1. Introduction
Walnuts (
Juglans regia L.) are grown in Serbia and are a traditional Serbian food [
1]. Walnuts are a nutrient-dense food containing many bioactive constituents which separately or together may produce beneficial effects on human health [
2]. Based on the analysis of the EuroFir food composition databases, walnuts were identified as major sources of
n-3 fatty acids among nuts in the Balkan region. Additionally, walnuts represent nuts rich in ALA. Maguire and coauthors, showed that walnuts have particularly high content of ALA (11.6% of total) compared to other nuts (i.e. hazelnuts and almonds) [
3,
4]. The ratio of ALA to linoleic acid is high among all the tree nuts [
5] meaning that walnuts have a beneficial
n-6/
n-3 ratio [
6]. Given the cultural acceptability of walnuts as a traditional food, walnuts are seen as an excellent candidate for improving
n-3 fatty acid intake [
7]. Limited consumption of
n-3 fatty acids and a Western type diet contribute to the unfavorable fatty acid profile in humans. The intake of
n-6 PUFA is significantly higher than the intake of
n-3 PUFA which results in a negative shift of
n-6/
n-3 ratio in plasma [
1]. In addition to
n-3 fatty acids, walnuts contain many different minerals [
8,
9]. The walnuts are a good source of manganese, a trace element important for maintaining cellular redox status [
10]. By virtue of the unique micro- and macro-nutrient composition of walnuts, an increased consumption of walnuts has been seen as beneficial for cardiovascular health [
11]. Prospective observational studies and large clinical trials demonstrate various health benefits of walnut consumption including a reduced risk of cardiovascular diseases, cholesterol-lowering effects, reduced oxidative stress, reduced inflammation, and blood pressure and an increased vascular reactivity [
12]. In order to overcome the low intake of
n-3 FAs walnuts could be a beneficial dietary choice. Therefore, we postulated the following aims: The first aim was to analyze the nutritional composition data, fatty acid and mineral content of walnuts grown in Serbia. The second aim was to evaluate the consumption of
n-3-fatty acids by Serbian residents using the 24-h dietary recalls. The third aim was to evaluate the fatty acid profile of an apparently healthy group of people. The fourth aim was to assess the effect of walnut consumption in the amount of 56 g daily on the plasma phospholipid fatty acid profile of participants involved in a randomized 4-weeks trial.
4. Discussion
This study confirmed that walnuts are a good source of n-3 PUFA. People living in Serbia have a tendency to have a low dietary intake of n-3 PUFA which was confirmed by FA profile analysis. Consumption of walnuts short term (4 weeks) could be beneficial for increasing the intake of n-3 PUFA.
Walnuts contain many nutrients that could be helpful for improving fatty acid, phytochemical and micronutrient intake of consumers [
27]. The variability between walnuts obtained from different markets is larger than for fatty acids, resulting in a higher coefficient of variation. The results obtained in the present study are similar to data presented within the European Food Information and resource database. With this study we added nutrient composition data for manganese, nickel and sodium. In comparison of the mineral content of Serbian walnuts with that of other European countries, using the simultaneous access to a multiple set of available Food composition data base (FCDBs) through the EuroFIR FoodEXplorer (2013) [
28], it has been shown that there are some differences, also similarity, which affect nutrition composition database.
Although the major fatty acid in walnuts is linoleic acid (
n-6), the content of linolenic acid (
n-3), is also substantial [
6]. Walnuts contain higher amounts of linolenic acid (
n-3) than any other nuts (almonds, hazelnuts) [
29]. According to the values presented in
Table 2, a 25-gram serving of Serbian walnuts would provide 1.9 g of linolenic acid which is an amount well within the recommended intake of 0.58 to 2.81 g/day [
30]. Data presented by Marangoni et al., [
31] indicate that the intake of slightly over 1 g ALA/day through walnut consumption significantly and rapidly increased blood ALA and EPA levels.
We evaluated the intake of foods rich in
n-3 fatty acids, precisely the intake of walnuts. Only 20% of the study participants consumed whole walnuts. Among our study participants, walnuts consumers ate on average 9.7 g/d. On the level of the whole study sample, the intake is significantly lower to 1.5 g/d. Among the US adult population, an average consumption of 10.3 g/d and 13.1 g/d of walnuts was reported for walnut consumers 29–59 years old and those older than 60 years, respectively (Arab, Ang, 2015). Authors of PREDIMED study, reported that the average intake of walnuts per day for all study participants was 5.9 g [
32]. Probably, the majority of the population consume walnuts incorporated in some other foods, such as cakes.
The mean dietary ratio of
n-6 to
n-3 fatty acids in a sample of Serbian individuals was high (21.3), confirming poor eating patterns of people in this region. Some evidence suggest that
n-6/
n-3 ratio may be a potential biomarker for the development of cardiovascular diseases and decreasing the ratio could improve health status of Serbian population [
1]. There are published data on the
n-6/
n-3 ratio in some other countries like Japan (4.00), United Kingdom and Northern Europe (15.00), the United States (16.74), and urban India (38–50) [
33]. In general people living in Serbia, a non-Mediterranean West Balkan country, have an inadequate intake of total
n-3 fatty acids [
34]. Data from the European Prospective Investigations into Cancer and Nutrition (EPIC)—Norfolk Study showed that the total dietary intake of
n-3 FAs in men was 1.5 g/d, while in women it was 1.22 g/d [
35]. PREDIMED study pointed out that in Spain, the average daily dietary
n-3 intake was 2.24 g (calculated as a sum of ALA and
n-3 LC PUFA [
32]. The intake of
n-3 FAs in our study group was 0.9 g/d, which is substantially lower from the intakes reported for aforementioned Mediterranean countries. The ability of people to meet the recommended daily intake of
n-3 fatty acids via consumption of walnuts is important in non-Mediterranean West Balkan countries where a Greek-Mediterranean diet is not typically consumed. As the incidence of cardiovascular diseases in Serbia is very high [
36,
37], an increased consumption of
n-3 fatty acid rich foods, such as walnuts, could be of great importance. Potential interventions aimed at increasing the consumption of walnuts might be more easily implemented in Serbia than in other countries, since the walnuts are a part of traditional Serbian food [
1].
Although the major fatty acid in walnuts is linoleic acid (
n-6), the content of linolenic acid (
n-3), is substantial [
6]. Walnuts contain higher amounts of linolenic acid (
n-3) than any other nuts (almonds, hazelnuts, chestnuts) [
38]. Using the values in
Table 2, a 25-gram serving of Serbian walnuts would provide 1.9 g of linolenic acid which is an amount well within the recommended intake of 0.58 to 2.81 g/day [
39]. Data presented by Marangoni et al., [
31] indicate that the intake of slightly over 1 g ALA/day through walnut significantly and rapidly increased blood ALA and EPA levels. The efficiency of the pathway is inherently low in humans, with an estimated conversion of ALA to EPA of 0.2–6% and <0.1% for DHA, and therefore any changes in bioconversion efficiency have potentially large impacts on LC PUFA status [
40]. We hypothesized that ALA from walnuts could correct the low
n-3 status and tested this by a short-term intervention pilot study. Our pilot study data showed the improvement in the percentages of ALA (
p ≤ 0.01), EPA (
p ≤ 0.01) and overall
n-3 (
p ≤ 0.05) FAs in plasma phospholipids of study participants, while the
n-6/
n-3 ratio was decreased (
p ≤ 0.05).
Regarding the impact of consumption of walnuts on anthropometric measures our study demonstrated (
Table 5), a significant reduction in BMI, percentage of body fat, increased lean body mass and an increased amount of water in the body. Similar results (i.e., weight reduction) was provided by a large population-based cohort study (−0.10 kg, 95% CI-0.15, −0.04) [
23,
41]. Regarding the impact of walnut consumption on blood pressure measurements, we detected a significant reduction in systolic blood pressure (
Table 5). The blood pressure lowering effect could be due to the increased content of minerals in walnuts, i.e., magnesium and potassium. The impact of walnuts nutritional intervention on decreasing effect of blood pressure is also observed in the previous intervention the “Health Track study”, which reported significant reductions of SBP in examined study participants, which was attributed to the intake of 30 g walnuts/d [
42].
Our study results suggest that the moderate consumption, of walnuts not only improves the
n-3 fatty acid status of consumers, people of West Balkan non Mediterranean countries, but could also have some additional beneficial effects on human health. It is important to point out that there was an increase in EPA indicating an elevated conversion of ALA to EPA (>10%),
Table 5.
Regarding the study limitations, we underline a short-term duration of our intervention study and a small number of the participants included in this sub-study, so in this context the data should be interpreted with caution. The higher conversion of ALA to EPA (>10%) in examined study participants underlines the importance of a moderate consumption of walnuts. In addition, the obtained data provide basis for further larger clinical trials. Future studies should be designed with a higher number of study participants and should be run over a longer period of time.