

We acknowledge that although the food frequency questionnaire has been validated using dietary records as gold standard, this is not the best method to validate some dietary fatty acids intake such n-3 PUFAs. The use of a validation method with biomarkers as gold standard is recommended. This could lead to certain, probably non-differential, misclassification bias in the dietary n-3 PUFAs assessment.

Another fact to take into account is that quality of life is a complex concept with various dimensions. Nevertheless, the use of the SF-36 questionnaire for evaluating the physical and mental dimensions of quality of life is generally accepted, and its validity and reliability have been demonstrated in many population-based studies [55].

PH is an Associate Professor of Preventive Medicine and Public Health at the Department of Clinical Science, University of Las Palmas de Gran Canaria, Spain. She belongs to the Nutritional Research group of this University.

MB-R is an Associate Professor at the Department of Preventive Medicine and Public Health, University of Navarra, Spain. She is a member of the Scientific Committee of the SUN project.

MR-C is an Associate Professor at the Department of Preventive Medicine and Public Health, University of Navarra, Spain, and also a researcher in the SUN project.

CL is an Assistant Professor at the Department of Preventive Medicine and Public Health, University of Navarra, Spain, and also a researcher in the SUN project.