by Janet L. Simpson M.S., R.D.
Sports Nutritionist
Hydration, Heat and Humidity
The summer days are upon us! Extra daylight,
extra training time…and extra heat and
humidity! Some of us are training for our
first triathlon; others have already completed
an Ironman and continue training because it is
simply “in our blood.”
The heat and humidity offers special
challenges whether we are training or
competing in an event; it may negatively
impact your ability and if you are not
acclimatized to the heat-can become dangerous.
In moderate summer heat of 80 degrees, or even
70 degrees in high humidity, our bodies cannot
cope. Signs of this are heat cramps, heat
exhaustion and even heat stroke! Heat injuries
are caused by three factors: loss of body
fluids, loss of electrolytes and increase core
temperature.
Ellen Coleman, RD, MA, MPH,
CSSD, in her book Eating for Endurance,
explains:
“Heat cramps, or
voluntary muscle spasms, occur during
prolonged exercise when there has been profuse
and repeated sweating. Heat cramps are
probably caused by an imbalance of the body’s
fluids and electrolyte concentrations.
Treatment: Rest, drink fluids containing
electrolytes such as sports drinks, and add
salt to foods.
Heat exhaustion is probably caused by a
reduced blood volume due to excessive
sweating. Blood pools in the extremities and
the person may faint or feel dizzy. The
symptoms of heat exhaustion also include
nausea and fatigue. Although sweating may be
reduced, the rectal temperature is not
elevated to dangerous levels-less than 104
degrees. Treatment: Rest in a cool place
and drink fluids containing electrolytes.
Medical attention may be required.
Heat stroke is a medical emergency
requiring immediate action. The body’s
temperature-regulating processes simply stop
functioning. Sweating usually stops and the
skin becomes dry and hot. The rectal
temperature is excessively high-over
105.8 degrees F. Other symptoms include:
disorientation, vomiting, headache, and loss
of consciousness. If untreated, death occurs
due to circulatory collapse and central
nervous system damage. Treatment:
Aggressive steps must be taken to immediately
lower the elevated body temperature. Until
medical help arrives, the person can be
covered with ice-packs, immersed in cold
water, or rubbed with alcohol.
Be aware of the symptoms of impending heat
illness. These include weakness, chills, goose
pimples on the chest and upper arms, nausea,
headaches, faintness, disorientation, muscle
cramping, and cessation of sweating.
Continuing to exercise when experiencing any
of these symptoms can lead to a heat injury.”
THANKS ELLEN!
The risk of heat related illness may also be
increased by alcohol, caffeine, and some
prescription or over-the-counter drugs. The
issue of heat + humidity = sweat rolls off
your skin rather than cooling you through
evaporation. Heat, instead, builds up and your
core temperature increases. A five-six percent
loss of body weight increases the risk of a
heat-related illness considerably.
The heat index is as important to consider as
the temperature itself.
Remember! The heat index is an index that
combines air temperature and relative humidity
to determine an apparent temperature; how hot
it actually feels. ...(wikipedia)
At an apparent temperature of:
90-104 F Heat cramps or heat exhaustion
possible
105-130 F Heat cramps or heat exhaustion
likely, heat stroke possible
130 F Heat stroke likely.
To calculate it yourself, the formula is:
Or use this chart:
Heat Index Chart (Temperature & Relative
Humidity)
RH
(%) Temperature (° F)
90 91 92 93 94 95 96 97 98 99 100 101
102 103 104 105
90 119 123 128 132 137 141 146 152 157
163 168 174 180 186 193 199
85 115 119 123 127 132 136 141 145 150
155 161 166 172 178 184 190
80 112 115 119 123 127 131 135 140 144
149 154 159 164 169 175 180
75 109 112 115 119 122 126 130 134 138
143 147 152 156 161 166 171
70 106 109 112 115 118 122 125 129 133
137 141 145 149 154 158 163
65 103 106 108 111 114 117 121 124 127
131 135 139 143 147 151 155
60 100 103 105 108 111 114 116 120 123
126 129 133 136 140 144 148
55 98 100 103 105 107 110 113 115 118
121 124 127 131 134 137 141
50 96 98 100 102 104 107 109 112 114 117
119 122 125 128 131 135
45 94 96 98 100 102 104 106 108 110 113
115 118 120 123 126 129
40 92 94 96 97 99 101 103 105 107 109
111 113 116 118 121 123
35 91 92 94 95 97 98 100 102 104 106 107
109 112 114 116 118
30 89 90 92 93 95 96 98 99 101 102 104
106 108 110 112 114
Note: Exposure to full sunshine can
increase HI values by up to 15° F |
So…what to do???
The American College of Sports
Medicine (ACSM) recommends that athletes drink
two cups of fluid two hours before the event.
This gives the fluid time to be absorbed and
excrete excess water. Your urine should be a
light yellow or clear color.
For running and biking, if you
lose two percent or less of your body weight,
it is considered safe to rehydrate after the
event. For over two percent, fluids and
electrolytes need to be replaced during the
event! The amount depends on how much you have
lost and your gastric emptying rate. About six
ounces of fluid every 15 minutes typically
works well for most athletes.
It is difficult to determine
the amount of sodium lost in sweat. Ellen
Coleman recommends:
“As a general guide, I
estimate that endurance athletes lose about 1
gram of sodium per quart (2 lbs) of sweat. If
the athlete loses 2 lbs of sweat per hour,
then consuming one quart of a sports drink
that contains a higher amount of sodium should
adequately replace both sweat sodium and fluid
losses.”
You may also want to choose a
sports drink that contains a small amount of
potassium. (I like Gatorade Endurance, but
that is ONLY a personal preference.)
An athlete would also do well
to determine their sweat rate in an exercise
intensity and approximate heat and humidity
that will simulate competition.
What about swimming? Is dehydration an
issue? Absolutely!
Bonnie Modugno,
MS,RD reports:
“I worked with a swimmer who I
suspected was dehydrating during her
workouts. She couldn't fathom sweating in the
water so we did the
simple experiment. She weighed herself before;
jumped in the pool, and then after the workout
another weigh in. She lost 2-3 pounds each
workout. Her coach started to leave water
bottles at the end of the
pool for her to hydrate with. Her energy,
stamina and times all
improved.”
For the IM swim, ACSM
recommends that the athlete drink 1 oz of
fluid 4 hours before the swim. This will give
adequate time to excrete any excess fluid. If
you do not want to wake up 4 hours earlier,
then two hours will work…it is easier to
eliminate the excess fluid during the swim
Thanks again, Ellen!
Val Richardson, my swim coach
adds:
“We do actually sweat in the
water but the pores are clogged with water and
harder to release. More than 50% of our heat
is penetrating off the top of our heads. Take
the cap off, especially if it is silicone. Get
out of your wetsuit. The body needs to
breathe.”
Thanks Val!
The opposite and just as
serious as being dehydrated is hyponatremia.
Hyponatremia is defined as:
“The normal concentration of sodium in the
blood plasma is 136-145 mM. Hyponatremia
occurs when sodium falls below 130 mM. Plasma
sodium levels of 125 mM or less are dangerous
and can result in seizures and coma.” (Health
A-Z)
The symptoms of hyponatremia
begin with confusion, and then progress to a
headache, unconsciousness, coma and possibly
death. It occurs from drinking excessive
amounts fluid with low sodium levels. ACSM
states: “inclusion of sodium (0.5-0.7 grams
per liter of fluid) in rehydration solution
ingested during exercise lasting longer than
one hour is recommended since it may be
advantageous in promoting fluid retention, and
possibly preventing hyponatremia in certain
individuals who drink excessive quantities of
fluid.”
Hope this is helpful as you
get out this summer to swim, bike and run.
Questions? I welcome you to
contact me at:
janet@nourishingathletes.com
Check out more info on my website
www.nourishingathletes.com
See you in the water, with our
bikes and on the run!
Eat well and play hard,
Janet
