Citation: Jost, Z.; Tomczyk, M.;
Chroboczek, M.; Calder, P.C.;
Laskowski, R. Improved Oxygen
Uptake Efﬁciency Parameters Are
Not Correlated with VO2peak or
Running Economy and Are Not
Affected by Omega-3 Fatty Acid
Supplementation in Endurance
Runners. Int. J. Environ. Res. Public
Health 2022, 19, 14043. https://doi.
org/10.3390/ijerph192114043
Academic Editors: Nicolas Berger
and Russ Best
Received: 14 October 2022
Accepted: 25 October 2022
Published: 28 October 2022
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International  Journal  of
Environmental Research
and Public Health
Article
Improved Oxygen Uptake Efﬁciency Parameters Are Not
Correlated with VO2peak or Running Economy and Are Not
Affected by Omega-3 Fatty Acid Supplementation in
Endurance Runners
Zbigniew Jost 1,*
, Maja Tomczyk 1, Maciej Chroboczek 2, Philip C. Calder 3,4
and Radosław Laskowski 2,*
1
Department of Biochemistry, Gdansk University of Physical Education and Sport, 80-336 Gdansk, Poland
2
Department of Physiology, Gdansk University of Physical Education and Sport, 80-336 Gdansk, Poland
3
Faculty of Medicine, School of Human Development and Health, University of Southampton,
Southampton SO16 6YD, UK
4
NIHR Southampton Biomedical Research Centre, University Hospital Southampton NHS Foundation Trust
and University of Southampton, Southampton SO16 6YD, UK
*
Correspondence: zbigniew.jost@awf.gda.pl (Z.J.); radoslaw.laskowski@awf.gda.pl (R.L.)
Abstract: Peak oxygen uptake (VO2peak) is one of the most reliable parameters of exercise capacity;
however, maximum effort is required to achieve this. Therefore, alternative, and repeatable sub-
maximal parameters, such as running economy (RE), are needed. Thus, we evaluated the suitability
of oxygen uptake efﬁciency (OUE), oxygen uptake efﬁciency plateau (OUEP) and oxygen uptake
efﬁciency at the ventilatory anaerobic threshold (OUE@VAT) as alternatives for VO2peak and RE.
Moreover, we evaluated how these parameters are affected by endurance training and supplementa-
tion with omega-3 fatty acids. A total of 26 amateur male runners completed a 12-week endurance
program combined with omega-3 fatty acid supplementation or medium-chain triglycerides as a
placebo. Before and after the intervention, the participants were subjected to a treadmill test to
determine VO2peak, RE, OUE, OUEP and OUE@VAT. Blood was collected at the same timepoints to
determine eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in erythrocytes. OUE corre-
lated moderately or weakly with VO2peak (R2 = 0.338, p = 0.002) and (R2 = 0.226, p = 0.014) before and
after the intervention, respectively. There was a weak or no correlation between OUEP, OUE@VAT,
VO2peak and RE despite steeper OUE, increased OUEP and OUE@VAT values in all participants.
OUE parameters cannot be treated as alternative parameters for VO2peak or RE and did not show
changes following supplementation with omega-3 fatty acids in male amateur endurance runners.
Keywords: peak oxygen uptake; oxygen uptake efﬁciency plateau; running economy; omega-3 fatty
acids; endurance runners
1. Introduction
There are many cardiopulmonary exercise tests (CPETs) that aim to assess parameters
related to human performance, such as peak oxygen uptake (VO2peak) or maximal oxygen
uptake (VO2max). VO2max is considered the best indicator of potential in endurance events,
being a ‘gold standard’ measurement of integrated cardiopulmonary-muscle oxidative
function [1–3]. Although heart rate (HR), respiratory exchange ratio (RER), and minute
ventilation (Ve) are considered cardiovascular, respiratory, and pulmonary parameters,
respectively, their comprehensive function is often difﬁcult to evaluate. Therefore, there is
a need to identify alternative validated and reliable parameters for assessing cardiorespira-
tory ﬁtness.
Sun and co-authors [4] determined the relationship between oxygen uptake (VO2)
and Ve, called oxygen uptake efﬁciency (OUE). They noted that OUE increases linearly
with time during early exercise, but becomes non-linear as Ve increases faster than VO2.
Int. J. Environ. Res. Public Health 2022, 19, 14043. https://doi.org/10.3390/ijerph192114043
https://www.mdpi.com/journal/ijerph
Int. J. Environ. Res. Public Health 2022, 19, 14043
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This curvilinear relationship during an exercise test is not as appropriate for assessing
aerobic capacity as VO2peak. Thus, the authors described other physiological parameters
that can be determined from respiratory gases during CPET, i.e., oxygen uptake efﬁciency
at the ventilatory anaerobic threshold (OUE@VAT) and oxygen uptake efﬁciency plateau
(OUEP) in healthy subjects. It was observed that both OUE@VAT and OUEP are simple
measurements that do not require maximum effort. Moreover, they are also easy to visualise,
recognise and calculate [4], making them potentially robust parameters for assessing
physical ﬁtness. It is worth noting that there is still scarce evidence of improvements in OUE
parameters after physical training and no evidence of improvements after supplementation
with bioactive compounds such as the omega-3 fatty acids (eicosapentaenoic acid (EPA)
and docosahexaenoic acid (DHA)).
Although some studies show no improvements in cardiopulmonary-muscle oxida-
tive function following supplementation with ﬁsh oil containing omega-3 fatty acids [5,6],
several studies do indicate a positive effect. For example, long-term EPA and DHA supple-
mentation may contribute to the improvement in VO2max [7] or to the reduction in the cost
of aerobic exercise in trained cyclists [8,9]. Moreover, our recent study showed that 12-week
supplementation with omega-3 fatty acids improved running economy (RE) in amateur
runners [10]. These studies focus mainly on VO2max and RE, and other submaximal oxygen
kinetics parameters need to be further explored.
The aim of our study was to determine whether OUE, OUEP and OUE@VAT can
be considered as a robust measurements of endurance capacity. Moreover, we verify if
those parameters are sensitive to changes after omega-3 fatty acid supplementation. The
main hypothesis of this research was that OUE will be sensitive to changes in VO2peak. We
also hypothesised that OUE@VAT and OUEP can be used as non-invasive, submaximal
parameters of oxygen kinetics replacing VO2peak and RE. We also evaluated whether twelve-
week of endurance training combined with omega-3 fatty acid supplementation can alter
these parameters in male amateur endurance runners.
2. Materials and Methods
2.1. Participants
A total of 26 male amateur runners (37 ± 3 years old; 77 ± 9 kg body weight; VO2peak:
54.2 ± 6 mL*kg−1*min−1) completed the 12-week experimental study as previously de-
scribed [10], which tested the effect of supplementation with omega-3 fatty acids on exercise
capacity in male amateur endurance runners. Participants were not taking medication and
all were in good health, as conﬁrmed by a medical check. The study was approved by
the Bioethical Committee of Regional Medical Society in Gda´nsk (NKBBN/628/2019) and
conducted according to the Declaration of Helsinki (2013). All participants provided their
written informed consent prior participating in the study. Detailed participant characteris-
tics and project design are shown in Table 1 and Figure 1, respectively.
Table 1. Characteristics of participants.
Variable
MCT
(n = 12)
Mean ± SD
OMEGA
(n = 14)
Mean ± SD
Age [y]
37 ± 4
37 ± 3
Body mass [kg]
78.0 ± 8
76.3 ± 11
Height [cm]
180 ± 4
181 ± 7
EPA [% of total erythrocyte fatty acids]
Pre
1.2 ± 0.3
1.1 ± 0.4
Post
1.2 ± 0.3
4.9 ± 1.1 *,ˆ
DHA [% of total erythrocyte fatty acids]
Pre
4.4 ± 1.1
4.7 ± 1.0
Post
4.5 ± 0.8
6.7 ± 0.8 *,ˆ
Int. J. Environ. Res. Public Health 2022, 19, 14043
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Table 1. Cont.
Variable
MCT
(n = 12)
Mean ± SD
OMEGA
(n = 14)
Mean ± SD
HRmax [beats*min−1]
Pre
186 ± 9
190 ± 9
Post
184 ± 7
189 ± 9
VO2peak [mL*kg−1*min−1]
Pre
54.7 ± 6.8
53.6 ± 4.4
Post
56.4 ± 5.9
56.0 ± 3.7 *
RE [mL*kg−1*min−1]
Pre
47.7 ± 3.3
47.6 ± 1.8
Post
48.7 ± 2.9
46.5 ± 2.4 ˆ
EPA—eicosapentaenoic acid; DHA—docosahexaenoic acid; HRmax—maximal heart rate; RE—running economy;
data are presented as mean ± SD; * p < 0.05 for post vs. pre value ˆ p < 0.05 for MCT vs. OMEGA.
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
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Post 
1.2 ± 0.3 
 
4.9 ± 1.1 *,^ 
DHA [% of total erythrocyte fatty acids] 
Pre 
4.4 ± 1.1 
 
4.7 ± 1.0 
Post 
4.5 ± 0.8 
 
6.7 ± 0.8 *,^ 
HRmax [beats*min−1] 
Pre 
186 ± 9 
 
190 ± 9 
Post 
184 ± 7 
 
189 ± 9 
VO2peak [mL*kg−1*min−1] 
Pre 
54.7 ± 6.8 
 
53.6 ± 4.4 
Post 
56.4 ± 5.9 
 
56.0 ± 3.7 * 
RE [mL*kg−1*min−1] 
Pre 
47.7 ± 3.3 
 
47.6 ± 1.8 
Post 
48.7 ± 2.9 
 
46.5 ± 2.4 ^ 
EPA—eicosapentaenoic acid; DHA—docosahexaenoic acid; HRmax—maximal heart rate; RE—run-
ning economy; data are presented as mean ± SD; * p < 0.05 for post vs. pre value ^ p < 0.05 for MCT 
vs. OMEGA. 
 
Figure 1. Procedure design. 
2.2. Supplementation 
Participants were randomly assigned to one of two groups with the final character-
istics as follows: OMEGA (37 ± 3 yr; 76.3 ± 11 kg body weight; VO2peak: 53.6 ± 4 
mL*kg−1*min−1) or medium-chain triglycerides (MCT) as placebo (37 ± 4 yr; 78 ± 8 kg body 
weight; VO2peak: 54.7 ± 7 mL*kg−1*min−1). The division of participants into two groups was 
performed to check the difference in the response of OUE parameters to supplementation. 
Hence, participants supplemented four capsules per day, providing a total of 2234 mg of 
EPA + 916 mg of DHA (OMEGA group) or 4000 mg of MCT (MCT group). The capsules 
were provided in coded, identical-looking packages to avoid a potential recognition. To 
maintain the quality of supplements consisting of omega-3 fatty acids and their respective 
dosages, materials adhering to the International Fish Oil Standard (IFOS) were used. 
2.3. Treadmill Exercise Testing 
Exercise tests were conducted under controlled environmental conditions (18–20 °C 
and humidity 40–45%) and were performed at similar time of day ± 2 h. Before carrying 
out the exercise tests, the participants performed a familiarization trial. The participants 
were informed to refrain from strenuous exercise for 24 h and from caffeine or alcohol 
consumption for 12 h prior to the tests. Before and after twelve weeks of the training pro-
gram, participants undertook a ramp exercise test to volitional exhaustion on a treadmill 
(h/p Cosmos, Saturn, Nussdorf-Traunstein, Germany). First, participants stood on the 
treadmill for 2 min to make sure the measuring equipment was ready and to measure the 
resting parameters. Thereafter, runners walked for 5 min at 5 km/h speed and with a 1.5% 
inclination as a warm-up prior to starting the test. Every next stage lasted 3 min, and the 
Figure 1. Procedure design.
2.2. Supplementation
Participants were randomly assigned to one of two groups with the final characteristics as
follows: OMEGA (37 ± 3 years; 76.3 ± 11 kg body weight; VO2peak: 53.6 ± 4 mL*kg−1*min−1)
or medium-chain triglycerides (MCT) as placebo (37 ± 4 years; 78 ± 8 kg body weight;
VO2peak: 54.7 ± 7 mL*kg−1*min−1). The division of participants into two groups was per-
formed to check the difference in the response of OUE parameters to supplementation. Hence,
participants supplemented four capsules per day, providing a total of 2234 mg of EPA +
916 mg of DHA (OMEGA group) or 4000 mg of MCT (MCT group). The capsules were
provided in coded, identical-looking packages to avoid a potential recognition. To maintain
the quality of supplements consisting of omega-3 fatty acids and their respective dosages,
materials adhering to the International Fish Oil Standard (IFOS) were used.
2.3. Treadmill Exercise Testing
Exercise tests were conducted under controlled environmental conditions (18–20 ◦C
and humidity 40–45%) and were performed at similar time of day ± 2 h. Before carrying
out the exercise tests, the participants performed a familiarization trial. The participants
were informed to refrain from strenuous exercise for 24 h and from caffeine or alcohol
consumption for 12 h prior to the tests. Before and after twelve weeks of the training
program, participants undertook a ramp exercise test to volitional exhaustion on a treadmill
(h/p Cosmos, Saturn, Nussdorf-Traunstein, Germany). First, participants stood on the
treadmill for 2 min to make sure the measuring equipment was ready and to measure the
resting parameters. Thereafter, runners walked for 5 min at 5 km/h speed and with a 1.5%
inclination as a warm-up prior to starting the test. Every next stage lasted 3 min, and the
treadmill belt was accelerated starting from 8 km/h by 1 km/h per stage up to 12 km/h.
Int. J. Environ. Res. Public Health 2022, 19, 14043
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Then, the inclination of the treadmill was increased to 5%, 10% and 15% at 12 km/h
speed until volitional exhaustion, despite strong verbal encouragement. During both
tests, heart rate (HR) was monitored (Polar RS400, Kempele, Finland). RE was measured
as an oxygen cost from last 50 s of each stage to 12 km/h speed and was expressed
as mL*kg−1*min−1 [11].
2.4. Respiratory Gas Measurements
During both laboratory tests, the exhaled air was continuously measured using a
breath-by-breath analyser (Oxycon Pro, Jaeger, Hoechberg, Germany). Before the tests, the
analyser was calibrated in accordance with the manufacturer’s instructions. All measure-
ments were averaged to 10 s intervals and included: oxygen uptake (VO2), carbon dioxide
output (VCO2), minute ventilation (Ve), end-tidal pressure of oxygen (PETO2) and end-tidal
pressure of carbon dioxide (PETCO2).
2.5. Determination of Oxygen Uptake Efﬁciency and Ventilatory Thresholds
The OUE was individually determined for each participant by calculating the regres-
sion slope from the linear relationship of absolute VO2 (mL*min−1) plotted as a linear
function of Ve (L*min−1) (VO2 = Ve + b), as previously described by Sun et al. [4]. Af-
ter calculating the OUE individually for each participant from the formula, the OUE
was correlated with the true VO2peak and normalized, and the original OUE values (“b”)
were compared for the slope of the linear regression of the oxygen uptake efﬁciency.
OUEP was calculated as the 90 s average of the highest consecutive measurements of VO2
(mL*min−1)/Ve (L*min−1) and OUE at the ventilatory anaerobic threshold (OUE@VAT),
as the 60 s average of consecutive measurements at and immediately before the VAT ac-
cordingly to Sun et al. [4]. First, ventilatory threshold (VT1) was determined as increase
in both the ventilatory equivalent of oxygen (Ve/VO2) and end-tidal pressure of oxygen
(PETO2) with no concomitant increase in the ventilatory equivalent of carbon dioxide
(Ve/VCO2) [12]. The ventilatory anaerobic threshold (VAT) was measured by the V-slope
method [13]. Peak oxygen uptake (VO2peak) was obtained as the last 30 s oxygen uptake
mean value recorded during the test [14].
2.6. Training Program
All participants underwent 12 weeks of an endurance training program. The partici-
pants performed endurance training of varying intensity three times a week according to
Costa et al. [15] with slight modiﬁcations. Additionally, participants performed training
once a week, which aimed to strengthen the central stabilization muscles and to reduce
the risk of injury [16]. The training intensity was distributed among 3 heart-rate zones
(Z1-Z2-Z3). They were determined according to the ﬁrst ventilatory threshold (VT1), ven-
tilatory anaerobic threshold (VAT) and the corresponding values of the heart rate [Z1:
≤HR@VT1 + 5 bpm; Z2: (>HR@VT1 + 5 bpm) to (≤HR@VAT-5 bpm); Z3: >HR@VAT-5
bpm]. Average training times spent in every mesocycle were (~80%-15%-5%) in zones
(Z1-Z2-Z3), respectively. In the last week, the training volume was reduced to reduce the
accumulated fatigue. All trainings were monitored by Polar M430 (Kempele, Finland)
wrist watches and H9 heart-rate chest sensor and the supervision over the participants was
carried out by a certiﬁed track and ﬁeld coach.
2.7. Erythrocyte Fatty Acid Analysis
Sample collection and fatty acid determination were outlined elsewhere [10]. In brief,
blood samples were collected into 4 mL sodium citrate vacutainer tubes and centrifuged at
4 ◦C (4000× g for 10 min). After centrifugation, plasma was collected with a disposable
Pasteur pipette, transferred into separate Eppendorf probes and stored in a −80 ◦C freezer
until further analysis. Erythrocyte lipids were extracted into chloroform:methanol and fatty
acid methyl esters (representing the erythrocyte fatty acids) were formed by heating the
lipid extract with methanolic sulphuric acid. The fatty acid methyl esters were separated
Int. J. Environ. Res. Public Health 2022, 19, 14043
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by gas chromatography on a Hewlett Packard 6890 gas chromatograph ﬁtted with a BPX-70
column using the settings and run conditions described by Fisk et al. [17]. Fatty acid methyl
esters were identiﬁed by comparison with runtimes of authentic standards and data were
expressed as weight % of total fatty acids.
2.8. Statistical Analysis
Statistical analysis was performed using GraphPad Prism 7 (San Diego, CA, USA).
Arithmetic means, standard deviation (SD), and signiﬁcance levels of differences between
means were calculated. Two-way analysis of variance (ANOVA), with repeated measures,
was used to investigate the signiﬁcance of differences between groups and time. Signiﬁcant
main effects were further analyzed using the Bonferroni corrected post hoc test. Correlations
between variables were evaluated using the Pearson and Spearman correlations coefﬁcients.
All analyses used a signiﬁcance level of p < 0.05.
3. Results
3.1. Predicted VO2peak from OUE Equation
Predicted VO2peak calculated from the OUE formula both before and after the supple-
mentation intervention was moderately correlated with peak oxygen uptake (R2 = 0.338,
p = 0.002; Figure 2A) for all participants before the study. Moreover, the results without
grouping also showed a correlation after 12 weeks of intervention (R2 = 0.226, p = 0.014;
Figure 2B), but the correlation was weak.
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
5 of 10 
 
 
fatty acid methyl esters (representing the erythrocyte fatty acids) were formed by heating 
the lipid extract with methanolic sulphuric acid. The fatty acid methyl esters were sepa-
rated by gas chromatography on a Hewlett Packard 6890 gas chromatograph fitted with 
a BPX-70 column using the settings and run conditions described by Fisk et al. [17]. Fatty 
acid methyl esters were identified by comparison with runtimes of authentic standards 
and data were expressed as weight % of total fatty acids. 
2.8. Statistical Analysis 
Statistical analysis was performed using GraphPad Prism 7 (San Diego, CA, USA). 
Arithmetic means, standard deviation (SD), and significance levels of differences between 
means were calculated. Two-way analysis of variance (ANOVA), with repeated measures, 
was used to investigate the significance of differences between groups and time. Signifi-
cant main effects were further analyzed using the Bonferroni corrected post hoc test. Cor-
relations between variables were evaluated using the Pearson and Spearman correlations 
coefficients. All analyses used a significance level of p < 0.05. 
3. Results 
3.1. Predicted VO2peak from OUE Equation 
Predicted VO2peak calculated from the OUE formula both before and after the supple-
mentation intervention was moderately correlated with peak oxygen uptake (R2 = 0.338, p 
= 0.002; Figure 2A) for all participants before the study. Moreover, the results without 
grouping also showed a correlation after 12 weeks of intervention (R2 = 0.226, p = 0.014; 
Figure 2B), but the correlation was weak. 
 
Figure 2. The linear relationship between VO2peak and predicted VO2peak before (A) and after (B) 
twelve weeks of combined endurance training and supplementation (OMEGA and MCT groups; n 
= 26). 
3.2. Oxygen Uptake Efficiency Plateau 
Pre-intervention OUEP values weakly correlated with VO2peak (R2 = 0.247, p = 0.01; 
Figure 3A). After twelve weeks of intervention, no correlation was found between these 
two indicators (R2 = 0.077, p = 0.17, Figure 3B). 
Figure 2.
The linear relationship between VO2peak and predicted VO2peak before (A) and af-
ter (B) twelve weeks of combined endurance training and supplementation (OMEGA and MCT
groups; n = 26).
3.2. Oxygen Uptake Efﬁciency Plateau
Pre-intervention OUEP values weakly correlated with VO2peak (R2 = 0.247, p = 0.01;
Figure 3A). After twelve weeks of intervention, no correlation was found between these
two indicators (R2 = 0.077, p = 0.17, Figure 3B).
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 
6 of 10 
 
 
Figure 3. The linear relationship between VO2peak and OUEP before (A) and after (B) twelve weeks 
of combined endurance training and supplementation (OMEGA and MCT groups; n = 26). 
3.3. Oxygen Uptake Efficiency at the Ventilatory Anaerobic Threshold 
OUE@VAT poorly correlated with the peak oxygen uptake (VO2peak) before the study 
(R2 = 0.179, p = 0.031, Figure 4A) and there was no correlation after the 12-week interven-
tion (R2 = 0.082, p = 0.154, Figure 4B) in all participants. 
Figure 3. The linear relationship between VO2peak and OUEP before (A) and after (B) twelve weeks
of combined endurance training and supplementation (OMEGA and MCT groups; n = 26).
Int. J. Environ. Res. Public Health 2022, 19, 14043
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3.3. Oxygen Uptake Efﬁciency at the Ventilatory Anaerobic Threshold
OUE@VAT poorly correlated with the peak oxygen uptake (VO2peak) before the study
(R2 = 0.179, p = 0.031, Figure 4A) and there was no correlation after the 12-week intervention
(R2 = 0.082, p = 0.154, Figure 4B) in all participants.
 
 
Figure 3. The linear relationship between VO2peak and OUEP before (A) and after (B) twelve weeks 
of combined endurance training and supplementation (OMEGA and MCT groups; n = 26). 
3.3. Oxygen Uptake Efficiency at the Ventilatory Anaerobic Threshold 
OUE@VAT poorly correlated with the peak oxygen uptake (VO2peak) before the study 
(R2 = 0.179, p = 0.031, Figure 4A) and there was no correlation after the 12-week interven-
tion (R2 = 0.082, p = 0.154, Figure 4B) in all participants. 
 
Figure 4. The linear relationship between VO2peak and OUE@VAT before (A) and after (B) twelve 
weeks of combined endurance training and supplementation (OMEGA and MCT groups; n = 26). 
3.4. Correlation between OUEP, OUE@VAT and RE 
The changes observed in RE (presented as VO2 delta [%] at 12 km/h) were not corre-
lated with the change in OUEP (R2 = 0.018, p = 0.511; Figure 5A). Similar results were ob-
served in the correlation between RE and OUE@VAT (r = 0.079, p = 0.699; Figure 5B) in all 
participants. 
 
Figure 5. Correlation between changes in RE and OUEP (A) and OUE@VAT (B) after twelve weeks 
of combined endurance training and supplementation (OMEGA and MCT groups; n = 26). 
Figure 4. The linear relationship between VO2peak and OUE@VAT before (A) and after (B) twelve
weeks of combined endurance training and supplementation (OMEGA and MCT groups; n = 26).
3.4. Correlation between OUEP, OUE@VAT and RE
The changes observed in RE (presented as VO2 delta [%] at 12 km/h) were not
correlated with the change in OUEP (R2 = 0.018, p = 0.511; Figure 5A). Similar results were
observed in the correlation between RE and OUE@VAT (r = 0.079, p = 0.699; Figure 5B) in
all participants.
 
 
Figure 3. The linear relationship between VO2peak and OUEP before (A) and after (B) twelve weeks 
of combined endurance training and supplementation (OMEGA and MCT groups; n = 26). 
3.3. Oxygen Uptake Efficiency at the Ventilatory Anaerobic Threshold 
OUE@VAT poorly correlated with the peak oxygen uptake (VO2peak) before the study 
(R2 = 0.179, p = 0.031, Figure 4A) and there was no correlation after the 12-week interven-
tion (R2 = 0.082, p = 0.154, Figure 4B) in all participants. 
 
Figure 4. The linear relationship between VO2peak and OUE@VAT before (A) and after (B) twelve 
weeks of combined endurance training and supplementation (OMEGA and MCT groups; n = 26). 
3.4. Correlation between OUEP, OUE@VAT and RE 
The changes observed in RE (presented as VO2 delta [%] at 12 km/h) were not corre-
lated with the change in OUEP (R2 = 0.018, p = 0.511; Figure 5A). Similar results were ob-
served in the correlation between RE and OUE@VAT (r = 0.079, p = 0.699; Figure 5B) in all 
participants. 
 
Figure 5. Correlation between changes in RE and OUEP (A) and OUE@VAT (B) after twelve weeks 
of combined endurance training and supplementation (OMEGA and MCT groups; n = 26). 
Figure 5. Correlation between changes in RE and OUEP (A) and OUE@VAT (B) after twelve weeks
of combined endurance training and supplementation (OMEGA and MCT groups; n = 26).
3.5. Omega-3 Fatty Acids Supplementation
Baseline levels of EPA and DHA did not differ between the groups (OMEGA group:
1.1% EPA, 4.7% DHA; MCT group: 1.2% EPA, 4.4% DHA, both p > 0.999). Post-intervention
values of EPA and DHA increased in OMEGA group (4.9% EPA, 6.7% DHA, both p < 0.001).
Changes were not observed in MCT group (1.2% EPA, p > 0.999; 4.7% DHA, p = 0.551). All
results are provided in Table 1.
3.5.1. Oxygen Uptake Efﬁciency
At the end of the 12-week supplementation period, there was an increase in the slope
of oxygen uptake efﬁciency in the OMEGA group from 35.4 ± 3.3 to 37.6 ± 3.0 and in the
MCT group from 35.5 ± 3.7 to 37.2 ± 3.1; (both p < 0.001). OUE increased when groups
were combined from 35.5 ± 3.4 to 37.4 ± 3.0; (p < 0.001, Table 2).
Int. J. Environ. Res. Public Health 2022, 19, 14043
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Table 2. Comparison of effects omega-3 fatty acid supplementation with placebo controlled on
cardiorespiratory ﬁtness (CRF) parameters.
Variable
MCT
(n = 12)
Mean ± SD
OMEGA
(n = 14)
Mean ± SD
ALL
(n = 26)
Mean ± SD
Pre
Post
Pre
Post
Pre
Post
OUE [mL*L−1]
35.5 ± 3.7
37.2 ± 3.1 ***
35.4 ± 3.3
37.6 ± 3.1 ***
35.5 ± 3.4
37.4 ± 3.0 ***
OUEP [mL*L−1]
41.8 ± 5.2
42.9 ± 3.8
41.3 ± 4.6
43.6 ± 4.0 *
41.6 ± 4.8
43.2 ± 3.9 **
OUE@VAT [mL*L−1]
33.2 ± 3.8
35.4 ± 3.5 **
32.7 ± 3.6
35.9 ± 4.7 *
32.9 ± 3.7
35.7 ± 4.1 ***
Ve [L*min−1]
93.8 ± 11.6
90.7 ± 9.3 **
92.9 ± 20.4
87.4 ± 20.2 *
93.3 ± 16.4
88.9 ± 15.7 *
OUE—oxygen uptake efﬁciency; OUEP—oxygen uptake efﬁciency plateau; OUE@VAT—oxygen uptake efﬁciency
at the ventilatory anaerobic threshold; Ve—minute ventilation; * p < 0.05 for post to pre value; ** p < 0.01 for post
to pre value; *** p < 0.001 for post to pre value; data are presented as mean ± standard deviation (SD).
3.5.2. Oxygen Uptake Efﬁciency Plateau
Oxygen uptake efficiency plateau values increased in the OMEGA group from 41.3 ± 4.6
to 43.6 ± 4.0; (p = 0.017). There were no changes in the MCT group (p = 0.2). Moreover, the
analysis of the two groups together (regardless of the supplementation that was undertaken)
showed that OUEP increased from 41.6 ± 4.8 to 43.2 ± 3.9; (p = 0.007, Table 2).
3.5.3. Oxygen Uptake at Ventilatory Anaerobic Threshold
There was an increase in OUE@VAT in the OMEGA group from 32.7 ± 3.6 to 35.9 ± 4.7;
(p = 0.012) and in the MCT group from 33.2 ± 3.8 to 35.4 ± 3.5; (p = 0.003). The results,
regardless of the supplementation undertaken, showed that OUE@VAT increased from
32.9 ± 3.7 to 35.7 ± 4.1; (p < 0.001, Table 2).
4. Discussion
This is the ﬁrst study to report the correlations between OUE, OUEP, OUE@VAT and
VO2peak as well as OUEP and OUE@VAT and RE. They were analyzed in terms of reliability
and repeatability, and whether they could be non-invasive substitute measurements for
VO2peak and RE. Additionally, we investigated whether these parameters were altered
following supplementation with omega-3 fatty acids.
The true VO2max value is mainly achievable during a laboratory progressive exercise
test to exhaustion where large muscle groups are involved. Simultaneously, the observed
kinetics of oxygen supply/utilization in the muscles must be without signiﬁcant changes:
the so-called plateau [18]. It is known that this phenomenon occurs when a high intensity
is met, and the primary criteria for achieving this parameter (VO2max) during CPET are:
(1) reaching a VO2 plateau or (2) levelling-off the oxygen uptake (VO2) [19–21]. Thus, in
Sun and co-authors’ study, OUE, OUEP and OUE@VAT comprehensively reﬂected cardio-
vascular functions as an alternative for parameters assessing CRF without the need for
maximum effort [4]. A steeper OUE (VO2/Ve) and higher values of OUEP and OUE@VAT
show more efﬁcient oxygen uptake and utilization in the working skeletal muscles. OUE
showed an improvement, but, for both groups, this occurred after 12 weeks of interven-
tion. Hence, it is believed that the increase in slope/higher OUE values was the result of
endurance training. Moreover, in our study, weak or no correlation was observed between
OUEP and peak oxygen uptake. In a study by Bongers et al. [22], in which 214 children
participated, OUEP was weak-to-moderately correlated with VO2peak (r = 0.646), which
is inconsistent with our results. However, children and adults respond differently to ex-
ercise, which might explain this difference. Another study also conﬁrms that OUEP does
not accurately predict VO2max in male adolescents and should not replace VO2max, when
assessing CRF [19]. In our study, OUE@VAT also demonstrated no correlation with VO2peak
before and after 12 weeks of intervention. In contrast to our results, one study revealed
that ventilatory anaerobic threshold (VAT) strongly correlated with VO2peak (r = 0.831) [23].
However, there is a difference between the compared parameters, because OUE@VAT is the
Int. J. Environ. Res. Public Health 2022, 19, 14043
8 of 10
60-s average of consecutive measurements at and immediately before the VAT. On the other
hand, VAT is a single measurement and is not free from intra-observer and inter-observer
variability [24]. Hence, both OUEP and OUE@VAT may be more stable measurements than
VAT; however, the results of our study did not conﬁrm this.
Endurance capacity also has a stable predictor in the form of RE [25]. However, as
earlier authors suggest, an accurate measurement of RE can be carried out with the use
of invasive lactate measurement, which is one of the disturbances in VO2 steady-state
indicators [26,27]. Therefore, in this study, an attempt was made to replace RE with OUEP
and OUE@VAT and to check whether they can be a solid, non-invasive predictor of RE
in recreational runners. Despite the increase in the efﬁciency of oxygen uptake in all
participants, the linear regression did not show any correlation between OUEP, OUE@VAT
and RE. Hence, the RE measurement should not be replaced with OUEP and OUE@VAT, as
they are not related.
The assessment of adaptive changes following supplementation with omega-3 fatty
acids is also not fully known. The health-promoting effects of n-3 PUFA supplementation
are well-established [28–30]. These effects are related to the incorporation of EPA and DHA
into the erythrocyte cell membrane [31], skeletal muscles [32] and heart [33]. Furthermore,
the systemic response to supplementation with omega-3 fatty acids as exempliﬁed by
maximum oxygen uptake [7], exercise economy [9,10] or anaerobic endurance capacity [34]
is well-known. Nevertheless, in our study, for the ﬁrst time, an attempt was made to link
the effect of supplemental EPA + DHA to changes in OUEP and OUE@VAT. However, the
OUE parameters increased in both groups. Therefore, changes in OUEP and OUE@VAT
following 12 weeks of intervention are dictated by adaptation to endurance training rather
than changes caused by EPA and DHA supplementation.
Limitations and Future Perspectives
Despite some valuable information coming from this study, there are some limitations.
First, the small number of participants could distort the estimate of correlations between
the variables. Second, this study was conducted in male runners only; therefore, these
ﬁndings cannot be generalized and extrapolated to females. Future studies should include
a larger number of participants and include females.
5. Conclusions
In conclusion, the results obtained in this study do not support the use of OUEP
and OUE@VAT as an alternative parameter to VO2peak and RE. Additionally, the 12-week
supplementation of omega-3 fatty acids at a dose of 2234 mg of EPA and 916 mg of DHA
daily did not reveal changes in OUEP and OUE@VAT. Hence, the suitability of using
OUEP and OUE@VAT as alternative, non-invasive CRF parameters for VO2peak and RE can
be questioned.
Author Contributions: Conceptualization, Z.J.; methodology, Z.J. and M.C.; software, Z.J., M.T. and
M.C.; validation, R.L.; formal analysis, Z.J. and M.C.; investigation, Z.J., M.T. and M.C.; resources,
M.T.; data curation, M.C.; writing—original draft preparation, Z.J., M.T., P.C.C. and R.L.; writing—
review and editing, Z.J., M.T., M.C., P.C.C. and R.L.; visualization, Z.J. and P.C.C.; supervision, P.C.C.
and R.L.; project administration, M.T.; funding acquisition, M.T. All authors have read and agreed to
the published version of the manuscript.
Funding: This research was funded by National Science Center (Poland), grant number 2018/31/N/N
Z7/02962.
Institutional Review Board Statement: The study was conducted in accordance with the Declaration
of Helsinki and approved by the Bioethical Committee of Regional Medical Society in Gda´nsk
(NKBBN/628/2019).
Informed Consent Statement: Informed consent was obtained from all subjects involved in the
study.
Int. J. Environ. Res. Public Health 2022, 19, 14043
9 of 10
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Conﬂicts of Interest: The authors declare no conﬂict of interest.
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Improved Oxygen Uptake Efficiency Parameters Are Not Correlated with VO<sub>2peak</sub> or Running Economy and Are Not Affected by Omega-3 Fatty Acid Supplementation in Endurance Runners.
Jost, Zbigniew,Tomczyk, Maja,Chroboczek, Maciej,Calder, Philip C,Laskowski, Radosław