Nutritional considerations during prolonged exposure to a confined, hyperbaric, hyperoxic environment: recommendations for saturation divers

The effect of dietary intake on metabolic and physiological processes is vast, with
the application of personalised nutritional strategies deemed to be most effective
in promoting performance and health. Accordingly, the current review aims to synthesise
the available literature related to the physiological responses to saturation diving,
and to consider these in relation to commonly accepted nutritional principles 20]–22]. Recommendations have been made within 4 key categories including: (1) maintenance
of energy balance; (2) macronutrient composition of dietary intake; (3) micronutrient
requirements and (4) hydration. These recommendations are made based on the available
evidence, but remain cognisant of the practical challenges associated with achieving
appropriate nutritional intake in this unique environment.

Energy balance

Consistent reports of body mass loss while in saturation 12], 13], 23] suggests that divers may be in an energy-deficient state, and this loss includes
muscle mass 13]. Chronic energy deficit while in saturation may contribute to a number of adverse
health outcomes, including immunosuppression and compromised bone health (10, 2).
It has been suggested that energy deficiency during saturation may, at least in part,
be due to an increased energy cost of activities performed under saturation, relative
to the energy cost of those same activities under usual environmental conditions 15]. For example, helium (which is often used as the inert gas required to create the
hyperbaric environment) possesses a conductivity six times greater than oxygen 3], which increases heat exchange from the body, therefore increasing the energy cost
of thermoregulation. In addition, hyperbaria results in a greater gas density thereby
increasing airway resistance and subsequently the energy cost of respiration 24]. Using doubly labelled water, a significant increase from 3105 ± 95 to 3534 ± 119 kcal
was reported when comparing energy expenditure at the surface and 317 m depth, respectively,
in US navy saturation divers 15]. A comparison between 50 and 317 m revealed no significant difference in energy expenditure,
indicating the increase in energy expenditure may be due to the composition of the
gaseous mix as opposed to pressure differences associated with depth. Further research
by Busch-Stockfisch and Bohlen 13] indirectly supported this notion of increased energy expenditure though comparison
of nutritional intake and changes in body composition in operational saturation divers.
This study reported a mean 1.98 kg loss in muscle mass and 2.65 kg loss in total body
mass over the course of an extended operational saturation dive between 30 and 44 days,
despite dietary intakes remaining comparable with onshore intakes.

Energy intake recommendations

Paciorek 25] calculated that saturation divers required approximately 53 kcal/kg body mass (BM)
a day. This calculation was based on maintenance of body mass of six divers during
a 17 day working dive at 350 m which involved 6 days of 8 h working shifts, a working
pattern which would not be uncommon with current operational practices. Using Paciorek’s
25] proposed figure, an 80 kg diver would require 4249 kcal daily, a figure that would
seem appropriate considering the work of Seale et al. 15] who reported expenditure of 3534 ± 119 kcal in 79.5 ± 2.5 kg navy divers at 317 m
during a dry simulated dive (i.e. no underwater work was performed). Using the available
evidence, the proposed lower limit of required energy intake is 44 kcal/kg BM a day,
which is based on a 79.5 kg diver requiring 3534 kcal on average as reported by Seal
et al. 15]. Therefore, saturation divers should aim to consume between 44 and 53 kcal/kg BM
a day dependent on the duration and intensity of work performed on a given day. As
energy requirements will vary daily amongst individuals, divers should monitor their
daily energy intake against subjective feelings of fatigue, energy and hunger to assist
in achieving an energy balance.

Macronutrient composition

Carbohydrates and dietary fats

Research regarding the utilisation of metabolic substrates at rest and during physical
activity during hyperbaria or hyperoxia is sparse 17], 26], 27], although the available evidence supports the notion of increased fat utilisation.
It was deemed important to address the issue of substrate utilisation due to the potential
influence on macronutrient recommendations during work and rest periods. For example,
research conducted under hypoxia has demonstrated altered substrate utilisation 28], leading to tailored nutritional recommendations for increased carbohydrate ingestion.
It is documented that exercise under acute hyperoxic conditions lowers the respiratory
exchange ratio (RER) of exercise compared to normoxia 17], 26], 27] indicating increased fat utilisation. An earlier study by Dressendorfer et al. 16] conducted under hyperbaric conditions assessed maximal oxygen uptake in three different
conditions, including an atmosphere equivalent to the ambient surface air, a hyperoxic
heliox atmosphere and a normoxic heliox atmosphere during a seventeen-day simulated
saturation dive. Although the main purpose of the study does not provide relevant
data for the purposes of this section, further analysis of the presented raw data
of the four subjects does. The Vo
₂
and VCo
₂
data provided during submaximal exercise allowed calculation of fat and carbohydrate
oxidation using stoichiometric equations 29]. The analysis revealed that energy expenditure was greater in both heliox atmospheres,
as supported by subsequent research 15]. In addition, carbohydrate oxidation within both heliox conditions was over twofold
smaller (0.83 vs. 1.70 g/min) and fat oxidation was four-fold greater (0.50 vs. 0.14 g/min)
in hyperoxia compared with normoxia. These results suggest that chronic exposure to
hyperoxia, irrespective of hyperbaria, results in increased fat oxidation. However,
these data were taken from 5 min of submaximal exercise using the Douglas bag technique,
which suffers from increased error when conducted in hyperoxia 30]. Therefore, these data should be used only as an indication of substrate use during
saturation, while further research is required to better understand the metabolic
implications of prolonged hyperoxic exposure.

Carbohydrate and dietary fat recommendations

The information presented on fuel utilisation under hyperoxia demonstrates a trend
towards increased fat oxidation, but no study has directly assessed fuel use during
an extended saturation dive either in the chamber or during an underwater excursion.
Therefore, the current research may not be considered strong enough to provide recommendations
that greatly deviate from general guidelines proposed by the Institute of Medicine
(IOM) 20]. Current guidelines for dietary fat propose total intake should comprise 20–35 %
of total calorie intake 20]. Due to the potential shift in fuel utilisation under hypoxia, which results in increased
fat oxidation, we suggest saturation divers should consume dietary fats at the upper
end of the range set out by the IOM. Furthermore, fat is more energy dense than carbohydrate
(~9 vs. 4 kcal/g, respectively), therefore consuming a higher proportion of dietary
fat may also facilitate saturation divers to meet the increased energy demand required
under the heliox atmosphere. Catering provision on diving support vessels should endeavour
to provide good dietary fat options on the menu so divers can consume higher intakes
without compromising health. Table 1 provides examples of recommended dietary fat sources.

Table 1. Recommended sources of dietary fat which should be made available to saturation divers
to attain the nutritional recommendations outlined for fat and total energy intake

Current carbohydrate recommendations for the general population are between 45 and
65 % of total calorie intake 20]. To allow sufficient intake of all three macronutrients and to allow for fat intake
towards the top end of the recommended range, divers should be encouraged to target
the low to mid carbohydrate intake of the current IOM guidelines. It should be made
clear that we are not recommending a low-carbohydrate diet for saturation divers,
but rather an appropriate balance of all three macronutrients to support metabolic
function. Despite these suggestions, we believe that achieving energy balance is the
most essential component of nutritional intake in this group in order to support physiological
functions and maintain energy availability during lockout. Along with the absence
of strong evidence on fuel use, it is suggested the appropriate balance of these two
macronutrients should primarily be determined by the individual’s preferences, so
to enable divers to achieve energy balance.

Protein metabolism

Knowledge of protein metabolism is important to provide guidelines on appropriate
protein intakes, due to the implications on health and body composition 31], 32]. A series of simulated dives at various depths (200–600 m) revealed divers experienced
reductions in lean body mass of between 1 and 5 kg over dives lasting 26–44 days 13]. This suggests divers may find themselves in a catabolic state, in which case, a
higher protein intake is required to reduce skeletal muscle loss 33]. To the authors’ knowledge, only a single study has investigated whole body protein
synthesis (WBPS) in this environment 14]. A significant twofold reduction in protein synthesis was reported between the surface
and a simulated 45 m dive. Declines of this magnitude are typically associated with
disease states, inadequate protein intakes, or negative energy balance 34]. However, in the study conducted by Conway et al. 14], subjects were healthy US navy divers who were in a positive nitrogen balance, consuming
a protein rich diet equivalent to 1.5 g/kg BM a day throughout the trial. In addition,
data from two adjunctive studies 15], 23] using the same sample revealed that the subjects also attained a balance between
energy expenditure and intake. The results from these studies demonstrate that diminished
WBPS may be attributed to dive conditions.

Protein recommendations

It appears appropriate to propose that under the unique environmental and physiological
challenges, protein intake for saturation divers should be above the recommended daily
allowance for the general population, which is currently 0.8 g/kg BM a day. An appropriate
recommendation for this population would be a minimum of 1.3 g/kg BM a day, aligning
with current recommendations to maximise muscle protein synthesis 35]. It may be conceivable that higher protein intakes are required to maximise WBPS
under saturation based on the data reported by Conway et al. 14] but further research is required to define optimal protein requirements from a molecular
perspective. Research in athletic populations suggests protein intake of between 1.8
and 2 g/kg BM a day would be appropriate to prevent muscle loss while in an energy-deficient
state 35]. However, a main consideration when setting this protein intake was the contribution
to achieving total energy requirements. Protein has high-thermogenic properties and
promotes feeling of satiety, which will reduce appetite and subsequent energy intake
36] thus increasing the difficulty in achieving energy balance. Therefore, protein intake
should be kept at a moderately increased level (1.3 g/kg) to help divers achieve a
daily energy balance.

Micronutrient requirements

Micronutrients are involved in various physiological functions in the body and are
essential to the overall health of an individual. It is likely that if the higher
energy intake described previously is achieved through a balanced and varied diet
then the current IOM guidelines for micronutrient intakes will be met 20]. For saturation divers, the role of certain micronutrients may become even more relevant,
particularly for issues related to bone health, oxidative stress and haematological
processes, and these will be discussed throughout the forthcoming section.

Vitamin D

Brubakk et al. 2] identified bone health as one of the few documented long-term disease states linked
with saturation diving. Bone is a nutritionally modulated tissue meaning dietary intake
can have a significant impact on overall bone strength and structure 37]. Vitamin D has a pivotal role in regulating calcium homeostasis and therefore overall
bone health. Vitamin D is also proposed to have implications on other physiological
aspects such as immune function 38], insulin resistance 39] and physical performance 40]. We have focused on the role of vitamin D in these guidelines due to the probability
that insufficient or deficient levels of serum 25[OH]D, a biomarker of vitamin D,
may be prevalent amongst saturation divers due to lack of sunlight exposure within
the saturation chamber.

Vitamin D is unique compared to other essential vitamins, predominantly as the main
source is through ultraviolet B (UVB) radiation from sunlight as opposed to dietary
intake 41]. According to some researchers, 42] in the absence of adequate sunlight exposure, it is unlikely that individuals can
obtain sufficient vitamin D through diet alone. In saturation divers, this has obvious
concerns as they are deprived of sunlight exposure for up to 28 days at a time. It
appears reasonable to postulate that divers may have inadequate or deficient levels
of serum 25[OH]D. Only one study has measured serum 25[OH]D concentrations, before
and after a saturation dive lasting 14 days 7], during which serum 25[OH]D fell by 11 nmol L
^?1
by day 12 (from 92 ± 23 to 81 ± 18 nmol L
^?1
) in a sample of six divers. If this trends were to continue over a full 28 day dive,
concentrations could drop by more than 20 nmol L
^?1
. Smith et al. 7] conducted the study in America, which may explain the high levels of baseline serum
25[OH]D.

Vitamin D recommendations

Due to the difficulty of obtaining sufficient vitamin D through dietary intake alone
42], along with the absence of UBV radiation, supplementation is deemed an appropriate
strategy while in saturation. The aim for vitamin D recommendations should to be to
maintain serum 25[OH]D  50 nmol L
^?1
, the current IOM threshold for vitamin D adequacy 43]. We recommend that saturation divers should receive regular blood analysis of vitamin
D status so appropriate supplement strategies can be determined. Vitamin D is correlated
with seasonal variations 43], so we suggest testing in the autumn and winter months when levels are likely to
be at their lowest. Regardless of testing it would be prudent for divers to supplement
whilst in the chamber. It has been proposed that supplementation with 2000 IU daily
is sufficient to maintain serum 25[OH]D levels 44] whilst 4000 IU is defined as the upper tolerable limit by the IOM 45]. Therefore, we recommend that saturation divers can supplement between 2000 and 4000 IU
safely when in saturation to maintain 25[OH]D concentrations. Supplementation can
continue at this level onshore without any concern but may not be necessary, depending
on the climate in the location divers reside, time of year, outdoor sunlight exposure
and serum 25[OH]D test results.

Antioxidants

The term oxidative stress refers to a state caused by the generation of oxidised molecules
such as reactive oxygen species (ROS) and free radicals that are greater than the
ability of the body’s antioxidant system to reduce them, and may result in damage
to cellular proteins, lipids and DNA. Research has implicated this imbalance between
ROS and antioxidant reserves with the pathogenesis of various disorders, including
cardiovascular and neurodegenerative diseases, diabetes and several types of cancer
46]–48]. While intermittent exposure to oxidised particles may elicit a net positive adaptive
response, chronic exposure to hyperoxia within the saturation chamber has been reported
to elicit a greater magnitude of oxidative stress compared to the ambient surface
atmosphere, resulting in increased lipid peroxidation and DNA damage 7], 8]. In addition to the reported oxidative damage, endogenous antioxidant enzyme activity
has also been reported to be attenuated, specifically related to superoxide dismutase
(SOD) activity 7]–9]. Despite this, the full effect of saturation diving on redox physiology is yet to
be ascertained, in particular other factors such as the chronic high pressure exposure
and the effect of inert gas mixtures, which may contribute to oxidative stress 2]. It is however, unknown if the associated long-term health implications of chronic
oxidative stress are prevalent in this population, as no epidemiological data exist
2]. Consumption of exogenous antioxidants can attenuate the magnitude of oxidative stress
experienced under hyperoxic conditions 49]. Results from this study showed that supplementation of vitamin C (600 mg) and E
(150 mg) diminished oxidative damage in the liver; however, to the authors’ knowledge,
this has been the only study to investigate antioxidant supplementation in this environment
49].

Antioxidant recommendations

Exogenous antioxidants obtained through a healthy diet are associated with low levels
of oxidative stress. A number of randomised controlled trials have shown that fruit
and vegetable intake, which are antioxidant rich, can reduce markers of oxidative
stress in healthy individuals 50]–52]. Therefore, a varied diet high in fruit and vegetables is recommended during a dive.
It is not known, however, if a varied dietary intake alone will be sufficient to attenuate
oxidative damage during a saturation dive. Additional supplementation is not likely
to cause any harm to individuals; therefore, supplementation of antioxidants at the
same dosage used by Ikeda et al. 49] is deemed a safe and appropriate recommendation for saturation divers. Another consideration
of redox homeostasis is the reduced endogenous antioxidant enzyme activity 7]–9]; therefore, dietary intake to elevate both SOD and glutathione peroxidase may be
prudent. Dietary intake can support these enzymes such as selenium intake to increase
GPX activity 53] and zinc intake, which may elevate SOD 54]. It is prudent for saturation divers to meet the IOM guidelines on and offshore for
selenium and zinc, which are 55 and 11 mg/day, respectively.

Vitamin B12, folate and iron

Haemoglobin concentrations throughout the course of a saturation dive have consistently
been shown to decline 7], 9], 55], 56]. Nakabasy et al. 55] suggested the primary cause of reduced haemoglobin concentration is the attenuation
of the rate of red blood cell production, which is also supported by the finding of
reduced erythropoietin (EPO) activity during a dive 9]. In these guidelines, we focus on vitamin B12, folate and iron due to their involvement
in haematological processes.

Vitamin B12 and folate recommendations

Vitamin B12 and folate are involved in the production red blood cells 57] and are linked to EPO activity 58], 59]. It is therefore deemed appropriate that saturation divers should consume sufficient
levels of both nutrients to maintain normal circulating levels. Recommendations for
vitamin B12 from the IOM are 2.4 ?g/day, which is deemed appropriate for saturation
divers. Circulating levels of folate are lowered after a saturation dive 9], and therefore it is suggested that folate intake corresponding to the RDA of 400
µg/day is appropriate 20] but higher intakes may be prudent. However, it is advised that intake does not exceed
the recommended upper tolerable limit of 1000 ?g/day 20]. Recommendations for each nutrient can be achieved through a varied diet; nevertheless,
there may be some instances where supplementation may be required, in particular folate,
if availability is insufficient from on-board catering provision.

Iron recommendations

Iron also has an important role in haematological processes. However, iron has been
reported to accumulate within the body during saturation, which is likely to be due
to the decline in red blood cell content 11]. Excessive iron can result in the formation of powerful ROS 60] and is associated with a greater risk of type 2 diabetes 61]. For this reason, we suggest dietary intake of iron should be monitored. Current
dietary recommendations of iron are 18 mg/day, there is not sufficient evidence to
lower this recommendation but saturation divers are not advised to increase intake
beyond this or supplement with iron in the chamber.

Hydration

Maintenance of fluid and electrolyte homeostasis is required to ensure usual physiological
function, due to the wide range of functions which water fulfils within human metabolism
62]. Hypohydration occurs during the process of dehydration whereby fluid loss is greater
than fluid intake, and is thought to cause a range of adverse health and performance-related
implications 62], 63]. Many decrements are thought to commence at approximately 2 % body mass loss 63]. The hyperbaric hyperoxic environment of saturation diving induces a number of physiological
changes, which may contribute to a state of hypohydration.

The primary factor that may challenge fluid homeostasis within the saturation chamber
is hyperbaric diuresis. This refers to an excess production of urine, which has been
reported to occur under conditions of hyperbaric hyperoxia 4]–6]. Sagawa et al. 6] reported a significant twofold increase in daily urine production (1032 ± 140 to
2100 ± 105 ml, p  0.05), a 54 % increase in sodium excretion (163 ± 24 to 251 ± 14 meq/24 h, p  0.05) and a 32 % reduction in urine osmolality (799 ± 78 to 541 ± 29 mosmol, p  0.05) during a 7-day simulated saturation dive. Furthermore, the increased sodium
loss 6] may contribute to a state of hypovolemia (i.e. decreased plasma volume). This is
supported through reported plasma volume reduction of 20 ± 9.3 and 30.1 ± 9.1 % at
450 and 230 m, respectively, found during a simulated hyperbaric dive 64].

In addition to the hydration challenges associated with the hyperbaric environment,
underwater work periods may also compromise the body’s ability to maintain euhydration.
Performing physically demanding work while immersed in water and wearing a hot water
diving suit may create a state of thermal stress 65], which is likely to increase fluid loss due to increased sweat rate and production.
A number of studies have reported that seawater immersion lasting between 4 and 6 h
results in significant fluid loss 65]–68] due to the hyperosmolarity of seawater, which exacerbates fluid loss. In a study
evaluating body mass loss during lockout in 128 divers, Hope et al. 64] reported that subjects lost ?2 % body mass during one-third of dives (corresponding
to the threshold typically reported as resulting in performance decrement), with losses
up to 6 % body mass loss reported, representing a state of severe dehydration 69].

Hydration recommendations

The ultimate goals of the hydration recommendations made are to encourage maintenance
of in-chamber euhydration, and to ensure sufficient fluid replacement following lockout.
Prescriptive recommendations related to hydration strategies are difficult due to
the highly individual nature of the body’s response. In accordance with guidelines
from the British Dietetics Association, a minimum of 2 L/day of water is recommended.
This recommendation aligns with those provided for astronauts, a group who experience
similar environmental challenges to saturation divers, including changes to atmospheric
pressure, and confinement for prolonged periods of time.

Given the large fluid losses reported during lockout, along with the limited opportunities
to drink, more extensive hydration strategies may be required in order to counteract
these factors. The primary pre-lockout goal should be to ensure that divers enter
lockout in a euhydrated state, with the consumption of food and fluid containing sodium
to be used prior to lockout to facilitate fluid retention 70], 71]. Due to the iso-osmotic composition of fluid loss underwater 65], an isotonic electrolyte drink is recommended during a dive to replace electrolyte
losses, thus reducing the likelihood of hyponatremia or large reductions in plasma
volume. Post lockout, it may be advantageous to practice more extensive hydration
strategies to facilitate rapid recovery in preparation for the next lockout period,
typically within the next 12–24 h. Sodium intake in this instance is critical for
the effective restoration of fluid balance within euhydration ranges 70], 71]. It is important to consider that these recommendations may inadvertently increase
the risk of over-hydration and the development of hyponatremia, which are associated
with adverse health 72]. As such, in the absence of direct measurements of fluid balance, divers are encouraged
to dictate fluid intake with sensation of thirst when consuming fluids above the minimum
2 L recommended.

Additional nutritional considerations

The aspects highlighted within these guidelines have primarily focused on the role
of nutrition on the health and performance of saturation divers. There is, however,
emerging evidence to suggest that nutritional strategies may also enhance safety of
divers during decompression. Administration of exogenous nitric oxide (NO) reduces
microbubble formation after decompression 73] in divers, whilst knockout of nitric oxide synthase (NOS) in mice increased microbubble
formation 74]. The presence of gaseous microbubbles is positively correlated to the risk of decompression
sickness 2], for that reason dietary strategies to increase nitric oxide availability for decompression
may be beneficial. Manipulation of dietary intake can augment nitric oxide bioavailability
through exogenous nitrate intake 75], which is predominantly obtained through vegetables such as spinach, beetroot and
rhubarb. We recommend divers should be encouraged to consume a healthy diet rich in
nitrates throughout a saturation dive. If practical barriers in catering provision
prevent divers from obtaining dietary nitrates, supplementation with L-arginine can be used as an alternative to increase NO production 75]. Nevertheless, these recommendations are somewhat speculative given the limited scientific
evidence in the area. Thus, further research is required to ascertain the clinical
relevance of such recommendations. That said, dietary intake of nitrates is deemed
a safe but potentially effective recommendation.

Practical challenges and recommendations to nutritional intake

In addition to the physiological factors described above, there are also a number
of practical challenges associated with saturation diving that may present barriers
to achieving appropriate nutritional intake. Therefore, nutritional recommendations
for this population should be cognisant of both physiological and practical considerations,
thereby allowing greater opportunity to practice appropriate dietary strategies within
this challenging environment. The greatest practical barriers to nutritional intake
include shift patterns and food provision, lockout and food palatability. These areas
will be discussed in more detail below, along with recommendations to facilitate nutritional
intake to overcome practical barriers.

Shift patterns and food provision

Saturation divers are allocated 12 h shift patterns, during which they can be asked
to enter lockout. This necessitates the adjustment of daily routines to the allocated
time periods and may distort perception of normal night and day. It has been suggested
that shift work (within the general population) can impact upon the ability to maintain
usual eating patterns, particularly if food choices are limited 76]. At any given time of day, divers may have different dietary needs that will have
implications for catering provision on board the vessel. As one diving team is returning
from an intense underwater excursion, another team may be waking from sleep and desire
breakfast food items. In this case, we suggest that catering providers work in conjunction
with an appropriately qualified nutrition professional (e.g. dietitian or registered
nutritionist) to develop menus that will enable saturation divers to meet their specific
nutritional requirement regardless of shift allocation. On larger diving support vessels
that may have eight diving teams on different shift patterns, this will require appropriate
planning and communication between the catering providers, diving support team and
nutritional professional.

Lockout

During lockout saturation, divers are isolated from the main vessel for up to 8 h,
6 of which may involve underwater immersion and the completion of physically demanding
tasks whilst wearing a hot water diving suit. Previously, divers had limited opportunity
to refuel or rehydrate during lockout, although recent legislative changes stipulate
a mandatory break if they are to be immersed for 4 h or longer, which may have a positive
influence on ability to maintain fluid and energy balance during lockout. Simple steps
can be taken to encourage dietary intake in this situation, for example, provision
of energy dense snacks in the bell may encourage intake. These snacks can be consumed
while the divers are being transported to and from the working depth or during the
mandatory 30-min break mid-lockout. The use of a carbohydrate-based sports drink may
be most appropriate for this situation as it can provide the energy, fluid and electrolytes
that are required when working. Sport nutrition research has shown that the minimum
rate of carbohydrate ingestion to elicit physical performance benefits is 16 g/h 77], which is a suitable minimum recommendation for underwater excursions in order to
maintain performance. Ingestion at higher levels, between 30 and 60 g/h, in line with
current sports nutrition recommendations 78] should be based on the individual tolerance and the intensity of work undertaken.
Previously, a water carriage system integrated within diving suits has been piloted
with some success 79], providing a potentially suitable and effective method of energy intake and maintaining
fluid balance while underwater; however, the use of such apparatus is thought to be
limited amongst saturation divers due to debates over their practicality and safety
underwater.

The timing of energy intake around lockout is also deemed pertinent. Increased energy
availability has been suggested to have a considerable impact upon physical performance
80], more specifically, the consumption of carbohydrates and dietary fats 3–4 h prior
to exercise is suggested to have an ergogenic impact on exercise performance 81]. Additionally, the restoration of muscle glycogen and intramuscular triglyceride
stores following lockout may be beneficial, particularly if subsequent shifts are
within the next 12–24 h. A meal rich in moderate to high glycaemic index carbohydrates,
along with dietary fats, are recommended 82] post lockout to facilitate recovery. For this to be practiced, we recommend that
the diving support and catering teams communicate appropriately, so food is available
for each diver within a reasonable time of entering and leaving lockout. In addition,
it is likely that the intensity and duration of an underwater excursion will be known
prior to the event. Communication of the intended work schedule to the catering team
will allow menus to be developed that provide the appropriate amount of energy for
the proposed work. Where possible both teams should plan ahead, however this will
require a certain degree of flexibility on behalf of the catering team as schedules
are susceptible to last minute change. Effective implementation of this recommendation
requires the use of a qualified nutrition professional to facilitate the communication,
in order to ensure the energy expended during lockout is matched with the calories
available in food provision, whether this is through energy dense meal options or
varying portion sizes.

Food palatability and appetite

All food that enters the saturation chamber is compressed to the ambient pressure,
which in certain instances appears to distort the perception of food palatability
18], 83]. Thorp and Doubt 83] recorded the perception of different foods at varying depths with results indicating
that certain foods were distorted and not accepted for consumption by divers. Milk,
for example, became warm and unpalatable, only fresh vegetables were found to maintain
their taste, and meat had to be of good quality and cooked well to maintain palatability
in the chamber. Little is known about the contributing factors to the taste and smell
distortion, although it is likely a combination of factors originating from the increased
pressure and the helium-rich environments. Further research to elucidate the contributing
factors may be warranted given the sensory perception of food has a critical role
in regulating appetite and dietary intake 84]. Along with taste and smell, sensory perception is influenced by the food texture
and aesthetics, both aspects that can be controlled during food preparation. We recommend
that the catering professionals on the vessel should be made aware of the significant
role they have in promoting the health and performance of saturation divers and be
encouraged to place emphasis on the texture and the visual presentation of the food.

Personal communication between the authors and saturation divers has also revealed
that appetite may be suppressed after an intense shift underwater. This phenomenon
warrants scientific investigation; however, the theory is plausible as participation
in physically demanding activities has been reported to transiently reduce hunger
sensations 85], representing an additional challenge to appropriate dietary intake. Due to this,
we recommend that during incidences of suppressed appetite, the availability of a
liquid based energy dense meal may be a suitable alternative.