Introduction
Olive oil is the main cornerstone of the Mediterranean diet and its consumption, specifically the extra virgin variety, is associated with a reduced inflammation and a diminished risk of cardiovascular disorders and mortality [
1‐
3]. These benefits may be related to its source of polyphenols, which have been shown to possess antimicrobial, antioxidant and anti-inflammatory systemic properties [
4,
5]. Despite the large amount of beneficial effects on health, the impact of olive oil supplementation on endurance performance is still unclear [
6]. These effects have been measured through specific aerobic physiological markers like maximal oxygen uptake (VO
2max), but not according to variables able to capture the dynamic interactions among physiological systems.
On the basis that diets high in unsaturated fatty acids and endurance exercise share different positive effects on metabolic and cardiovascular health, and given that both seem to increase fat oxidative capacity, earlier research hypothesised that their combination may have synergic effects [
7]. The main results revealed that unsaturated fatty acids supplementation tended to slightly increase fat oxidation after training compared to control conditions. However, these changes were not reflected in VO
2max or in other performance and physiological parameters [
6‐
8]. Moreover, regarding the antioxidant effect of beverages containing polyphenols on physical performance and physiological markers, the results are not yet entirely clear, with studies reporting controversial effects [
9]. These unclear results suggest that VO
2max and other commonly registered physiological and performance variables might not be sensitive enough to detect specific exercise-related changes as has been suggested by other authors [
10‐
12]. A feasible explanation for the lack of sensitivity may stem from the fact that they provide little information on the nature of the dynamic interactions between physiological systems and their common role in an integrated network [
13,
14]. More specifically, cardiovascular and respiratory systems change their interaction as a consequence of exercise [
10]. Therefore, to capture such specific interactions, the time series analysis and the detection of coordinative variables [
15] seems a recommendable strategy. These research approaches can detect not only quantitative differences related to maximal physiological values (e.g., VO
2max), but also qualitative changes related to the coordinated activity among physiological systems, and their changes under exercise-related constraints [
16,
17]. This coordinative changes occurring at systemic level as a consequence of exercise, although much less studied than those occurring at cellular or subcellular level, are not less relevant.
Since the anti-inflammatory and antioxidant effects of olive oil are observed at a systemic level [
5], and its effectiveness may not be precisely measured through the commonly registered physiological and performance parameters, we proposed here to use a recently investigated coordinative variable (cardiorespiratory coordination; CRC) which has shown a higher responsiveness to training [
10] and workload accumulation [
11], in contrast to VO
2max and other markers of aerobic fitness. CRC is a novel variable informing about the co-variation of cardiovascular and respiratory variables during cardiorespiratory exercise testing [
10,
11]. It is estimated through principal components analysis (PCA), performed on the time series of selected variables. In contrast to isolated cardiorespiratory outcomes, the cardiorespiratory response to exercise can be represented through PCA, i.e., by a short set of principal components (PCs) extracted in decreasing order of importance (the first PC accounting for most of the variation). Principal components represent the maximum possible fraction of the variability from the original data, so that the total number of PCs reflects the degree of coordination among the selected variables. As pointed out by Kelso [
18], a dimension reduction is a hallmark of formation of coordinative structures, and so, the decrease in the number of PCs and/or the increase in PC eigenvalues can be interpreted as an improvement in the efficiency of CRC (see Balagué et al. [
10] for a detailed explanation).
Accordingly, the aim of the current research was to assess the effect of an acute fatty acid supplementation, in the form of extra virgin olive oil rich in polyphenols, on CRC and physiological and performance variables, compared to palm oil rich in unsaturated fatty acids and without polyphenols. We hypothesized that the positive effects of olive oil supplementation would be mainly reflected on CRC.
Discussion
The present research was conceived to assess the effect of an acute fatty acid supplementation, in the form of extra virgin olive oil rich in polyphenols, on CRC and performance, compared to palm oil and placebo. An increase in CRC under olive oil supplementation, probably provoked by its high content in polyphenols, was observed. However, these improvements were not reflected on the commonly evaluated maximal performance and physiological variables. These results suggest that CRC could be a sensitive tool to detect systemic effects linked to dietary supplementations.
The improvement in CRC under olive oil supplementation was solely observed at moderate intensity during the incremental test. Specifically, PC
1 eigenvalues were significantly higher compared to palm oil and placebo supplementations. Since PC
1 eigenvalues show the ratio of explained variance by PC
1 [
10], these results, indicating an increase in the degree of co-variation among the selected physiological variables, informed about an improvement in CRC. In other words, under olive oil supplementation, the ventilatory efficiency of the cardiorespiratory system seemed larger. These results might be explained by the high content of olive oil in phenolic compounds, tocopherol, or carotenoids, which have been shown to possess antioxidant and anti-inflammatory properties, by producing beneficial effects on lipid oxidation and in general oxidative stress [
4,
5]. As pointed out by Sallam and Laher [
24], acute bouts of exercise provoke transient damage to contracting skeletal muscles, triggering an inflammatory response that increases the levels of proinflammatory cytokines and reactive oxygen species (ROS) production. Thus, olive oil supplementation could have increased the antioxidant capacity while performing at moderate exercise intensity, reducing the negative effects of ROS accumulation [
25]. A feasible explanation for the absence of differences in CRC among dietary supplementations at low and high intensity, might stem from the fact that markers of lipid peroxidation seem to be lower during mild and high intensity compared to moderate exercise [
26]. The concentration of lipid peroxidation markers at low and high intensities was not probably enough to impair antioxidant capacity and, therefore, ROS clearance did not occur at a high enough rate to affect cardio-respiratory function. As a result, the antioxidant and anti-inflammatory effects of olive oil could be detected through CRC solely during moderate intensity exercise. Another explanation for the lack of differences at low and high intensity might be related to the Fat
max zone [
27], which seems to range from 40 to 75% VO
2max [
28]. The cutting points delimiting the moderate intensity interval in the current study were located at 40.29
+ 10.57 and 70.81
+ 7.08% VO
2peak. Thus, the previously observed slight effect of olive oil on fat oxidation [
7], could have been magnified at moderate intensity (i.e., Fat
max), and this might have been detected by CRC analysis.
As depicted in Table
2, while comparing the projections of the selected cardiorespiratory variables onto PC
1 at moderate intensity, VE projection was shown to be significantly higher in olive oil compared to palm oil supplementation. This means that the increment in the degree of CRC observed under olive oil supplementation was mainly provoked by a change in VE behaviour (i.e., a decrease in the absolute values of VE while performing at moderate intensity and at AT; see Fig.
1 and Table
2), which subsequently led to an increase in co-variation between VE and the other cardiorespiratory variables. The decreased VE under olive oil supplementation could be related to the high content in polyphenols [
5], specifically, hydroxytyrosol [
29], which may have some peripheral effects on mitochondrial function. As detailed in Hao et al., [
29] relatively low doses of hydroxytyrosol increase the expression of all mitochondrial respiratory chain complexes, including ATP synthase, and stimulates mitochondrial biosynthesis pathway. This fact could let to an improvement of central control of VE and, thus, a reduction in ventilatory demands and an increase in ventilatory efficiency (i.e., a reduced VE for the same workload). In contrast, since palm oil and placebo supplementations contained no polyphenols [
30], participants probably presented higher ventilatory demands. Accordingly, at moderate intensity (i.e., at AT), PETO
2 was lower and PETCO
2 was higher under olive oil, compared to palm oil supplementation (Table
3). Although VE and PETO
2 were lower and PETCO
2 was higher under olive oil, compared to both palm oil and placebo supplementations at moderate intensity, the reduced sample size of this study can probably explain why statistically significant differences were only found between olive oil and palm oil supplementations, but not with respect to placebo conditions.
In agreement with Boss et al. [
7] and Capó et al. [
6,
8], athletic performance (measured through exercising time) was not altered by olive oil supplementation, probably because the improvement on CRC observed in the current study was not strong enough to positively affect performance. Since changes associated to olive oil supplementation seem to be not only happening at cellular or subcellular level but also at systemic levels, and given that CRC is a coordinative variable which might be more sensitive to exercise and dietary supplementations than commonly registered physiological and performance variables, the tracking of changes in CRC may contribute to shed light on other unclear questions regarding the effectiveness of different fatty acid supplementations, such as omega-3 polyunsaturated fatty acids [
31] or conjugated linoleic acid [
32]. Further research should be conducted to provide an accurate picture of the effectiveness of some controversial dietary supplementations throughout CRC evaluation, such as the physiological mechanisms underlying the impact of beetroot juice supplementation on cardiorespiratory function, in highly-trained endurance athletes [
33].
Our findings should be discussed in the light of our methodological limitations. First, oxygen and CO
2 content and air flow rate were monitored using a low frequency (i.e., every 30 s), and continuous blood pressure monitoring could not be provided in this study. However, the findings obtained in the current research (i.e., high sensitivity and responsiveness of CRC, number of PCs, PC projections) are in full accordance and reinforce those published previously, where respiratory gas exchange was recorded breath by breath and blood pressure was monitored continuously [
10,
11]. This means that CRC might be also studied using lower sampling rates when tests have longer duration and provide enough data sets. Second, since inflammatory and lipid peroxidation markers could not be assessed in this research, we cannot guarantee that the changes observed in CRC under olive oil supplementation were provoked by physiological adjustments at this level. Therefore, further research is warranted analysing CRC together with inflammatory and oxidation markers to confirm this hypothesis.