When an individual exercises, there is a 3 to 72 hour subsequent period known as the ‘open window’ where they are susceptible to illness due to altered immune function and activity [1
]. Consequently, upper respiratory illness (URI) is a prevalent health complaint for athletes [2
]. URI episodes have a significant impact on training and performance with 19% and 31% of athletes reporting a decrease in or complete cessation of training, respectively [4
]. School athletes are a specific cohort who, in addition to high training loads, have external contributors (e.g., academic and social demands) that are associated with increased stress and URI risk [5
]. This is illustrated by the increased illness rates in the 2012 Winter Youth Olympic Games (8.4%) [2
]. In addition, it has been reported that the respiratory illness rates in elite high school athletes is higher than other levels in the same sport [7
With no effective front-line cure, treatment plans for URIs are centred on symptom alleviation [8
]. Sports teams are therefore focusing on novel nutritional supplements that may decrease URI incidence [9
]. A recent meta-analysis reported that flavonoids, a sub group of polyphenols, decreased URI incidence by 33% after at least 7 days’ supplementation [9
]. Olive leaf extract (OLE) is an over-the-counter supplement containing a range of polyphenols, predominately oleuropein and hydroxytyrosol (HT). Oleuropein and HT have a range of mechanisms that may reduce URI incidence and duration. Firstly, oleuropein acts as an antioxidant with dose-dependent inhibition of the copper sulphate-induced oxidation of low-density lipoproteins (LDLs) [13
]. Furthermore, oleuropein increases nitric oxide production in macrophages, increasing functional activity [14
]. Oleuropein also has an effect on the aetiology of URIs by producing strong antimicrobial activity against gram-positive and gram-negative bacteria and antiviral activity against respiratory syncytial virus (RSV), a common URI virus [15
Scientific and anecdotal evidence of the health benefits of OLE supplementation have led to significant attention and use [18
]. To date, there is no research investigating the effect of OLE on URI incidence or duration. The objective of this study is therefore to determine the effect of OLE supplementation on URI incidence and duration in high school athletes.
To the authors’ knowledge, this is the first study investigating the effects of OLE supplementation on any athlete cohort relating to health and illness indices. The primary outcome of the study suggests that OLE has little effect on URI incidence but reduces sick days by 28% in high school athletes. Secondly, both cohorts had sub-optimal carbohydrate intake, which may have mitigated any further effects of OLE supplementation [22
]. This study highlights that URIs are a problem in high school athletes, with at least 50% experiencing one or more URIs over a nine-week period, and 10% experiencing three. Although OLE did not decrease URI incidence, further research should investigate other individual and polyphenol combinations that may decrease incidence.
Although both groups had nearly equal gender distribution, overall the study had more women (66%) than men, which may have influenced the outcomes. One previous study on athletes reported that the URI incidence in females was insignificantly higher than in males (0.6 ± 0.8 in males and 0.8 ± 1.0 in females); however, the duration was longer for females (6.8 ± 7.1 days for females and 4.7 ± 5.0 days for males (p
= 0.016)) [25
]. Conversely, in the 2012 Winter Youth Olympics women had an increased overall illness rate (risk ratio = 1.84 (95% CI 1.21 to 2.78)); however, this was not separated into URIs [6
]. In this study, there was no significant effect of OLE on incidence when gender was controlled for (p
> 0.05), but interestingly there was a significant reduction in illness days in the females supplemented with OLE, but an increase in males (OR: 0.57 (95% CI 0.43–0.77) and 1.96 (95% CI 1.01–3.85), respectively). Stress and menstrual cycle are known influencers of immune parameters, including natural killer cells and CD4+
ratio, and it is possible OLE has an improving effect on URI risk in women, ameliorating menstrual immune dysfunction [26
]. Additional research should be conducted in single gender populations to elucidate this potential effect.
Interestingly, the majority of studies conducted investigating whether other nutritional supplements (vitamin D, vitamin C probiotics, bovine colostrum protein, flavonoids and β-glucan) decrease URI in athletes have shown no significant decrease in duration irrespective of a decrease in incidence [9
], although vitamin C appears to decrease sick days while not influencing the incidence [31
]. This difference could be explained by the effects of OLE, which include an increase in the functional activity of macrophages and in antimicrobial and antiviral activity [14
]. Rather than alleviating the altered immune function that occurs post-exercise, OLE may have an effect once a pathogen has entered the host, preventing further replication and improving the host’s defence, thus reducing duration.
Four participants on OLE supplementation experienced side effects. No serious adverse effects have been reported in OLE studies; however, other side effects have been reported, notably allergic reactions in those who have an allergy against the plant species (Oleaceae) [32
]. Studies in mice have reported no adverse effects with acute oleuropein doses at 1000 mg·kgBW−1
, including no toxicity of its breakdown products (HT and elenolic acid) at this dosage [33
]. Despite recent studies on OLE supplementation for humans, which reported no increased adverse effects, more research is needed examining the safety in humans and potential long-term effects [35
]. The authors recommend that the product is tolerable to the majority; however, as sport requires optimum performance, if any side effects are perceived by the athlete, supplementation should cease immediately.
Nutritional intake can also be an important confounding variable when it comes to athletes and illness rates, especially in sports that require elements of leanness. Recent statements by the International Olympic Committee (IOC) and American College of Sports Medicine (ACSM) have remarked that athletes with low energy availability (LEA) have an increased risk of infections and illnesses, especially noting women and the female triad [22
]. Both cohorts consumed less than the 5 to 7 g·kg−1
of carbohydrates recommended by the IOC for athletes exercising at this level [24
]. The authors propose that the athletes may be in an immune-compromised state due to a lack of carbohydrates and subsequent LEA, which OLE supplementation is unlikely to offset. In addition to potential OLE supplementation, there is a need to educate sports people at this level regarding adequate macro and micronutrient intake to ensure optimal performance and health.
There are limitations in this study—notably, training intensity was not measured. Training intensity is an important variable in the extent and duration of the ‘open window’ [1
]. This study included team sports, therefore an individual’s game time may have been the same but they had different demands and intensity profiles based on their position. If accounted for, this may have elucidated an effect of OLE not otherwise detected.
It is also noted that compliance is not perfect in either group. As compliance analysis was undertaken at completion, the authors were not able to track what days or periods that participants missed. OLE has a rapid bioavailability with conjugated HT metabolites peaking approximately 1 hour after consumption [39
]. Once ingested, oleuropein undergoes extensive hydrolysis during Phase I metabolism, releasing more HT, and then undergoes sulfation and glucuronidation during Phase II [40
]. Consequently, the majority of oleuropein consumed circulates as other conjugated metabolites [40
]. Research using another OLE product reported that consuming 76.6 or 51.1 mg of oleuropein (in OLE capsule form) resulted in peak plasma concentrations of 0.60 ± 0.37 and 0.52 ± 0.24 ng·mL−1
, respectively [39
]. It is noted that the liquid version of OLE has a significantly higher peak of metabolites; however, the liquid has a very strong and bitter taste which is likely to decrease compliance even further [39
]. Both oleuropein and HT metabolites are excreted in urine within 24 h, so daily supplementation is needed to ensure circulating levels [41
]. If a participant missed a few days consecutively it may have attenuated any effect of OLE supplementation.