Dr. Iñigo San Millán on Hydration
Sweating is the physiological process of dissipating heat in response to an increase in our body’s core temperature. As blood flow to the skin is increased, the sweating process begins. However, as we sweat, and thermoregulation begins, it can result in excessive fluid and electrolyte loss. This leads to dehydration and a drastic decrease in our ability to pedal a bike.
Decreases in performance starts with a sweat loss of about 2% of body weight. Sweating that results in a 5% loss of body weight can decrease performance up to 30%. When in competition, or exercising at a high intensity a mere 2.5% loss in body weight can decrease performance by about 45%.
When dehydration starts and it is not corrected, things get worse, fast. A chain of events begins to unfold: As you become dehydrated there is a decrease in blood volume in your body, along with a decreased cardiac output (your heart’s capacity to pump blood), then a decreased skin blood flow, which decreases the rate you can sweat, which in turn decreases your ability to dissipate heat and ultimately results in a further increased core temperature. Short answer: When you’re dehydrated you get hotter and hotter until you overheat.
When cycling you sweat at a rate of 0.7-1.3 Liters/hour under heated conditions, so it is obvious that we need to control fluid replacements in the heat as much as possible.
Electrolytes. When it comes to dehydration it is not just that we lose body water; we also lose electrolytes, which are crucial for the neuro-electrical stimulation of muscle contraction and other important physiological processes during exercise. Simply stated, electrolytes make your nerves conductive and are responsible for making your muscles contract. A decrease in electrolytes due to sweat sweat can decrease the capacity of muscles to contract. Muscle cramping is one of the most common signs of a decrease in electrolytes.
There are three main types of electrolytes: Sodium, Chloride, Potassium and Magnesium. Sodium is probably the most important one. Sodium is key for electrical activity and signaling in the nervous system as well as it is necessary for muscle contraction. Average Sodium loss per each liter of sweat is about 900mg (or 0.9g) so if we lose about 3L (6.6Lbs) in a 3h race or training ride we will be losing about 2,700mg of sodium. At this much loss, performance is drastically affected.
Lower than normal sodium levels results in a condition called hyponatremia, which along with decreasing your performance, can also be dangerous and even life threatening in certain extreme conditions. This is why replacing lost electrolytes is crucial. When we drink water we only replace fluid. We do not replace electrolytes lost from sweat. Someone only drinking water will be at a clear performance disadvantage over someone who is replacing electrolytes correctly with a hydration mix.
In some very hot conditions drinking too much plain water can be dangerous because it dilutes the electrolyte content in the blood, mainly sodium, exacerbating hyponatremia into a dangerous condition. Every year too many athletes are taking to hospitals after endurance events because they’re suffering from hyponatremia. A few have even died simply from drinking too much water (also called water intoxication) and diluting the sodium content of their blood, thus exacerbating their hyponatremia.
Performance Benefits: Water vs Electrolyte Fluid. When it comes to performance, the benefits of electrolyte fluid replacement vs plain water are loud and clear. Take 2 cyclists with the same sweat rate, using different hydration methods, but drinking the same amount of fluids. During a 3h race, cyclist A chooses a traditional/classic approach of mixing water with sports drinks so he drinks about 6 bottles of water and 4 bottles of sports drinks during the race. Cyclist B however, drinks 9 bottles of sports drinks and 1 bottle of plain water. If both lose 3L of fluids during the race cyclist A would be losing about 2,700mg of sodium and only replacing with about 750mg. Meanwhile cyclist B would be replacing about 2,250mg of sodium -- assuming they both drink the same sports drink brand.
Clearly cyclist A won’t be replenishing enough sodium and other electrolytes and will be at an drastic disadvantage, even though he consumed the same amount of fluid as cyclist A. When exercising in hot environments, a recommendation of about 500-700mg (0.5-0.7g) of sodium per each liter should be considered. During extremely hot conditions, even the electrolytes contained in your regular sports drink may not be enough to replace the amount of electrolytes you lose in your sweat. In these cases use extra electrolytes. In these situations it is great to use hydration powders and tablets. They let you control the amount of sodium and electrolytes you add to your water. One scoop of Clif Shot Electrolyte powder has 200mg of sodium and Cytomax has 283mg of sodium in a single scoop. Carry the single serving packets of powder or electrolyte tablets with you and add them to your sports drink when you refill. 1 pouch of powder should be enough. This is usually what we do with professional cyclists at the Tour de France, and it works quite well.
How to Calculate Sweat Rate. Weigh yourself without clothes on before your exercise and then afterwards weigh yourself again without clothes on. Record the duration of exercise, the amount of fluid you consumed and number of times you urinated while exercising (approximately 0.5L/urination). Then use this equation to calculate Sweat Rate:(Before weight - After weight) + Fluid consumed during exercise - Urination = Sweat Loss/Time Example: -Pre Ride weight: 70Kg (154Lbs) -Post Ridee Weight: 66.5Kg (149Lbs) - Fluid consumed during exercise: 1000ml (1L) - Urination times: 1time (500ml Approx) - Ride duration: 4h - Fluid Deficit : 70Kg-66.5Kg= 3.5Liters - Total Sweat Loss: 3.5L+1L-0.5L= 4L - Sweat rate: 4L/4h= 1L/h
Carbohydrates. When it comes to fluid replacement it is not just about hydration and electrolyte replacement. It is also about replacing carbohydrates. Fluids are a great way to replace carbohydrates during exercise. A combination of solid and liquid forms of carbohydrates are ideal during a cycling race. It is very difficult to eat all the carbohydrates you need during a race or ride. For this reason your sports drink is a great vehicle for carbohydrate delivery. Going back to the above example of the two cyclists, cyclist A would be replacing about 100g of carbohydrates through his sports drink whereas cyclist B would be replacing about 225g of carbohydrates (assuming they both use the same brand). So like in the electrolyte example, when it comes to performance, cyclist A will again be at a disadvantage compared to cyclist B.
Timing. It is recommended to drink 300-600ml during the hour before exercise. Then the recommended intake during exercise should be about 150-200ml ( 5-7fl) every 10-15 minutes although tolerance and consumption may vary depending on exercise intensity and weather. After exercise it is recommended to replace the amount of fluid lost (body weight) within the first 2 hours after exercise. Many sports bars, gels and chews have electrolytes and are very helpful in replacing the electrolytes lost in your sweat. However when it comes to pure hydration, sports drinks are a much better option.If you have any questions about hydration, e-mail us at firstname.lastname@example.org.