Our study was conducted with a sample of 815 randomly selected subjects, which was representative of the adult population according to data from the Statistical Institute of Catalonia (IDESCAT). The publicity strategies used to announce the study and the personalized service provided to each of the participants are directly linked to the high level of participation achieved (61.5%), which is similar to that in other general population studies .
Food intake was assessed using 3-day dietary records internationally recognized as a quantitative method for estimating subjects’ regular habits with enough accuracy to permit the inference of partnerships with their nutritional status , despite that this method does not take into account seasonal changes. Due to differences in the consumption pattern found in men and women in our sample, we performed all analyzes separated by gender. In 2005, Tur et al. published the results of a food and nutrient intake study in a population similar to ours, and found different behaviors between men and women .
The characteristics related to lifestyle (Table 1) also show significant differences in some parameters between both genders. In our sample, men drink more alcohol and smoke more but do more physical exercise than women. In terms of BMI, we found the highest percentage of obesity in men. These results are consistent with the findings of the latest Catalan Health Survey for the adult population (ENCAT). We did not find differences between men and women with regard to education level, which is consistent with the findings published by the statistics office of the Spanish Ministry of Education in 2010 (INE).
Iron levels were determined using levels of transferrin saturation as an indicator of circulating iron, and SF as an indicator of iron stores. According to oxidative theory, elevated levels of either of these pose a health risk . SF is an acute phase reactant and can be increased under inflammatory conditions. Hence, C-reactive protein was analyzed to control the confounding effects of inflammation on serum ferritin.
To determine the parameters for the degree of oxidative stress we used two widely used biomarkers. TBARS assay is inexpensive, fast and very easy to perform. Even though there is controversy regarding the specificity of TBARS toward compounds other than MDA, the fluorometric measurement of TBARS is one of the most commonly used biomarkers for detecting the oxidative damage to lipids, of which MDA is the major compound . The ORAC assay is one of the most reliable methods used to measure the total antioxidant capacity. This assay has frequently been used to provide information about the antioxidant capacity of foods . ORAC is a biomarker that that does not distinguish between the quality and quantity of antioxidants present in plasma; however, ORAC is useful for studies such as ours which aim to determine the overall antioxidant capacity and correlate it with the intake. The use of these two different biomarkers yields a comprehensive view of oxidation in both directions: pro-oxidant and preventive.
In the descriptive analysis, we found that men had a higher energy intake and therefore higher consumption of most of the nutrients studied than women; however, we found no gender difference in the consumption of fruit and vegetables. On the other hand, body iron levels were higher in men than in women, evidence that has been widely connected to higher energy intake, non-menstrual blood loss and hormonal factors.
Our data revealed that the ORAC value was similar in the plasma of the younger and the older subjects and that there were no differences between the ORAC values of men and women. These gender results are in agreement with previous studies , although some authors have found differences between age groups . The use of this biomarker to determine the total antioxidant level in plasma has only been used to compare pathology groups with healthy controls, but studies assessing healthy subjects separated by age and sex are scarce. The role of oxidative damage in normal aging has been substantiated in studies with experimental animals, but there is limited evidence in humans. In our study, TBARS levels were found to increase with age, which is consistent with the results of other researchers . Furthermore, our results support the “free radical theory of aging” proposed in 1956 by Harman based on the accumulation of oxidatively damaged molecules.
Voss et al. reviewed the behavior of oxidative stress, the glutathione system and antioxidant systems related to diet and lifestyle in relation to age . They predicted an increase in oxidative stress, a decline in the glutathione system, and the lifetime preservation of antioxidant systems related to diet. In this respect, our results are consistent with this reasoning, as we found an increase in TBARS and similar ORAC values at different ages.
To study the association between diet and biochemical iron levels with oxidative stress, we used dietary factors associated with oxidation or dietary iron absorption, adjusting for confounding factors.
There are two forms of dietary iron: heme and non-heme. Iron in meat, fish and poultry is found in a chemical structure known as heme. The body absorbs heme iron very efficiently. Iron in vegetables and legumes is arranged in a different chemical structure called non-heme iron. Non-heme iron is not as well absorbed as heme iron . As shown in Table 3, dietary heme and non-heme iron have differing relationships with oxidative stress. Non-heme iron is related to a decrease in oxidative damage as measured by TBARS, whereas the opposite is true of heme iron. These results reflect the effect of diet on oxidative stress, i.e. the consumption of products rich in non-heme iron (vegetables) linked to a diet rich in antioxidants, and the consumption of heme iron (from meat and fish) associated with a pro-oxidant diet. The possible pro-oxidant properties of heme iron have been the subject of study in the past  and a close relationship was identified between heme iron intake and the risk of mortality from cardiovascular disease, lung cancer or upper digestive tract cancer and general mortality . In contrast, non-heme iron intake is negatively associated with this peroxidation. This may be because non-heme iron is found mainly in leafy green vegetables, fruit and vegetables, which are also a good source of vitamins and antioxidants which may decelerate the negative effect of iron on human health.
Comparing the results for men and women separately, it becomes clear that in men smoking is associated with oxidative stress, a connection which is widely known. It also seems that increased oxidative stress in women is related to higher transferrin levels. Iron can damage tissues by catalyzing the conversion of hydrogen peroxide to free-radical ions which attack cellular membranes, proteins, and DNA. Proteins sequester iron to reduce this threat. Iron ions circulate bound to plasma transferrin and accumulate within the cells in the form of ferritin . The presence of free iron has been associated with low concentrations of transferrin accompanied by an increase in its saturation in an attempt to prevent the accumulation of that free iron in plasma. Many studies have linked high and even moderate levels of iron with many chronic and common diseases in the population, such as cardiovascular disease and cancer . According to these studies, iron, as an oxidant mineral, could lead to increased oxidative stress and therefore promote all types of diseases related to oxidation. Given that the participants in this study are not patients but members of the general population, we believe that the increased transferrin saturation may be a reflection of higher levels of free iron and therefore of increased oxidative stress (TBARS).
When the same MLR models were applied to look at antioxidant capacity, many points of similarity with the previous study emerged. First, a diet rich in saturated fatty acids, which are a source of oxidizable lipids, is related to a decrease in antioxidant capacity and a diet rich in vegetables, which is a source of antioxidants, and vitamin C intake are related to an increase in ORAC. Both results are consistent with findings in the existing literature . This increase in antioxidant capacity along with increased SF supports the hypothesis posed by Juckett et al. . They suggested that ferritin may act as a protective mechanism against oxidative processes, and that oxidative stress induces the synthesis of ferritin to capture the oxidized iron, and thus preventing toxicity.
The positive relationship between increased iron storage and antioxidant status does not differ between men and women. However, in men ORAC increases significantly with C-reactive protein. Since CRP is a marker of inflammation, this could indicate a possible mechanism for defending the body against oxidative stress in situations such as inflammation. Peairs et al. , in an intervention study with a low-carbohydrate, weight-loss diet, found a significant decrease in some indicators of inflammation (e.g. MCP-1) but not others (e.g. IL-6, CRP) within seven days. Recently, Floegel et al.  concluded that the antioxidant intake from supplements may have additional beneficial effects on plasma Hcy but not on serum CRP concentrations in the US population. Therefore, the relationship between antioxidant status, CRP and SF is not clear and requires further research.
As mentioned earlier, an increase in the consumption of vegetables and vitamin C is associated with improved antioxidant capacity, but a regression analysis separating subjects by gender revealed that this improvement is exclusive to women. Although no differences were recorded in vegetable consumption between men and women, the significant increase of some antioxidants like beta carotene and vitamin C in women might indicate that the consumption of vegetables by gender does not differ in quantity but may differ in quality.