Slurpees for Science

We are getting some hot weather, which isn't a big deal because as it's summer holidays I do most of my workouts in the morning. But Thursday is run group, and run group is at 5:30 p.m. As in the hottest part of the day.

Last week I really struggled for the first half of the run, although I know my body was trying to cool itself because I was super sweaty betty. Dripping. Absolutely soaked. Then I remembered an article going around last year about "ice slurry pre-cooling" (a PhD thesis by Rodney Seigel from Edith Cowan University in Australia) and thought I'd do a little experiment.

Here is the abstract from the study: (you can scroll down, I summarize it)

The rise in body core, skin and muscle temperatures associated with exercise

in hot environments (~30°C and above) is known to impair performance over a

variety of exercise modes and durations. Precooling has become a popular strategy to

combat this impairment, as evidence has shown it to be an effective method for

lowering pre-exercise core temperature, increasing heat storage capacity and

improving exercise performance in the heat. To date, the majority of precooling

manoeuvres are achieved via external means, such as cold water immersion and the

application of cooling garments; methods which have been criticised for their lack of

practicality for use in major sporting competitions. However, recent evidence has

shown that internal or endogenous cooling methods, such as drinking cold fluids, are

able to lower core temperature and enhance endurance performance in the heat. This

method may be more advantageous than current forms of precooling, as ingesting

cold fluids is easily implemented in the field and provides the additional benefit of

hydrating athletes. Based on the law of enthalphy of fusion, which states that a

significantly greater amount of heat absorption is required for water to change phase

from solid to liquid (melt), the ingestion of an ice slurry mixture may be a more

powerful means for lowering pre-exercise core temperature. Therefore, the primary

focus of this PhD thesis was to determine the effectiveness of ice slurry ingestion as

a precooling manoeuvre for improving submaximal exercise performance in the heat,

as well as investigate the potential mechanisms behind the improvements observed.

Study 1 of this thesis was aimed at determining whether ice slurry ingestion

was able to significantly lower pre-exercise core temperature and increase

submaximal run time to exhaustion in the heat compared with the ingestion of a cold

(4°C) fluid. The results showed that ice slurry ingestion significantly reduced rectal

temperature compared with cold water ingestion (0.66 ± 0.14°C vs. 0.25 ± 0.09°C; P

= 0.001), and remained lower for the first 30 min of exercise. Running time was

longer (P = 0.001) after ice slurry (50.2 ± 8.5 min) versus cold water (40.7 ± 7.2

min) ingestion. During exercise, mean skin temperature (P = 0.992), heart rate (P =

0.122) and sweat rate (P = 0.242) were all similar between conditions; however,

mean ratings of thermal sensation (P = 0.001) and perceived exertion (P = 0.022)

were lower following ice slurry ingestion. An unexpected finding from this study

was that at exhaustion, rectal temperature was higher (0.31 ± 0.11°C; P = 0.001)

with ice slurry versus cold water ingestion. It was speculated that this may have been

due to the influence of ice slurry ingestion on lowering brain temperature or in

altering thermoreception.

After showing that ice slurry ingestion was an effective precooling

manoeuvre for improving endurance performance in the heat, Study 2 was conducted

to compare this method with the current “gold standard” method of cold water

immersion. Despite rectal (P = 0.001) and skin temperatures (P = 0.009), as well as

heart rate (P = 0.018) and sweat rate (P = 0.019) being significantly lower following

cold water immersion, ratings of thermal sensation (P = 0.750) and perceived

exertion (P = 0.278) were not different, and run times to exhaustion were similar

between conditions (CWI: 56.8 ± 5.6 min vs. ICE: 52.7 ± 8.4 min; P = 0.355).

Additionally, the result of a higher rectal temperature at the point of exercise

termination following ice slurry ingestion (0.28°C) was replicated. These findings

indicate that ice slurry ingestion is a comparable form of precooling to cold water

immersion, and provided further evidence that ice slurry ingestion may enhance

performance via thermoreceptive/sensory mechanisms.

As a result of the findings showing that the ice slurry precooling method was

consistently associated with higher end point rectal temperatures in Studies 1 and 2,

Study 3 was performed to determine whether ingesting a small bolus of ice slurry

(1.25 g·kg-1) was able to increase maximal voluntary isometric contraction (MVC)

torque under conditions of heat strain. The results showed that following exerciseinduced

hyperthermia, ice slurry ingestion significantly increased mean torque

production during a 2-min sustained MVC of the elbow flexors, compared with the

ingestion of 40°C fluid (30.75 ± 16.40 vs. 28.69 ± 14.88 Nm; P = 0.001). This was

despite run times to exhaustion (P = 0.530), end rectal (P = 0.934) and skin

temperatures (P = 0.922) as well as heart rate (P = 0.830) being similar between

trials. The mechanisms responsible for this improvement with ice slurry ingestion

may therefore be an adjustment in afferent feedback relayed from internal

thermoreceptors pertaining to the thermal state of the body, and/or

activation/suppression of brain regions associated with reward, pleasure, motivation

or fatigue.

The main findings from this PhD thesis were that ice slurry ingestion was an

effective, practical precooling manoeuvre for prolonging submaximal running time

in the heat, and comparable to the current “gold standard” cold water immersion

method. Furthermore, ice slurry ingestion was able to prolong running time in the

heat by increasing the rectal temperature tolerable before exercise termination.

Finally, ice slurry ingestion may enhance exercise performance in conditions of heat

strain via thermoreceptive/sensory mechanisms. Due to its’ practicality for use in the

field, ice slurry ingestion may be a more preferred form of precooling than

traditionally used strategies.

I made it small because it's really boring and wordy. It basically says that when it's hot, run performance increases when athletes can stay cool. Duh. They claim that ice water isn't cold enough to make a difference to core temperature, so they experimented with internal pre-cooling by having athletes consume an ice slurry before their workout. They found, through several methods - including rectal temperature measurements, which I have no intention of duplicating - that yep, runners stayed cooler and ran longer when they used this form of pre-cooling.

My experiment consisted simply of drinking a slurpee on my way to run group. I like slurpees, but don't drink a lot of them, so I was eager to take part in my informal study. I didn't read their whole thesis to see if there was a recommended dosage (i.e. mL to body weight ratio?), I simply bought a small coke slurpee and drank most of it. I didn't finish it because I was sick of it by the time I got about 3/4 of the way through.

The test subject.

Did I run better? I'm not sure... ah, science can be so inexact. I'd probably have to replicate this study many times, using more research subjects; I wonder if I can get a grant for that? I had an ok run, still super sweaty betty though. I did feel a bit nauseous at the start, probably because I had a stomach full of coke syrup. And I burped a few times. I did have a lot of kick at the end, maybe because of all the sugar? I don't have conclusive results; as I mentioned I skipped the before and after rectal temperature probe. Ha.

In the future, I think I'd rather have a glass of ice water before a hot run. Or maybe a snow cone, perhaps I should try this next week with a snow cone...