Hotter than ever…
July 2016 was officially the hottest month for the entire world, on record… ever! This may not have come as a surprise to anyone racing around parts of Northern and Southern Europe. However I can only comment on my personal experience. As mentioned in my previous post, I participated in the GF ‘Legendary’ Charly Gaul, Trento, Italy 10th edition…although the time trial was a qualifier for the amateur road championships, I was glad I missed this completely. My excuse: 7hrs 9minutes of 141km and 3,750m of 38 degrees average centigrade unrelenting tortuous heat. Like riding in a tarry sludge of hot pea-soup left on the hob, with added cheese. I kid you not. If anyone was on the continent crazy enough to be racing in those conditions….I admire their bravery to suffer it out.
After a week of temperatures in the high 30’s during my travels, including lugging big heavy bags around with what seemed a constant immersion in body water, my own, I had broken an important rule of race preparation….get hydrated before the start line! I rarely let school-boy errors ruin things, but sometimes even the best of us get caught out. So I’d like this post to serve as a reminder or lesson even to those who may find themselves in similar conditions (and myself).
At 8am the temperature felt like it was already well over 30C and didn’t drop below 34C for most of the day. The effect of heat on performance is well documented (Augmented supra-spinal fatigue following constant-load cycling in the heat; Scand J Med Sci Sports 2015: 25 (Suppl.1 164-172) PDF , but there seems to be some confusion on the importance and mechanisms of becoming and remaining hydrated during training and competition, especially with so many hydration products on the market.
Physiologically, severe fatigue and performance will suffer; firstly due to a drop in blood volume and then from depletion in carbohydrate. I was lucky to cross the line, many didn’t, however I salvaged a potential disaster through emergency hydration and nutrition, and would like to share with you the multiple aspects of hydration physiology and product choices we currently face, I’ll try and keep it simple and brief, yet cover the fundamentals.
Dehydrated or hypohydrated?
The main concern of staying hydrated is so that we can avoid a performance decline as well as heat-related issues, cramps, heat stroke allowing us to maximise training adaptations. A certain understanding of the science is required to appreciate the following elaboration but I expect it will be a learning experience for many. Thermoregulation, the role of the kidneys in maintaining body water homeostasis and the competition between skin (for sweating, and hence cooling) and muscle starts to get a little complex. Then there is the function of electrolytes and salts and their effect on these mechanisms, thirst, blood plasma volume and osmolality. Understanding how these mechanisms are controlled will help us develop slightly better personal strategies in achieving optimal hydration. I have listed a brief key word glossary at the bottom.
Endurance performance relies on the perfusion of muscles with water for the metabolic demands of contraction, heat dissipation and tissue nutrition. Hence blood volume is essential to maintain adequate conditions for sustained exercise. Sweating is predominantly more important as part of thermoregulation, than is blood flow to the muscles for exercise. Hence the consequential loss of blood plasma volume, and pooling of water under the skin, in turn, reduces stroke volume of the heart and therefore cardiac output. Even if water is freely available fluid loss (max sweat rates can be approx. 1.5-3.0L/hr) can exceed the rate of gastric emptying (0.8-1.3L/hr).
Around 63% of body weight in an average person is water. Any increase in adipose tissue (fat) influences total body water content. This is further partitioned roughly as 1/3 Intra-cellular fluid (ICF) and 1/3 Extracellular fluid (ECF). Where ECF accounts for both Interstitial Fluid (ISF), between organs and tissues)(3/4) and Plasma Volume (PV), excluding red blood cells) (1/4)
The main salt ions (Na+, K+, Cl- and HCO3–) other solutes and small metabolic substrate particles determine the osmolality of the fluid compartments, with little difference between ISF and PV. Water moves readily between various body fluid compartments across membranes and between cells, driven by a hydrostatic or osmotic pressure dictated by protein content of the fluid (oncotic pressure) and the gradient of the ion strength to achieve an equilibrium.
Is your body in balance?
Dehydration, in both cases, if water isn’t freely available for drinking, can be due to loss of plasma from the blood (bleeding or sweating). This causes the hypovolemic thirst mechanism to re-instate adequate balance. Or, dehydration may be due to presence of hypertonic solutions in the gut (either through food or liquid where water is drawn out from tissues and organs to balance a more concentrated salty meal or drink) which causes osmotic thirst.
Therefore plasma volume, pressure and osmolality through the control of ‘osmoreceptors’ and various hormones such as Vasopressin (ADH) and Aldosterone (an adrenal hormone) are important in regulating total body water, via mechanisms in both the gut and kidneys. These hormones affect the permeability of water (retaining water) through cells (proteins called acquaporins) in the gut lumen or collecting ducts of the kidney nephrons.
As the water gradient follows low salt (ionic) concentration to higher concentration, predominantly Na+ and Cl– the active transport of Na+ through these cells by Na+/K+ ATPase pumps these ions in different directions, extruding Na+ from the cell and into the blood plasma under ‘euvolemic’ conditions, setting up an electro chemical gradient for Cl– and HCO3– to follow increasing the osmolality of the plasma (Concentration of Na+ in the lumen (gut epithelial cells) 25mM vs plasma concentration 120mM) so water molecules follow via osmotic pressure.
Sodium ions are absorbed along the entire length of the intestine, and if the bowel contents are isotonic (same concentration of Na+) then this sodium and water are readily absorbed. Otherwise if the food contents of the gut lumen (chyme) have a higher salt concentration then water is pulled osmotically from the blood capillaries to dilute it before absorbing across the cell membrane. Also in the gut, end products of digestion such as glucose, galactose and amino acids facilitate and follow Na+ absorption and hence allow the osmotic flow of more water which ultimately lowers plasma osmolality and sodium concentration (Normal range Na+ 136 to 145mM). Dangerously low sodium levels may occur from lack of salt intake/ electrolytes in hypotonic solutions (hypotonic hyponatremia) or from loss of blood plasma volume and electrolytes (hypovolemic hyponatremia).
Introduction of a hypertonic solution into the gut decreases the water volume of both ECF and ICF (hypovolemia) and increases the osmolality in both compartments (hypertonicity). Introduction of a hypotonic solution reduces the osmolality and increases volume of both compartments. Ingestion of an isotonic solution increases the water volume in the ECF without affecting the volume in the ICF or the osmolality of either. Hypovolaemia and hypertonicity both contribute to reduced heat loss and increased heat storage. An increase in plasma osmolality stimulates the release of ADH/ Vasopressin resulting in a lower volume of urine excreted and increased desire to drink. A decrease in plasma osmolality inhibits the release of ADH resulting in a larger volume of urine excreted.
Aerobic exercise tasks are likely to be adversely effected by hypohydration with the potential affect being greater in hot conditions. Hypohydration interferes with thermoregulation as it increases heat storage by reducing sweat rate and skin flow responses for a given core temperature. Increased osmolality increases the rate of core temperature rise.
In addition to affecting the secretion of ADH, changes in the osmolality and volume or pressure of the blood, control the thirst centre of the brain. When plasma osmolality is increased, or when blood volume or pressure is decreased, the person experiences the desire to drink. If the individual has access to free water, the intake of water is increased; together with decreased renal excretion (urine) of water, the plasma osmolality and blood volume or pressure are restored to their normal values. A decrease in the plasma osmolality or an increase in the blood volume or pressure suppresses the urge to drink.
Strategy to combat hypohydration/ dehydration:
It is always best to start any exercise optimally hydrated to attenuate total body water loss during exercise. This will result in decreased heat load as more water will allow a greater sweat capacity and therefore a greater capacity to dissipate heat and supply blood to the working skeletal muscle.
Normal sports beverages focus on carbohydrate fuelling not hydration and are typically formulated to:
- Prevent dehydration
- Supply carbohydrate to increase available energy
- Provide electrolytes to replace losses via sweat
- Conform to regulations (FDA)
- Be highly palatable
Typical drinks can have a high carbohydrate content which not only might delay gastric emptying but also be hypertonic (high osmolality) and actively draw water (reverse water flux) from the body before being able to absorb its contents properly.
From the physiology text books we know that sugars, salt (sodium Na+ and chloride Cl– ions ) are needed to aid water absorption at isotonic concentrations as previously mentioned. High sodium chloride concentrations are associated with high osmolality and greater gastrointestinal distress, and are better provided in lower concentrations, but some drinks can be too low to replace salts lost via sweat (rate of Na+ loss 0.8-4.0g/hr). Sodium citrate is a good alternative to sodium chloride as it performs better in hydration, with reduced osmolality of hydration drinks, and it’s buffering capacity (similar to the action of bicarbonate) of low pH associated with muscle failure which also allows lactate to perform its functional and beneficial role.
So an optimal concentration of electrolytes, carbohydrates affects net water absorption, but the greatest absorption comes from solutions containing transportable substrates such as maltodextrin, glucose or amino acids. Ideally, a sucrose-maltodextrin (maltodextrin keeps beverage osmolality low, but affects the absorption of water due to its own breakdown) combination have the advantage of a higher carbohydrate load without impacting osmolality as much as glucose-fructose. Although the glucose-fructose combination has the highest carbohydrate solution, fructose provokes gastrointestinal distress and does not require sodium for absorption.
Thus a specific hydration beverage for before and after activity, with focus on increasing TBW/ fluid retention is an ideal strategy for maintaining optimal body water content. The electrolyte ions such as sodium and potassium, chloride are essential in rehydration. However a low concentration hypotonic drink (just under isotonic concentrations) should be enough for adequate hydration along with a glucose-maltodextrin composition just enough to facilitate optimal water absorption.
I have listed the electrolyte composition and concentration of some known electrolyte brands, and bottled/ tap water in comparison to blood plasma for an idea of electrolyte concentrations and composition. RA = the concentration of the recommended administration, or diluted stock solution. Although more in depth research would be warranted to assign merit on their individual benefit for optimal hydration.
Looking holistically at all of the info above, it is not hard to see how sufficient hydration may be missed if athletes are combining both carbohydrate intake (hypertonic solutions) along with inadequate water absorption, without effective electrolyte concentrations to meet hypohydrational demands or worse, adressing dehydration before and during exercise. It may well be that your current product has the necessary electrolyte, but you may not be taking it optimally, or not with enough carbohydrate, or too much, or the wrong type.
Taking calories from food is always the best option, as there is usually an adequate salt composition, when combined with an isotonically optimal drink should be a sustainable hydration strategy. Training the gut to process food and compatible osmolality drinks will make sure calorie intake are sufficient and thremoregulation is optimal.
As always, I endeavour to provide enough sound information for my clients and readers to make the right choices. Ultimately everyone wants to avoid disasters on race day and get the result they deserve. Thanks for reading my ‘school boy lesson’, and hope you have gained some new information on this hot topic, to avoid making the same mistake as myself, and perhaps as few others have done during the hottest ever July!
Wishing you safe riding!
GLOSSARY OF TERMS
Dehydration – also known as hypohydration can be defined by not enough body water (water loss exceeds intake), due to the loss of water (free water or with salt). This can cause hypernatraemia and hypovolaemia which results in the disruption of metabolic processes. In people over age 50, the body’s thirst sensation diminishes and continues diminishing with age. With exercise, exposure to hot environments, or a decreased thirst response, additional water may be required. An accurate determination of fluid volume lost during a workout can be made by performing weight measurements before and after a typical exercise session. A sweat test may be worthwhile to estimate loss of salt during exercise which is important for normal cellular function. In extreme cases, the losses may be great enough to exceed the body’s ability to absorb water from the gastrointestinal tract; in these cases, it is not possible to drink enough water to stay hydrated, and the only way to avoid dehydration is to either pre-hydrate. Resolved by adequate oral hydration with adequate electrolytes. When large amounts of water are being lost through perspiration and concurrently replaced by drinking, maintaining proper electrolyte balance becomes an issue.
Euvolemia – The presence of a normal amount of total body water.
Hydration – The supply and retention of adequate water in tissues and organs to maintain optimal total body water content.
Hyponatraemia – A term used to define low blood plasma Sodium concentration. Sodium levels can fall dangerously low (below 135mEq/L) in patients who eat a low-sodium diet and drink too much (low solute) water. Severe and prolonged diarrhea also can cause low sodium levels. In any case when sodium levels drop too low, can cause seizures or coma which needs to be treated as a medical emergency.
Hypovolaemia – a state of reduced volume of blood plasma due to blood loss or loss of fluids usually caused by dehydration. This can be hypo-, iso- or hyper-natraemic dehydration.
Hypertonicity – a hypertonic solution is one with a higher concentration of solutes outside the cell than inside the cell. When a cell is immersed into a hypertonic solution, the tendency is for water to flow out of the cell in order to balance the concentration of the solutes. (a fluid with a higher concentration of solutes than the remainder of the body).
Hypernatraemia – A term used to define low blood plasma Sodium concentration. If the amount of water ingested consistently falls below the amount of water lost, the plasma sodium level will begin to rise, leading to hypernatremia Severe symptoms are usually due to acute elevation of the plasma sodium concentration to above 157 mEq/L (normal blood levels are generally about 135-145 mEq/L for adults and elderly).
Osmosis- is the spontaneous net movement of solvent molecules through a semi-permeable membrane into a region of higher solute concentration, in the direction that tends to equalise the solute concentrations on the two sides.
Thermoregulation– The internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism’s internal conditions. Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. Most body heat is generated in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles when the surrounding temperature is higher than the skin temperature, anything that prevents adequate evaporation will cause the internal body temperature to rise. During sporting activities, evaporation becomes the main avenue of heat loss. Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss.
Total Body Water (TBW) – Is the water content contained in the tissues, the blood, the bones and elsewhere. This water makes up a significant fraction of the human body, both by weight and by volume. Water constitutes more than 55% of the total body weight. Body fat relative mass directly influences total body water. This explains the influence of age, gender and aerobic fitness on total body water.
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