Last week I sent a message requesting advice on current beliefs about
maximal rates of gastric emptying and intestinal absorption of fluids.
The relevant sections of all replies I received are below my signature block
in the order in which I received them.
In addition, the following is a summary of my understanding of the present
state of knowledge in this area, based on the information provided by
subscribers to Sportsci and some of the references they provided:
The importance of determining the maximal rate at which an athlete can
absorb water is illustrated by the sweat rate of 3.7 L/h reported by
Armstrong et al [Phys Sportsmed 1986;14:73-81] for Alberto Salazar at the
1984 Los Angeles Olympic Games. Other (anecdotal) reports of ~3 L/h sweat
rates suggest that Salazar may not be alone. If the commonly applied 'rule
of thumb' is right--that fluid can be absorbed only at a rate of about 1
L/h--then some athletes clearly cannot avoid a high level of hypohydration
within two hours of commencing very vigorous activity in the heat.
Although such athletes may not be able to replace all the water lost as
sweat through drinking, there are other factors of significance to hydration
status. Muscle glycogen is stored with water (which is released as glycogen
is mobilised) and the amount of metabolic water produced by the catabolism
of carbohydrate and fat (to H2O + CO2) also needs to be considered. Rogers
et al [Med Sci Sports Exerc 1997;29(11):1477-1481] found that
ultra-endurance athletes who had engaged in 21 km canoeing, 97 km cycling
and 42 km running were approximately in water balance despite drinking a
quantity of fluid equivalent to only ~80% of their sweat losses (mean value
of sweat rate was 940 mL/h).
Also, the common belief that fluid can be emptied from the stomach and
absorbed from the intestine at a rate of only about 1 L/h may be incorrect.
Gisolfi and Ryan [In: Buskirk ER, Puhl, SM, eds. Body Fluid Balance:
Exercise and Sport. Boca Raton, FL: CRC Press, 1996:19-51] discuss papers
reporting that both emptying and absorption can occur at ~2 L/h. Combined
with release of water from glycogen and the generation of metabolic water,
this should allow for water balance even if sweat rate is slightly in excess
of 2 L/h (perhaps up to ~2.5 L/h, extrapolating from the findings of Rogers
et al, 1997).
Nevertheless, Noakes [Exerc Sport Sci Rev 1993;21:297-330] states that "...
high rates of fluid intake (> 1 liter/hr) are achieved with difficulty
during exercise, especially when
running, and are likely to lead to feelings of abdominal discomfort,
possibly due to the accumulation of unabsorbed fluid in the small bowel or
colon."
It may be that the action of running leads to abdominal discomfort if large
quantities of fluid are consumed, while the same problem may not apply (or
not to the same extent) for athletes such as cyclists and canoeists. Gisolfi
and Ryan (see reference above) seem to imply that GE is not impaired during
cycling compared to resting when they state that "Cycling has not
consistently been shown to enhance GE ABOVE (my emphasis) resting values.
Data on other modes of exercise does not exist."
My (current) belief is that at least in theory, sweat rates of ~2 L/h or
even slightly more should not be incompatible with maintenance of
euhydration for several hours, through a combination of ingested fluid,
water released from glycogen mobilisation and the production of metabolic
water. This may be harder to achieve when running than when cycling. The
practicality of drinking at a rate approaching 2 L/h (from the point of view
of discomfort due to excessive stomach distension and/or accumulation of
water in the small intestine or colon) needs more research.
I would be pleased to receive constructive criticism of this (very brief)
summary.
Chris Forbes-Ewan
Task Coordinator, Nutrition
Defence Nutrition Research Centre
76 George St
SCOTTSDALE Tas 7260
AUSTRALIA
Phone: Int + 61 3 6352 6607 (03 6352 6607 in Australia)
Fax: Int + 61 3 6352 3044 (03 6352 3044 in Australia)
E-mail: [EMAIL PROTECTED]
The opinions expressed in this message are those of the author and should
not be taken to represent the official position of the Defence Science and
Technology Organisation or of the Australian Department of Defence
FIRST REPLY
You make a good point. I think that GE can reach rates higher than 1 l/h,
but don't have a reference that shows that . Indeed, Coyle and Montain
(1992) state that 1 L/h may be the limit, but I know that ultraendurance
athletes generally consume 1 to 2 L of fluid per hour depending on the
athlete's sweat rate and the temperature (Laursen and Rhodes, 1999; Clark et
al., 1992; Noakes, 1992). Personally speaking, as a former ultraendurance
triathlete, I'm pretty sure that I consumed greater than 1 l/h during
ultraendurance performance in high temperatures. Don't forget also that this
rate would also likely be affected by the physical size of the athlete, with
larger individuals likely having a larger pylorus.
Coyle EF, Montain SJ. Carbohydrate and fluid ingestion during exercise: are
there trade-offs? Med. Sci. Sports Exerc. 1992; 24(6), 671-687.
Clark, N, J Tobin, C Ellis: Feeding the ultraendurance athlete: Practical
tips and a case study. Journal of the American Dietetic Association. 1992,
92(10), 1258-63.
Noakes TD: The hyponatremia of exercise. International Journal of Sport
Nutrition. 1992, 2, 205-228.
Laursen, P.B., and E.C. Rhodes: Physiological analysis of a high-intensity
ultraendurance event. Strength Cond. J. 21(1):26-38, 1999.
SECOND REPLY
Nancy Rehrer did a good review on this in 1995 in MSSE?
The GSSI site has all the references on fluid absorption. One of the
references must reference the basic research.
FYI when I was training for Ironman, I rode in a heat chamber for 5 hours at
30C and 50-60% humidity at 270 watts on 4 occasions over 4 weeks. I measured
temperature, sweat rate, urine production and body weight. I drank 1.1 L per
hour of a 7% CHO solution with 30 mmol NaCl with a 300 ml start. I found
that in the first week my stomach was full and very uncomfortable. In the
second week it felt full but comfortable, in the third week it was fine and
in the 4 week I could have drunk a lot more.
Additionally if you are exercising you liberate H2O stored with Glycogen and
also produce metabolic water. There are equations on this. But I seem to
remember that 600g Glycogen has 1.2L of H2O and will also produce 1/2L of
water, but don't quote me on that.
So you do not need to equal sweat rates with fluid replacement (read emptied
from the stomach into your body's ECF and ICF).
Also check out the Ironman Nutrition article on the sport sci web site.
THIRD REPLY
I remember reading somewhere that elite marathon runners such
as Alberto Salazar had a sweat rate of more than 3 Liters per minute during
the
Los Angeles olympic marathon, but I cannot find the reference.
FOURTH REPLY
I've always been taught to use the 1L/h rule of thumb, but lately this has
been
shown to be inaccurate, so I've changed my thought. Teaching sport
nutrition
I've found a reference to a much higher level of gastric emptying in the
text.
In the text book, Sport Nutrition for Health and Performance, by Melinda
Manore,
they reference that rate to be approximately 40 ml per min translated to 2.4
L/h. This is referenced to Gisolfi and Ryan , 1996 (Ref follows).
Gisolfi CV, Ryan AJ. Gastrointestinal physiology during exercise. In:
Buskirk
ER, Puhl, SM, eds. Body Fluid Balance: Exercise and Sport. Boca Raton, FL:
CRC
Press, 1996:19-51.
FIFTH REPLY
I don't know the references, but I do know two of my friends who have done
triathlons and gained weight over the 3 hours.
SIXTH REPLY
Greater rates of fluid ingestion had no measurable effects on plasma volume
and osmolality and did not improve 2-h running performances in a 25 degrees
C environment: 150 or 350 mL x 70 kg(-1) body mass (approximately 130 or
300 mL) every 15-20 min, or 20 fluid ounces vs 40 fluid ounces per
hour....[Daries HN, Noakes TD, Dennis SC, Med Sci Sports Exerc 2000
Oct;32(10):1783-9.] Fluid intake of 20 fluid ounces/hour were satiating and
adequate.
I have not read anything indicating a faster fluid loss from sweat rate
than -1.2 liters/hr.
Noakes wrote: "At low sweat rates (< 1 liter/hr), it is probable that all
of the lost fluid can and should be replaced; rates of fluid ingestion
needed to offset higher sweat rates may exceed the maximum intestinal
absorptive capacity for water. Furthermore, high rates of fluid intake (> 1
liter/hr) are achieved with difficulty during exercise, especially when
running, and are likely to lead to feelings of abdominal discomfort,
possibly due to the accumulation of unabsorbed fluid in the small bowel or
colon." [Exerc Sport Sci Rev 1993;21:297-330]
Circulatory fluid shifts were studied in MIDDLE-AGED MARATHON RUNNERS (6
males and 5 females, ages 32-58 yr) during a 42.2-km marathon race run in
mild weather (dry-bulb temperature = 17.5-20.4 degrees C). Running times
for the subjects were 3:12-4:40 (mean values were 3:34 for males and 4:10
for females). Venous blood samples were taken without stasis in all
subjects seated at rest before the start of the race and within 3 min of
finishing; eight of the subjects also paused for samples at 6 and 27 km
during the race. At 6 km, body weight loss averaged less than 1%, whereas
plasma volume (PV) had decreased by 6.5% in male subjects and 8.6% in
female subjects. By the end of the race, hypohydration had reached 3.2% in
male subjects and 2.9% in female subjects, but PV in both groups remained
stable. SWEAT RATES DURING THE RACE AVERAGED 545 and 429 g X m-2 X h-1 for
male and female subjects, respectively, with ad lib. water intake replacing
21-72% of fluid loss. [J Appl Physiol 1985 Aug;59(2):559-63.]
At this point, I do not know how to rationally explain hyperthermic
exercise fluid balance by drinking in enough to replenish losses.
Other than slowing down the pace to a crawl, taking long walk breaks,
the rehydration problem during competitive exercise is not possible. If
rehydration is possible beyond the 21-72% observed, please inform me.
SEVENTH REPLY (FOLLOWUP TO SIXTH)
I have a file full of athletes who have suffered from muscle cramps, gastric
stress, from either dehydration or related hyponatremia. Their common report
include a fluid intake above 30 fluid ounces, a caloric intake above 300
calories, or a sodium intake below 300 mg each hour during hyperthermic
conditions in events lasting longer than 3 hours duration. When the fluid
intake is kept in the range of 16-30 fluid ounces, calories reduced to under
280, and sodium electrolyte increased to 300-600 mg/hour, 19 out of 20
resolve symptoms in follow-up similar conditions. Most of those who report
either hyponatremia or dehydration from any and all combinations of the
above were doing events in excess of 5 hours duration, with the bulk of them
doing triathlons in the 11 hour range or ultramarathon trail runs in the
50-100 mile range [9-29 hours].
If there is an exact position determining fluid replacement rate beyond 1
liter per hour, beyond 280 calories per hour, or one that gives sodium
electrolyte loss replacement beyond 600 mg. [2000 mg are lost per hour, but
the kidney recirculates cooperatively on less intake], I would be extremely
keen in participating in establishing reference range intake for endurance
athletes in hyperthermic exercise applications, basing the ranges on loss
vs repletion rate.
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