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Up in the Air: Evidence of Dehydration Risk and Long-Haul Flight on Athletic Performance

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The microclimate of an airline cabin consists of dry, recirculated, and cool air, which is maintained at lower pressure than that found at sea level. Being exposed to this distinctive, encapsulated environment for prolonged durations, together with the short-term chair-rest immobilization that occurs during long-haul flights, can trigger distinct and detrimental reactions to the human body. There is evidence that long-haul flights promote fluid shifts to the lower extremity and induce changes in blood viscosity which may accelerate dehydration, possibly compromising an athlete’s potential for success upon arrival at their destination. Surprisingly, and despite several recent systematic reviews investigating the effects of jet lag and transmeridian travel on human physiology, there has been no systematic effort to address to what extent hypohydration is a (health, performance) risk to travelers embarking on long journeys. This narrative review summarizes the rationale and evidence for why the combination of fluid balance and long-haul flight remains a critically overlooked issue for traveling persons, be it for health, leisure, business, or in a sporting context. Upon review, there are few studies which have been conducted on actual traveling athletes, and those that have provide no real evidence of how the incidence rate, magnitude, or duration of acute dehydration may affect the general health or performance of elite athletes.
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nutrients
Review
Up in the Air: Evidence of Dehydration Risk and
Long-Haul Flight on Athletic Performance
Damir Zubac 1, 2, * , Alex Buoite Stella 3and Shawnda A. Morrison 4
1Faculty of Kinesiology, University of Split, Teslina 6, 21000 Split, Croatia
2Science and Research Centre Koper, Institute for Kinesiology Research, 6000 Koper, Slovenia
3Clinical Unit of Neurology, Department of Medicine, Surgery and Health Sciences, Cattinara University
Hospital ASUGI, University of Trieste, Strada di Fiume, 447, 34149 Trieste, Italy; alex.buoitestella@gmail.com
4Faculty of Sport, University of Ljubljana, Gortanova 22, 1000 Ljubljana, Slovenia;
shawnda.morrison@fsp.uni-lj.si
*Correspondence: damir.zubac@kifst.hr; Tel.: +385-95-502-1976
Received: 14 July 2020; Accepted: 19 August 2020; Published: 25 August 2020
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Abstract:
The microclimate of an airline cabin consists of dry, recirculated, and cool air, which
is maintained at lower pressure than that found at sea level. Being exposed to this distinctive,
encapsulated environment for prolonged durations, together with the short-term chair-rest
immobilization that occurs during long-haul flights, can trigger distinct and detrimental reactions to
the human body. There is evidence that long-haul flights promote fluid shifts to the lower extremity
and induce changes in blood viscosity which may accelerate dehydration, possibly compromising
an athlete’s potential for success upon arrival at their destination. Surprisingly, and despite several
recent systematic reviews investigating the eects of jet lag and transmeridian travel on human
physiology, there has been no systematic eort to address to what extent hypohydration is a (health,
performance) risk to travelers embarking on long journeys. This narrative review summarizes the
rationale and evidence for why the combination of fluid balance and long-haul flight remains a
critically overlooked issue for traveling persons, be it for health, leisure, business, or in a sporting
context. Upon review, there are few studies which have been conducted on actual traveling athletes,
and those that have provide no real evidence of how the incidence rate, magnitude, or duration of
acute dehydration may aect the general health or performance of elite athletes.
Keywords: athletic performance; jet lag syndrome; fluid intake; hypohydration
1. Study Rationale
To compete internationally, elite athletes are frequently exposed to long-distance air travel, often
across multiple time zones. This results in a cluster of acute, detrimental, health-related symptoms,
commonly known as jet lag. Although there have been a number of major review articles on jet
lag published over the past ~10 years which suggest avoiding dehydration or advocating “staying
hydrated” whilst flying [
1
4
], none discuss the magnitude of expected levels of dehydration to be
experienced over a given flight duration, or whether athletes could be at greater risk for becoming
hypohydrated compared to other populations. Thus, there remains a need to determine the influence
transmeridian air travel has on the physical performance outcomes of athletes.
2. PART ONE: Mechanisms of Hydration State Changes during Long-Haul Flights
2.1. The Airline Cabin Environment
When flying, cabin environmental characteristics are dierent from the normal conditions on
land in that cabin air is characterized by reduced oxygen partial pressure and lower humidity [
5
].
Nutrients 2020,12, 2574; doi:10.3390/nu12092574 www.mdpi.com/journal/nutrients
Nutrients 2020,12, 2574 2 of 15
Additional factors which may impact human physiology, and have yet to be fully investigated, include:
the amount of air flow, uneven cabin air distribution, and the proportion of recycled air in the aircraft
environment [
6
]. During flight, the cabin is a ventilated, enclosed environment that is controlled by the
Environmental Control System (ECS) which provides the air to the passengers and crew. Pressurization
of the cabin is necessary since at most flight altitudes (e.g., 11,000 m), atmospheric pressure is associated
with an oxygen partial pressure nearly incompatible with human life. As such, the cabin pressure can
range from a maximum of 101 kPa at sea level, to a minimum of 75 kPa in flight, corresponding to
an altitude of ~2440 m [
7
,
8
]. Because of the density of the passengers and the limited volume inside
the aircraft, cooling is usually necessary to prevent uncomfortable heating of the cabin environment.
Circulated air is usually maintained to around 23
C by supplying the cabin with air at minimum
10
C [
7
]. Control of the relative humidity of the air inside the cabin is one of the main tasks of the
ECS, since humidity may aect both passengers’ comfort and the structural integrity of the aircraft.
Typically, at cruise altitudes, relative humidity is maintained at 10–20% by removing moisture from the
cabin air; this is to avoid structural damages to the aircraft, [7].
Taken together, these factors (i.e., dry air, low O2pressure) may result in an increased insensible
water loss, including evaporation of water at the skin surface [
4
] and respiratory water loss as a product
of the combination of increased ventilation (in particular, changes to tidal volume) [
5
] and increased
need to humidify the air that enters the lungs [
1
,
2
,
6
]. Indeed, increasing water intake by 15–20 mL
has been suggested for each hour of flight [
3
]; however, this recommendation is likely insucient
to prevent dehydration, since it is reported that resting ventilatory water losses can increase from
160 mL/hour to 360 mL/hour when relative humidity decreases from 60% to 12%, a drop consistent
with airline cabin environments. These calculations assume an ambient temperature of ~21
C and
ventilation rate of 7 L/hour [
4
,
9
,
10
]. It remains that prescribing fluid intake of only 15–20 mL/hour may
vastly underestimate the actual hydration needs of those flying for extended periods of time.
2.2. Fluid Intake during Long-Haul Flights
Due to the above-mentioned factors, it may be speculated that increased water losses are probable
during flight, and optimal fluid intake should compensate these expected losses in order to avoid
becoming hypohydrated [
1
,
2
]. However, only a handful of studies have investigated the fluid intake
habits of people during long-haul flights.
For example, one study investigated 10 healthy volunteers during a 10 h simulated flight at
2800 m
, and found that plasma volume decreased by 6% to 9%; the authors ascribed these dierences
to increased insensible water loss and decreased fluid intake that may have contributed to in-flight
hypovolemia [
11
]. Total fluid intake consisted of ~960 mL of nonalcoholic fluid. Pure water was
compared to a high-sodium–low-carbohydrate drink, or to a low-sodium–high-carbohydrate drink,
with the findings that plasma volume was better preserved using the electrolyte–carbohydrate solutions,
possibly due to the lower urine output [
11
] observed with that group. Despite the few studies assessing
hydration status during long-haul flights, some findings are available for aviators’ fluid balance during
military flights. Levkovsky et al. [
12
] investigated the hydration of 48 aviators who participated
in a total of 104 training flights. The authors found that their mean fluid loss rate was ~465 mL/h,
as evaluated by the dierence in body weight [
12
]; nevertheless, this data should be carefully interpreted
in the context of athletes traveling during a long-haul flight since the metabolic requirements and
protective equipment of military aviators are far dierent from the resting conditions of commercial
flight passengers.
In another study, Silva et al. [
13
] retrospectively queried the fluid intake habits of 94 male kite
surfers by asking them to recall the longest recent flight the athlete took for a competition [
13
]. During
their recalled flight, it was reported that 66% of the kite surfers self-reported drinking “some kind
of fluid” during the flight, and the majority (51%) self-reported consuming less than 0.5 L of water.
The authors did not report any association in fluid intake with flight duration (travel distance varied
greatly between 14.489
±
3629 km and 2264
±
781 km for the two subgroups). Although the research
Nutrients 2020,12, 2574 3 of 15
emphasis of these works focuses on amount of fluid intake during flights, overhydration has also been
mentioned to be avoided due to the possible disruptions frequently getting up to void the bladder will
have on sleep during the flight [14].
2.3. Eect of Jet Lag and Long-Haul Flight on Gastrointestinal and Renal Function
The expression of key genes in peripheral organs throughout the day is orchestrated by a central
pacemaker in the brain, namely the suprachiasmatic nucleus (SCN) of the anterior hypothalamus,
that takes cues from cycles of light and dark, food, hormone levels, or from the metabolic status of the
individual (e.g., exercise) [
15
]. Mechanistically, arginine vasopressin/V1 receptor signaling in the SCN
plays a critical role in the resilience of the circadian clock to jet lag [
16
,
17
]; however, to the best of the
authors’ knowledge, no study has investigated the associated between hydration status, vasopressin,
and jet lag symptoms.
Researchers do know that circadian rhythms control several gastrointestinal functions, including
gastric enzyme and fluid production, small intestine nutrient absorption, and gastric and gut motility.
Under normal physiological conditions, the gastrointestinal system is quiescent during the night,
rapidly increasing activity after awakening and throughout the day [
18
]. Amongst the primary
associations between circadian rhythm and gastrointestinal function, motility in the dierent tracts of
the gastrointestinal system is one of the key physiological functions that is altered due to circadian cycle
disruption [
19
]. Melatonin, the neuroendocrine clock factor produced by the pineal gland, is highly
present in the gastrointestinal tissue and therefore is implicated in digestive function [
20
]. Circadian
disruption shifts the timing of eating and normal gastrointestinal functions, such as secretions, enzyme
activity, intestinal motility, and the rate of nutrient absorption, often resulting in pathological conditions
such as abdominal pain, constipation, and diarrhea [
19
]. Suering from these side eects would have a
direct influence on fluid intake behavior and overall fluid balance of any traveler. If nutrient absorption
and gastric and gut motility are aected by circadian disruption, then it may be that the athlete cannot
absorb as much water as is needed to remain well hydrated, or they may simply lack the appetite to
consume additional liquids due to actual gut discomfort.
Appetite loss and gastrointestinal distress are often reported because of jet lag, and dietary
interventions may help modulate these symptoms, although further research is needed to definitively
establish the role of diet on jet lag adaptation [
21
]. If appetite loss is a direct consequence of circadian
rhythm disruption, then gastrointestinal distress may also be a consequence of poor hygiene practices
or food/water safety in the foreign location. Traveler’s diarrhea, especially when traveling to another
country where food contamination may be more likely to occur, is a common feature in as many as
60% of athletes who travel internationally [
22
]. This kind of disruption can obviously significantly
impair performance. On the other hand, reducing fluid intake during the flight to avoid frequent
trips to the bathroom, or to reduce the risk of consuming fluids that may be contaminated, may result
in constipation. In either case, it is important for the athlete not to overcorrect their fluid balance
situation, so to speak. To the best of the authors’ knowledge, there is a scarcity of studies investigating
renal function during long-haul flights or circadian rhythm disruption, with some results present for
chronic desynchronizations. In hamsters, for example, prolonged circadian disorganization may be
associated with proteinuria, tubular dilation, and cellular apoptosis [
23
]. More recently, in humans,
daytime napping has been suggested as being associated with negative eects on renal function,
namely hyperfiltration and microalbuminuria [
24
,
25
]. How these issues directly link to the traveling
athlete remains to be determined. Certainly, being aware that it is important to continue to consume
fluids regularly during long-haul flights is important to convey to traveling athletes.
Nutrients 2020,12, 2574 4 of 15
2.4. Other Medical Issues of Long-Haul Flights: Venous Thromboembolism
Among the medical issues that may be present during long-haul flights, and which may
undoubtedly benefit from a proper hydration protocol, is venous thromboembolism (VTE), a condition
which shows an increased risk on flights greater than >8 h duration. Indeed, there is some risk present
already after 4 h [
8
]. A population-based study including 9000 business travelers showed an absolute
risk for venous thromboembolism for one in every 4656 flights [
26
]. This increased risk is linked to the
reduced mobility during the flight [
27
]. Dehydration has been listed as one of the factors that may
predispose oneself to a higher risk of VTE [
9
,
27
]. Certainly, it has been established that as a consequence
of dehydration, hemoconcentration and increased blood viscosity may lead to hypercoagulability
which may be present also in athletes [
28
,
29
], although conflicting results are reported [
30
]. A further
risk factor to VTE may be represented by the fluid shifts that occur during prolonged chair rest mobility,
especially since dehydration was found to increase lower limb edema in otherwise healthy people
during long simulated flights [31,32].
In a particularly elegant study, one investigation looked at whether fluid loss occurred more in
individuals with coagulation activation after air travel and compared the responses to participants
without coagulation activation [
30
]. The secondary aim of the study was to examine fluid losses that
occurred during actual air travel. In their crossover study, 71 healthy volunteers were exposed to eight
hours of air travel, eight hours of immobilization in a cinema, and a daily-life control situation. Markers
of fluid loss (hematocrit, serum osmolality, and albumin) and of coagulation activation were assessed
before and after each exposure. There were 11 volunteers with, and 55 volunteers without, coagulation
activation during the flight. The authors found that fluid loss was not dierent in volunteers with
an activated clotting system from those without (dierence between groups in hematocrit:
0.6%,
95% confidence interval [CI]:
1.9 to 0.6). On a group level, mean hematocrit values decreased during
all three exposures, however, in some individuals, it increased; this occurred in more participants
during the flight scenario (34%; 95% CI 22 to 46) than during the daily-life situation (19%; 95% CI 10
to 28). Ultimately, these findings do not support the hypothesis that fluid loss itself contributes to
thrombus formation during air travel. However, one limitation to this study was that urinary hydration
markers were not assessed so a complete picture of hydration status was unavailable.
Based on all the mechanisms described above, increased fluid intake is often recommended as
a valid prophylaxis for VTE; however, these suggestions seem to be based more on common sense
than on actual evidence [
8
,
33
]. On a final note, the type of fluid ingested may also play a role in VTE
prevention. A research letter described their study which investigated the eects of an electrolyte and
carbohydrate beverage (ECB) compared to water on 40 healthy men during a 9 h flight. The authors
found that the ECB increased plasma volume compared to water, which was also associated with
reduced urine output and positive fluid balance. Interestingly, elevated foot blood viscosity was lower
in the ECB group, an encouraging finding that using such fluids during long-haul flights do contribute
to/modify blood viscosity [34,35] (Figure 1).
Nutrients 2020,12, 2574 5 of 15
Nutrients 2020, 12, x FOR PEER REVIEW 5 of 18
Figure 1. Air cabin environment creates a unique situation that promotes possible increased risk for dehydration
and fluid shift en route to the destination which may be further exacerbated by the behavior of the individual
and independent of jet-lag-induced alterations in circadian rhythm.
3. PART TWO: Effect of Long-Haul Flights on Physical Performance
Systematic [36,37] and narrative reviews [2,3,21] aimed to summarize the most recent findings
and provide practical guidelines on how to best minimize (handle/tackle) the consequences of the
air-travel-induced circadian rhythm desynchronization (e.g., jet lag) on various physical performance
indicators. Briefly, crossing multiple time zones via air travel and within a limited time frame is
instrumental to inducing the loss of synchronicity among the circadian rhythms, thereby affecting
sleep, core temperature, gastrointestinal function, and melatonin release, each of which can translate
into impaired physical performance in athletes [3].
3.1. Fluid Intake and Hydration Status of Athletes on Long-Haul Flights
There are only a limited number of investigations that have analyzed the fluid intake or
hydration status of athletes during transmeridian air travel. One study on kite surfers used a
questionnaire-based approach to self-report water intake among this population of internationally
competing athletes [13], as reviewed previously. A more objective study by Schumacher and co-
authors [28] examined the effects of an 8 h long-haul flight from Europe to the Middle East on
hematological indices of hydration status in 15 endurance-trained athletes. They found that
hemoglobin concentration was slightly more elevated before than after traveling in athletes (+0.5
g/dL, p = 0.038), and a similar pattern was noted three days after the athletes had reached their
destination. However, there were no differences observed in the hematological variables between
athletes and the nontraveling controls. Thus, Schumacher concluded that transmeridian air travel
does not influence overall fluid balance and hydration status among those athletes. And yet,
Schumacher and co-authors [28] did not measure any urinary indices of hydration status in their
work (Table 1).
Figure 1.
Air cabin environment creates a unique situation that promotes possible increased risk
for dehydration and fluid shift en route to the destination which may be further exacerbated by the
behavior of the individual and independent of jet-lag-induced alterations in circadian rhythm.
3. PART TWO: Eect of Long-Haul Flights on Physical Performance
Systematic [
36
,
37
] and narrative reviews [
2
,
3
,
21
] aimed to summarize the most recent findings
and provide practical guidelines on how to best minimize (handle/tackle) the consequences of the
air-travel-induced circadian rhythm desynchronization (e.g., jet lag) on various physical performance
indicators. Briefly, crossing multiple time zones via air travel and within a limited time frame is
instrumental to inducing the loss of synchronicity among the circadian rhythms, thereby aecting
sleep, core temperature, gastrointestinal function, and melatonin release, each of which can translate
into impaired physical performance in athletes [3].
3.1. Fluid Intake and Hydration Status of Athletes on Long-Haul Flights
There are only a limited number of investigations that have analyzed the fluid intake or hydration
status of athletes during transmeridian air travel. One study on kite surfers used a questionnaire-based
approach to self-report water intake among this population of internationally competing athletes [
13
],
as reviewed previously. A more objective study by Schumacher and co-authors [
28
] examined the
eects of an 8 h long-haul flight from Europe to the Middle East on hematological indices of hydration
status in 15 endurance-trained athletes. They found that hemoglobin concentration was slightly more
elevated before than after traveling in athletes (+0.5 g/dL, p=0.038), and a similar pattern was noted
three days after the athletes had reached their destination. However, there were no dierences observed
in the hematological variables between athletes and the nontraveling controls. Thus, Schumacher
concluded that transmeridian air travel does not influence overall fluid balance and hydration status
among those athletes. And yet, Schumacher and co-authors [
28
] did not measure any urinary indices
of hydration status in their work (Table 1).
Nutrients 2020,12, 2574 6 of 15
Table 1. Factors influencing fluid balance during long-haul flights.
Factor Expected Changes Measured Changes E.g. London–Tokyo
(~12 h)
Insensible and
ventilatory fluid losses
Increased due to reduced air
humidity and slightly increased
ventilation
Increased by 200 mL/h (360 mL/h
total) [4]
Increased fluid
consumption by at least
2.4 L (up to 4 L)
Gastrointestinal losses
Altered motility and absorption
due to circadian rhythm
alterations and dietary patterns
may aect both fluid intake and
fluid losses
Abdominal pain, constipation,
and diarrhea are often reported
[19]. No clear data on changes in
food and fluid intake or fluid
losses
Diarrhea or constipation
may increase or decrease
fluid losses, respectively
Urinary losses Circadian rhythm alterations may
influence kidney function
No clear studies are present,
possible hyperfiltration and
microalbuminuria [24,25]
No data are available to
make a clear estimation
Fluid intake
Reduced fluid intake may be
expected due to altered appetite,
gastrointestinal dysfunction,
changes in dietary habits, and
desire to reduce the need to void
the bladder
Few studies assessed fluid intake
during long-haul flights in
athletes. Less than 500 mL may be
consumed for long-distance
travel [13]
Fluid intake of minimum
200–250 mL/h may be
encouraged (consider
fluid from food)
Note:
Due to the lack of a common consensus on total fluid losses during long-haul flights, caution should be
applied when approximating/estimating fluid needs.
Urinary markers (e.g., urine specific gravity (USG) in particular) have been suggested by
Hamouti et al.
[
38
] to be an adequate index of hydration status compared to other methods, such as
using a color chart or analyzing blood markers [
38
]. Briefly, under well-controlled conditions [
38
],
USG is argued to be a superior dehydration marker (at lower levels of dehydration, ~2%), compared to
blood indices primarily due to the body fluid regulatory homeostatic mechanisms, including plasma
volume defense pathways [
39
]. Having said this, there remains no industry gold standard for assessing
hydration standards [
40
,
41
]. More specifically, because a large portion of fluid remains trapped in
plasma following fluid consumption, and after exercise-induced dehydration remains, this state can
“trick” the antidiuretic hormone (ADH)-osmoreceptor feedback system, leading to the incomplete fluid
restoration of interstitial compartments [
39
], especially in athletes [
42
]. Subsequently, an athlete’s thirst
sensation and fluid retention can be attenuated, which is then translated into an overall suboptimal fluid
restoration [
42
]. To address some of the unresolved issues, Cotter et al. [
35
] conducted a double-blind,
placebo-controlled, crossover study which examined the eects of a commercially available beverage
during a 7 h long transmeridian flight and its eects on the hydration status of 12 healthy volunteers.
Hydration status was measured via bioimpedance and urinary output measures. Their findings
indicated that consuming 330–400 mL of the commercially available drink during the long-haul
transmeridian flight significantly lowered urine output [1.05 (0.48) vs. 1.28 (0.34) L, mean (CI)] and
increased plasma volume by ~4% (bioimpedance) compared to the placebo drink.
Although the above-mentioned studies have attempted to analyze the eects of long-distance air
travel on hydration status, and many studies did evaluate the potential influence of dehydration on
physical performance or its associated physiological mechanisms, there is no clear study investigating
these eects in a consistent manner. Indeed, the importance of utilizing a proper hydration strategy
was recently discussed by van Rensburg et al. [
36
], who examined the eects of jet lag on physical
performance in athletes in their systematic review. They looked at a number of factors and concluded
that remarkably, there are still no studies looking at the combined eects of hydration status on physical
performance of athletes traveling across multiple time zones.
3.2. Eects of Caeine Consumption on Hydration Status during Long-Haul Flights
To overcome the acute, jet-lag-induced symptoms of fatigue and potential declines in physical
performance, caeine consumption during transmeridian flights is a widely accepted strategy [
13
,
43
,
44
].
Current guidelines from the European Food Safety Authority conclude that caeine ingestion (of up to
6 mg kg
1
body weight) will not induce diuresis; however, their conclusion is only related to caeine
Nutrients 2020,12, 2574 7 of 15
ingestion coupled with endurance exercise involvement. To consider these points,
Seal et al.
[
44
]
studied the eects of dierent caeine dosage on fluid balance and hydration status. Subjects completed
three trials on separate occasions at least five days apart in a counterbalanced, crossover study design.
All participants consumed 269
±
45 and 537
±
89 mg of caeine for LOW-CAF and HIGH-CAF trials,
respectively, representing a low and high dose of caeine. Urine was collected at 60, 120, and 180 min
after the test drink ingestion and urine volume was measured. The data indicate that caeine intake of
6 mg kg
1
in the form of coee can induce an acute diuretic eect, while 3 mg kg
1
does not disturb
fluid balance in healthy casual coee-drinking adults at rest.
These findings are in line with original work reported by Killer et al. [
45
] and a meta-analysis [
46
]
where authors demonstrated that the substantial fluid loss associated with lower concentrations of
caeine consumption is unjustified, especially when caeine is consumed prior to exercise. Specifically,
Killer et al. [
45
] used a counterbalanced crossover design in 50 male coee drinkers who habitually
consumed 3–6 cups per day. They participated in two trials, each lasting three consecutive days.
During the study, their physical activity, food, and fluid intake were controlled; participants consumed
either 4
×
200 mL of coee containing 4 mg/kg caeine (C) or water (W) [
47
]. Deuterium oxide was
used to assess the total body water (TBW) pre- and post-trial via ingestion. Urinary and hematological
hydration markers were recorded daily in addition to nude body mass measurement (BM). Plasma
was analyzed for caeine to confirm compliance. Their findings showed no significant changes in TBW
from beginning to end of either trial and no dierences between trials (51.5
±
1.4 vs.
51.4 ±1.3 kg,
for C and W, respectively). No dierences were observed between trials across any hematological
markers or in 24 h urine volume (2409
±
660 vs. 2428
±
669 mL, for C and W, respectively), USG,
osmolality, or creatinine. Mean urinary Na (+) excretion was higher in C than W (p=0.02). Collectively,
data by
Killer et al.
[
47
] suggest that coee, when consumed in moderation by caeine-habituated
males, provides similar hydrating qualities to water. Finally, Zhang et al. [
46
] summarized that
caeine ingestion (~300 mg) was translated into a minor diuretic eect; these were negated by exercise.
Concerns regarding unwanted fluid loss associated with caeine consumption might be unjustified,
particularly when ingestion precedes exercise. The caeine ingested was not corrected for body mass,
and both men and women were included in this work, making it dicult to compare with other
literature, since data on anthropometrics were not reported.
3.3. Hypohydration, Jet Lag, and Performance
It is still unknown whether hypohydration per se accelerates declines in physical performance
or imposes additional fatigue development following transmeridian air travel, since rapidly
crossing multiple time zones is known to impair physical performance. Jet lag side eects and
subsequent performance outcome parameters that have been assessed are diverse, including jump
performance [4850]
, sprint performance [
48
,
49
], isometric adductor strength [
47
], and handgrip
strength [
51
,
52
], all presented in Table 2. Performance decrements were predominantly observed
throughout the explosive movement of the lower limbs (e.g., countermovement jump decreases
immediately upon arrival by 7–10%, and a similar pattern was observed in sprint performance), but not
during isometric contractions of the lower or upper limbs. The mechanisms governing neuromuscular
function after long-haul travel remain unknown, since the above-mentioned measures nearly entirely
use gross estimates of neuromuscular function. More comprehensive insight via EMG or interpolated
twitch techniques is still needed. Interestingly, there are as yet no data on the combined eects of
jet lag and dehydration on neuromuscular performance in weight-class athletes, including combat
sports athletes, weightlifters, gymnasts, or sailors [
53
56
]. This is rather unexpected, since it is well
known that these athletes will typically restrict their food and fluid intake ahead of competition to
maintain lower body weight in an attempt to increase their likelihood of competitive success. These
athletes typically have suboptimal fluid intake [
53
], and food and fluid deprivation are known to
negatively aect neuromuscular performance in combat sport athletes (e.g., Olympic-style boxers) [
55
],
and sailors also tend to dehydrate during competition [56].
Nutrients 2020,12, 2574 8 of 15
Table 2. Summary of the physiological eects of jet lag and transmeridian travel on actual athletes traveling across multiple time zones.
Study Participants Population Flight Details Design Testing Methodology Performance Outcomes Fluid Intake? Hydration
Status
Assessed?
Chapman et al.
[48]
N=5
experimental
group (4 female)
N=7 control
(6 female)
Age: 25 ±6 y
Skeleton
athletes
Australia and
Canada national
team members
AUS to CAN
8 time zones
WEST
TT: ~24 h
Cross-sectional
cohort
Travel team
(AUS)
compared to
nontravel team
(CAN)
Data collection:
2 d before, immediately
after travel, and 6 times
in the 10 d postflight
duration
Lower body power tests
(power and velocity)
Dec in peak and mean
squat jump velocity; CMJ
velocity did not change;
CMJ jump height
decreased; squat
movement NS
Not reported No
Bullock et al.
[49]Same as above Same as above Same as above Same as above Test sampling as above
30 m sprint performance NS for performance time
in travel group;
saliva cortisol decreased
67%; no change in
nontravel group
Not reported USG measured;
NS on any
postflight
measurement
day; NS
dierent from
nontravel group
Broatch et al.
[50]
N=12 females
25 ±2 y Volleyball Canberra, AUS
to Manila,
Philippines
2 time zones
NORTHWEST
TT: 6.5 h
RCT
compression
garments
(randomized
design, n=6 per
group)
Data collection 1 d before,
12, 24, and 48 h postflight,
resting BP and HR during
flight
CMJ; NS time or
interaction eect;
mean velocity higher for
compression group by 4%
24 h post; relative power
8% higher 24 h post;
resting BP and HR NS
main eects; SaO2less
attenuated with
compression at 6.5 and 9
h postflight; calf girth NS;
markers of blood clotting
NS
Not reported No
Fowler et al.
[47]
N=18 males
24 ±3 y Professional
AUS Rugby
League
World series
2015
AUS to UK
11 time zones
WEST
TT: 24 h
Cross-sectional
pre/post Data collected:
1 d before, +2, +6, and +8
d post-travel
Muscle function
(isometric force via
adductor squeeze
dynamometry, flexibility,
and ROM)
Self-reported: sleep, jet
lag questionnaire
NS across any dependent
measure at any
time-point except
self-reported upper
respiratory symptoms 6 d
post-travel in n=6 (30%)
of athletes
Not reported No
Nutrients 2020,12, 2574 9 of 15
Table 2. Cont.
Study Participants Population Flight Details Design Testing Methodology Performance Outcomes Fluid Intake? Hydration
Status
Assessed?
Lemmer et al.
[51]WEST
N=13 male
25 ±2 y
EAST
N=6 male
23 ±2
N=4 athletes
participated in
both directions
Control data
collection on
same subjects
Elite gymnasts Frankfurt,
Germany to
Atlanta, USA
6 time zones
WEST
TT: 10 h
AND
Munich,
Germany to
Osaka, Japan
8 time zones
EAST
TT:12 h
Cross-sectional
with follow-up
assessments
Data collected:
As control in Germany,
then
+1, 4, 6, and 11 d after
arrival to destination for
each direction
24 h before HR, BP; oral
temperature; handgrip
strength; saliva for
cortisol and melatonin
Self-reported: jet lag
symptoms
All functions were
disturbed on the first day
on arrival at destination
(both directions) and
remained until 5–6 d
(WEST) and 7 d after
EAST
BP and HR aected on
first day also interaction
eect
WEST
SBP inc. ~5–8 mmHg
DBP no sig. changeHR
inc ~16 bpm
EAST
SBP dec ~11 mmHg
DBP dec ~7 mmHg
HR no sig. change
Handgrip not aected in
either direction
Not reported No
Kraemer et al.,
[52]
N=10 athletes
(compression)
23 ±2 y and
N=9 athletes
(matched
controls)
23 ±2 y
“Recreational”
athletes,
unspecified
Hartford CT,
USA to Los
Angeles, CA,
USA (return)
3 time zones
WEST and EAST
5–6 h total travel
time each
direction
RCT
Compression
garments
(worn on upper
and lower body
for entire testing
time, incl.
sleeping)
Data collected:
6 total test days
1 d before (pm)
(west-east) and on arrival,
Day 2 pre-/postexercise
testing, +1 and +2 days
after return flight
Blood +urine sampling:
markers of inflammation
and hormonal
disturbance
Physical performance
indicators:
handgrip strength, CMJ,
40-yard sprint, pro-agility
drills
Self-reported: jet lag
questionnaire
Compression garments
maintained lower body
indicators; no changes in
isometric strength of
upper limbs
Not reported
Methodology
specified that if
USG was >1020,
athletes were
instructed to
“drink water”
until USG was
<1020
Yes
USG measured
throughout, but
results not
reported or
correlated to
any
performance
outcomes.
Nutrients 2020,12, 2574 10 of 15
Table 2. Cont.
Study Participants Population Flight Details Design Testing Methodology Performance Outcomes Fluid Intake? Hydration
Status
Assessed?
Geertsema et al.,
[57]
N=48 athletes
and N=18
controls
Sex not reported
26 ±3 y
“Athletes” not
specified.
In-text defined
as “involved at
the senior level
in an aerobic
sport”
Pooled data
from N=10
flights
Range: 2 flights
3h and 8 flights
>10 h
Prospective
cross-sectional Data collected:
preflight 3 h, 7 h during
flight, at destination (in
the airport);
HR and O2saturation
NS HR
O2decreased 4% inflight,
and returned to baseline
immediately upon arrival
Not reported No
Schumacher
et al., [28]
N=15 male
28 ±4 y
N=11 male
controls
25 ±3 y
Endurance
athletes Germany to
Doha, Qatar
2 time zones
(EAST)
TT: 14 h
Prospective
cross-sectional Data collected:
1 d before (am)
+1 d on arrival (am)
+3 d after (am and pm);
Bloodwork
None per se
[Hb] and [Hct] were each
sig. lower on +1 d am
sample, but no dierent
between groups and
resolved by +3 d post
No correlation between
blood markers or change
in weight on travel day
Yes
Fluid intake
recorded on the
travel day
(2750 ±521 mL)
of “dierent”
beverages
consumed
Urine output:
1768 ±621 mL
No change in
weight
Indirectly yes
Changes in
weight and
overall fluid
intake were
measured
during travel.
Blood markers
were assessed,
but plasma
volume not
calculated from
these variables.
Stevens et al.,
[58]
N=12 male
48 ±14 y Triathletes
(Masters)
Hawaii Ironman
Sydney, AUS to
Kona via
Honalulu, USA
NORTHEAST
Flight time 12 h
TT: 23 h,
Prospective
cohort Data collected:
10 d, 7 d, and 4 d
preflight, 2 d, 3 d, 4 d, 1 d
before race day, during
race, 1 d after race
Sleep quality (actigraph)
Saliva: (sIgA, sCort)
Self-reported: sleep diary,
illness, jet lag, physical
preparedness
No changes in sleep
quality or mucosal
measures across the
study
No No
Notes: CMJ, countermovement jump; TT: total time traveled; NS, not significant; RCT, randomized control trial; HR, heart rate; BP, blood pressure; SBP, systolic blood pressure; DBP,
diastolic blood pressure; USG, urine specific gravity.
Nutrients 2020,12, 2574 11 of 15
In terms of the cardiorespiratory fitness and the cardiovascular response to long-distance air
travel, data on maximal oxygen uptake and/or oxygen uptake kinetics in elite athletes are sparse.
One study evaluated the eects of 10 h airline travel on oxygen saturation (SpO
2
, %) and heart rate
(HR) in 45 national-level athletes [
57
]. They found a significant acute decrease in the SpO
2
(by ~4%)
during the transmeridian air travel with readings returning to baseline values within 7 h after landing;
HR was
unaected throughout. Montaruli et al. [
59
] studied the eects of transmeridian air travel from
Italy to the USA in marathon runners. These runners were further separated into two experimental
and one control group to examine whether a training schedule (i.e., time of the day) aected sleep
patterns (assessed via actigraphy). They found sleep quality was mostly preserved with a preplanned
training routine, but unfortunately, they failed to report any data on the physiological profiles of these
runners, or their performance outcomes at the New York marathon. Lemmer et al. [
51
] evaluated
the HR rhythms and blood pressure (BP) response in elite German athletes traveling both eastward
and westward to compete internationally. In brief, they found that the HR and BP were not aected
immediately after arrival at their destination, but on day two after arrival, BP increased in those
athletes traveling westwards, and decreased in those traveling eastwards. Similar adjustments to HR
were noted, lasting up until day 11 for those athletes traveling westward. Lemmer et al. [
51
] also
reported the negative influence of transmeridian air travel on core temperature fluctuations and saliva
melatonin and cortisol concentrations, but again no data were shown on the hydration status of the
participants included or the potential influence of acute dehydration on any of the above-mentioned
physiological indicators.
The hormonal response and inflammation in athletes traveling and competing across dierent
time zones are often evaluated across investigations [
49
,
51
,
52
]. Salivary cortisol and melatonin
concentrations are each aected by transmeridian travel, and apparently, to a greater extent for athletes
traveling eastward [
51
]. Similar findings have been observed by Bullock et al. [
49
] who reported a ~67%
lower resting salivary cortisol level immediately after reaching their competition destination, whereas
no changes were observed in the nontraveling control group. Contrary to the above-mentioned, recent
work of Kraemer et al. [
52
] reported that plasma epinephrine, testosterone, and cortisol concentrations
are significantly elevated above baseline before and after a stimulated competition, irrespective of the
actual 6 h long trans-American air travel time. Interestingly, none of these investigations explored the
potential influence of hydration status on the hormonal disturbance following long-haul air travel.
Finally, subjective, self-perceived data on sleep quality, overall fatigue, muscle soreness,
and mental
preparedness uniformly suggest an impaired physical capacity in athletes immediately after the
long-haul flights which include large time zone changes [
13
,
47
,
49
,
57
]. Unfortunately, data from these
subjectively measured scales have not been combined with data on actual fluid intake, self-perceived
thirst scales, or USG readings in any of the above-mentioned papers. A possible explanation for
the lack of data on the combined eects of jet lag and acute dehydration may be primarily from a
methodological perspective. It seems rather challenging to isolate the independent and combined
eects of acute hypohydration from other long-haul, transmeridian, travel-induced symptoms and
side eects. This is especially true when no study has evaluated the hydration status of its traveling
athletes despite significant evidence that the air cabin environment creates an environment promoting
fluid loss of the individual by the very nature of the air quality, composition, and sedentary physical
inaction of air travel. Thus, both structural and functional aspects of human physiology are aected
by long-haul flights and the air cabin environment but to what extent dehydration is a risk factor for
traveling athletes remains to be determined.
4. Future Directions and Practical Recommendations
Despite the lack of studies directly investigating how modulating hydration status may prevent
or reduce jet lag symptoms, some recommendations may be proposed based on preliminary findings
and translating results of other research in this field.
Nutrients 2020,12, 2574 12 of 15
Good hydration should be an everyday practice; be sure to start travel in an optimally hydrated
state. General recommendations for everyday fluid balance may dier when applied to athletes.
During long-haul flights, fluid intake should be between 100 and 300 mL/h (including that which
is derived from food). These values are based on estimated fluid losses [
4
] and from preliminary
research findings [
11
,
35
]. Adapting one’s fluid intake timing in order to avoid disruption during
sleep is highly recommended.
Hydration strategies which incorporate electrolytes paired with carbohydrate solutions may be
beneficial, in addition to pure water [11,35].
If not habituated, avoid caeine intake since it may (i) increase diuresis and (ii) impair sleeping.
If habituated, a total caeine ingestion
300 mg has been shown to have minimal adverse eects
on travelers [44].
Future studies should focus on determining exact changes in hydration status across long-haul
flights and juxtapose these with severity of jet lag symptoms. Only until the magnitude of the
dehydrating eect of flying is quantified will it then be possible to design appropriate guidelines
and countermeasures for travelers. In particular, our analysis of the literature has highlighted some
questions and issues that may warrant further investigation, such as (i) what is a common consensus
on increased fluid losses during long-haul flights (assuming similar cabin conditions), (ii) determining
prevalence of hypo/underhydration during long-haul flights and its relationship with the severity of
jet lag symptoms, and (iii) the need for more placebo-controlled, double-blind studies to determine the
ecacy of nutrition/hydration protocols which can be used to maintain fluid balance and mitigate jet
leg symptoms.
5. Conclusions
There is a lack of randomized controlled trials exploring the eects of acute hydration status on
various physiological performance indicators following long-distance air travel. After reviewing the
limited studies which investigated any physiological variable on actual traveling athletes, there is no
consensus or real evidence of how the incidence rate, magnitude, or duration of acute dehydration due
to air travel aects the general health or performance of elite athletes. It remains that the air cabin
environment does provide a situation where significant changes in fluid balance may occur.
Author Contributions:
Conceptualization, S.A.M.; Methodology, D.Z., A.B.S., S.A.M.; Investigation, D.Z., A.B.S.,
S.A.M; Writing – Original Draft Preparation, D.Z., A.B.S., S.A.M; Writing – Review & Editing, D.Z., A.B.S., S.A.M;
Supervision, S.A.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments:
The authors completed this investigation with no external support (financial or otherwise).
We thank our colleagues who have contributed to numerous interesting conversations and personal reflections on
their own travel histories and experiences.
Conflicts of Interest: The authors declare no conflict of interest.
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... Therefore, adding an additional load to the standard tests could allow the assessment of the mechanical muscle capacities (i.e., F, V and P), providing a deeper insight into the function of the tested muscles and resolve a number of questions questioned in the literature. In addition, such knowledge could also improve the outcomes of muscle testing in different environmental scenarios and physiological conditions to understand the human body's adaptations and reactions to temperature [24][25][26], altitude [27] or dehydration [28][29][30]. ...
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The present study investigated the effect of a 3% rapid weight loss (RWL) procedure on neuromuscular performance in elite, Olympic-style boxers. Nine boxers were randomly assigned to two experimental procedures (RWL and control, in a randomized counterbalance order) to perform 5-s maximum isometric voluntary contractions (MVC) of the dominant leg knee extensors prior to (MVC1), and following (MVC2), a sustained, isometric contraction at 70% MVC until exhaustion. The voluntary activation (VA) was determined using percutaneous muscle stimulation and interpolated twitch technique. High (at 80 Hz) and low (at 20 Hz) frequency tetanic impulses were also delivered before and after the sustained 70% MVC to assess peripheral fatigue. Hydration status, hemodynamic parameters, and lactate concentration were assessed throughout the study. Body-mass was reduced by ~3% (during RWL) compared to control (p=.001). As a result of the RWL protocol, MVC1 force output was 12% lower and VA deficits of 7% were observed after the fatigue protocol compared to control (p=.001). Following RWL, time to exhaustion for the sustained 70% MVC was 69±20 s compared to 86±34 s for control (p=.020). Peak lactate production was 53% lower in RWL compared to control (p=.001). In conclusion, the 3% RWL procedure translated into significant decline in neuromuscular performance for both brief and sustained contractions in competitive boxers.
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The purpose of this investigation was to assess the merit of sports compression socks in minimizing travel-induced performance, physiological, and hematological alterations in elite female volleyball athletes. Twelve elite female volleyballers (age, 25 ± 2 years) traveled from Canberra (Australia) to Manila (Philippines), and were assigned to 1 of 2 conditions; compression socks (COMP, n = 6) worn during travel or a passive control (CON, n = 6). Dependent measures included countermovement jump (CMJ) performance, subjective ratings of well-being, cardiovascular function, calf girth, and markers of blood clotting, collected before (−24 hours, CMJ; −12 hours, all measures), during (+6.5 and +9 hours, subjective ratings and cardiovascular function), and after (+12 hours, all measures except CMJ; +24 hours and +48 hours, CMJ) travel. When compared with CON, small-to-large effects were observed for COMP to improve heart rate (+9 hours), oxygen saturation (+6.5 hours and +9 hours), alertness (+6.5 hours), fatigue (+6.5 hours), muscle soreness (+6.5 hours and +9 hours), and overall health (+6.5 hours) during travel. After travel, small-to-moderate effects were observed for COMP to improve systolic blood pressure (+12 hours), right calf girth (+12 hours), CMJ height (+24 hours), mean velocity (+24 hours), and relative power (+48 hours), compared with CON. COMP had no effect on the markers of blood clotting. This study suggests that compression socks are beneficial in combating the stressors imposed by long-haul travel in elite athletes, and may have merit for individuals frequenting long-haul travel or competing soon after flying.
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Jet-lag symptoms arise from temporal misalignment between the internal circadian clock and external solar time when traveling across multiple time zones. Light is known as a strong timing cue of the circadian clock. We here examined the effect of daily light on the process of jet lag by detecting c-Fos expression in the master clock neurons in the suprachiasmatic nucleus (SCN) under 8-hr phase-advanced jet lag condition. In WT mice, c-Fosimmunoreactivity was found at 1–2 hours on the first day after light/dark (LD) phaseadvance. This induction was also observed on the second and third days, although their levels were diminished day by day. In contrast, c-Fos induction in the SCN of V1a–/–V1b–/– mice, which show virtually no jet lag symptoms even after 8-hr phase-advance, was only detected on the first day. These results indicate that external light has affected SCN neuronal activity for 3 days after LD phase-advance in WT mice suggesting the continuous progress of activity change of SCN neurons under jet lag conditions. Noteworthy, limited c- Fos induction in V1a–/–V1b–/– SCN is also consistent with the rapid reentrainment of the SCN clock in mutant mice after 8-hr LD phase-advance.