Optimizing Performance in the Heat II: Sodium and Sweat
With any topic that involves heat regulation, hydration is part and parcel. On one side of the road, the endurance community has a "fluid plan" where the aim is to prevent dehydration, the dreaded 2%!, while on the other side of the road, "drink to thirst" (of Waterlogged by Tim Noakes ...Hey Tim! BIG fan! Remember me from your one lecture at Wits University where you spoke about your new book "Challenging Beliefs" and signed my copy! See photo at the end of the blog) has also become an equally popular approach.
This blog post explores the relationship between fluid and sodium and offers an introduction to the topic of hydration. It is not intended to be a comprehensive exercise physiology textbook chapter but a starting point that may be further developed in future posts. A Hors d'oeuvre!
But first,
in the previous blog post here. we had the following summary:
The hypothalamus regulates heat and cold responses in the body
The anterior hypothalamus regulates heat response
The posterior hypothalamus regulates cold response
The set point of temperature in the hypothalamus is 37 degrees centigrade
1ml of sweat loss is equivalent to 0.58kcal of energy
Optimal temperatures for road racing are between 10-30 degrees centigrade
Pre-cooling vests have a significant performance-enhancing effect for temperatures above 27 degrees centigrade
Questions we will answer:
1.Fluid Loss and thermoregulation
2.Function, role, concentration of sodium and sweat rates
3.Role of estrogen and progesterone in plasma volume and sodium regulation in Female Athletes
4.Summary
Fluid loss and thermoregulation
In 2007, Sawka and colleagues released a position statement indicating that a 2% body weight loss in fluid could compromise an athlete's performance, emphasizing the importance of drinking to prevent dehydration.
Several years later, Tim Noakes joined the conversation not convinced that maintaining fluid was the answer through his publication Waterlogged (2012), which centered on hyponatremia. Noakes supported a strategy of "drinking to thirst," proposing that athletes should prioritize avoiding hyponatremia rather than fixating on the amount of weight they lose. Hyponatremia is characterized by low levels of sodium in the blood, leading to an increase in the body's water content and subsequent cell swelling. This swelling can give rise to various health issues such as confusion, seizures, and even in some cases coma.
It would seem that somewhere between fluid balance and salt intake is potentially a key idea.
Heat Stress accumulation
Maintaining blood volume is a key determinant of performance. In contrast, maintaining carbohydrate intake is a second key determinant as the replacement of glucose in the blood can be solved within a few minutes, whereas replacing blood volume can take hours to fix.
Blood volume is a combination of red blood cells (hematocrit) and plasma (see figure 2 above), being the fluid component of blood. During exercise, both the muscle and skin compete for blood flow, with muscle blood flow being critical for the delivery of energy substrate to be used in the production of ATP (muscle contraction currency). Skin blood flow is used to dump heat created from muscle contractions (via sweating). As body-water drops, the competition between these two processes becomes problematic as one will win out over the other. Blood flow to the skin does decrease in favor of distribution to the working muscles which increases heat production and potentially heat illness.
Remember our set point is around 38 degrees and there are two levels that the body goes through with heat stress where performance begins to decline with the first being muscle temperature over this 38 degrees centigrade where proteins begin to break down and the second point being 40-41 degrees centigrade in the core temperature when the central nervous system starts to limit performance output.
Thus, physiological strain comes in the form of hot skin (less sweating), less sweating from a drop in blood volume (and blood distribution), and an increase in core temperature because of reduced thermal offload, and why fluid replacement is a serious consideration to maintaining performance.
Sodium
The main electrolyte found in the extracellular fluid is sodium. The high sodium concentration is what gives the fluid its characteristic 'salty' taste, and the amount of extracellular fluid in the body is closely connected to the levels of sodium. Higher sodium levels lead to increased fluid volume, whereas lower sodium levels cause a decrease in fluid volume.
During exercise, sodium content in sweat can range from 250 to 2,000 mg/l. Sweat rates vary among individuals and can fluctuate depending on conditions, with cooler environments seeing much less sweat rate and hot environments being in the liters per hour. This difference in sodium concentration and sweat rates can result in significant variations in total net sodium losses among athletes. Due to the individual nature of sweat and sodium losses, it's important to tailor a program of fluid intake that matches the athlete's individual variation in this sweat/sodium loss equation.
The main factors influencing personal net sodium losses include your total sweat volume, which is influenced by your sweat rate and the duration of sweating, and your sweat sodium concentration, which indicates the amount of salt lost in your sweat. Below is a description of determining your sweat rate per hour. However, your sweat sodium concentration is largely genetically determined and remains relatively constant.
The point where fluid and sodium loss becomes an issue for athletes isn't precisely known, but knowing sweat rate as well as potentially getting sodium concentration of sweat measured (knowing its relatively constant) allows the creation of a specific protocol tailored to the exact needs of the athlete. See the table below for concentrations of minerals in sweat (Barnes et al. 2019).
Blood and Sweat mineral concentrations
Body Fluid Concentration (L) | ||
Mineral | Blood | Sweat |
Calcium | 85-105mg | 0-40mg |
Chloride | 3.4g | 0.7-1.2g |
Magnesium | 16-30mg (not red blood cell) | 4-15mg |
Potassium | 130-220mg | 160-320mg |
Phosphorus | 20-50mg | 3-6mg |
Sodium | 3-3.5g | 0.46-0.1.84g |
Determining Sweat Rate
Below is taken from Asker Jeukendrup and his research:
Calculate Sweat Rate (can be done in a 45-60min session) |
|
|
|
|
|
|
|
C+Z divided by time = liters/hour |
Tadej weighs 80kg before starting the session. After completing his 1hour session, he weight 78kg. 2kg weight loss He drinks two bottles of 500ml. 1000ml= 1kg Therefore total weight loss is 3kg in 60mins or 3liters/hour |
The average sweat rates of athletes from Baker. 2017
Sodium Concentrations
The last piece of the puzzle is to figure out the sodium concentration of the sweat. As mentioned above, once you have determined this rate, it is fixed and does not change regardless of the acclimatization of the athlete. So sweat rate can vary quite a bit because of the weather and temperature, however, salt concentration tends to stay fixed at a specific X mg per Liter for an athlete. Determining your sweat salt concentration requires specialized testing and collection of a sweat sample.
Sodium Concentrations | ||
LOW | Medium | High |
<0.5 grams/L | 0.5-1.3 grams/L | >1.3 grams/L |
How to determine if you are a medium to high sodium sweater without specialized testing:
Salt marks on your clothing after a session
Sweat tastes salty and tends to sting your eyes during a session
Above-average loss of sodium can result in muscle cramping or predisposition (Schwellnus Theory is a topic for another blog)
Head Rush after standing up too quickly after an exercise session (blood volume and sodium loss impact whats called orthostatic hypotension)
Craving very salty food after exercise
Not feeling great after exercise in the heat where large sweat rates are achieved
Female Athletes
A special mention goes out to female athletes who because of progesterone and estrogen, have different challenges when it comes to fluid and sodium balance
Estrogen
From day 5 till around day 12, estrogen production increases and impacts anti durectic hormone or ADH (vasopressin hormone) which is responsible for retaining water which impact and alter blood plasma volume and osmolality (or how much sodium is dissolved in the fluid portion of blood)
Progesterone
Is released in the second half of the menstrual cycle and inhibits the receptor in the kidneys to aldosterone which helps to retain sodium in the kidneys and the result is more sodium loss through the kidneys during this phase
Excessive sodium loss can pose challenges to the athletic performance of women as there is a strong relationship between blood sodium levels and blood plasma volume. Inadequate total-body sodium can cause a decrease in blood plasma volume leading to reduced cardiovascular pressures and an increased heart rate as a compensatory mechanism during physical activity.
A decrease in plasma volume also impacts the body's ability to regulate temperature (see discussion above). With less available fluid for sweating, there is a more significant shift in core body temperature during exercise.
In the 2007 position statement by Sawka et al. cited above, they felt that there were no differences in renal water and electrolyte retention and not consequential.
F(luid)ood for thought!
Summary
Sodium is the largest electrolyte outside the cell
Maintaining blood volume is a key determinant of performance (then comes carbohydrate intake)
Two schools of thought: Maintain fluid balance and avoid dehydration (ACSM)
Or "drink to feeL" advocated by Professor Tim Noakes
Sweat rates are highly variable in athletes are respond to environmental conditions ie higher sweat rates for higher temperatures
Sodium concentration in sweat stays relatively the same in an athlete and is largely genetically determined
Sodium in sweat can range from 250mg to 2000mg/Liter
Female menstrual cycle can impact sodium and fluid loss and retention depending on where in the cycle the athlete is.
Till next blog...
Train Hard and Prosper!
Darrin Jordaan
MSc (Med) Biokinetics WITS
HMS (Hons) Sports Science UP
BK 0016934
CSCS
UCI Level 1 Cycle Coach
IronMan certified coach
References
Sawka, M. Burke E. Eichner, R. Maughan, S. Montain, S. Stachenfeld, S. 2007. American College of Sports Medicine Position Stand. Exercise and Fluid Replacement. Medicine and Science in Sports and Exercise, 39: 377-390
Noakes, T. 2012. Waterlogged: The serious problem of overhydration in endurance sports. Champaign, IL: Human Kinetics.
Cheung, S. Advanced Environmental Exercise Physiology. Champaign, IL: Human Kinetics.
Barnes KA, Anderson ML, Stofan JR, Dalrymple KJ, Reimel AJ, Roberts TJ, Randell RK, Ungaro CT, Baker LB. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. Journal of Sports Science. October 2019, volume 37, issue 20, pages 2356-2366.
L, Baker. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine. Volume 47 Pages 111-129. 2017.
Comments