As one of the three important Fuel and Hydration, sweat loss, and more specifically, the replacement of fluids, is an incredibly important aspect of endurance racing, and is strongly correlated to race day survival, performance and recovery. The other two aspects, fuel (primarily carbohydrates) and electrolytes (primarily sodium) are the other two components. Without any one of the three, long distance and endurance events, or races, become considerably more challenging, and the chances of you finishing, drastically diminish.
Understanding how much fluid you have to consume during an event is determined by a variety of factors and variables. A few of the variables:
Time of Day, Heat, Humidity, Diet, and Effort. In addition, your sweat rate is unique and may also change throughout your life. Unlike your relative sodium concentration, which generally is constant throughout your life and in the majority of circumstances.
Understanding your sweat rate in various conditions helps you understand what your hydration needs are, and what you strategy needs to be during endurance events. Generally you want to replace at least 50 – 60% of sweat loss, and you never want your body weight to drop more than 2 or 3%, which indicates dehydration and has proven to deteriorate your performance. Anything over 5% can lead to dizziness, headaches and severe fatigue. And anything over 10% can result in death! 🙁
There the first step of this puzzle is to calculate your sweat rate, the easiest way is by weighing yourself before and after your exercise, track the temperature and start to build a small chart/database/table of your typical sweat loss, and convert that to a rate. Below is one of the tables I have been using to keep track of this. From this data and for cycling, I typically estimate around 1.2 liters per hour sweat loss and do my best to replace ~ 80% during “regular” weather events where my power output is at or above 70%. This is around 950ml per hour. As my expected power output increases, decreases, or the temperature changes, I can adjust accordingly.
| Date | Avg Power (Unnormalized) | LPH | Indoor Temp | Intensity | Exercise Length | Liquids Consumed | Notes |
| 6/23/21 | 199W | 1.87 | 80 | 60 | 0.67 | High Humidity, no fan, Medium to High intensity effort for full time | |
| 6/24/21 | 188W | 1.5 | 80 | 60 | ~0.6 | Low Humidity, fan and AC | |
| 6/25/21 | 158W | 1.2 | 80 | 60 | ~0.6 | Low Humidity, fan and AC | |
| 6/26/21 | 107W | 1.04 | 78 | 90 | 0.7 | Outdoors | |
| 6/28/21 | 170W | 2.0 | 80 | 40 | 0.63 | High Humidity, no fan, Medium to High intensity effort for full time | |
| 6/30/21 | 186W | 1.58 | 80 | 55 | 0.65 | High humidity, fan on | |
| 7/1/21 | 202W | 1.7 | 80 | 55 | 0.65 | As above. Fan on 2. | |
| 7/5/21 | 181W | 1.7 | 80 | 55 | 0.67 | As above. Fan on 2. | |
| 7/7/21 | 197W | 1.6 | 80 | 40 | 0.6 | Morning session. As above. Fan on 2. | |
| 7/10/21 | 177W | 1.6 | 80 | 120 | 1100 | No AC, Fan on 2, was true 80F per Smartthings sensor. | |
| 8/5/21 | 191W | 1.55 | 76 | 56 | 0.65 | Fan on Level 1, not much humidity, see sweat patch. 40 mins, 16oz, fluid loss 10oz, sodium loss 661.1146ml per hour | |
| 8/14/21 | 202W & 163W estAvg 180 | 1.59 | 82 | 245 | 2300 | Ride – run – ride – run, hot on second set | |
| 9/11/21 | Multi Sport (Ride & Run) | 1.15 | 60 – 80 | 220 | 1400 | 2:30 Ride, 1:15 run, hot on run, but only drank ~4oz | |
| 9/5/22 | Ride: 172W | 1.4 | 81 | 73 | |||
| 9/13/22 | Ride: 170W | 1.1 | 81 | 60 | 700 | ||
| 9/22/22 | Ride: 183W | 1.4 | 80 | 90 | 1000 | 1.13KG lost | |
| 9/28/22 | Ride: 215W | 1.4 | 65 | 105 | 1240 | 1.3KG lost, 50 mmol NaCi = (23 / 58.5) * 50 | |
| 2/7/23 | Ride: 178W | 1.4 | 66 | 67% | 80 | 700 | 1.1KG lost, Cool indoors, no fan |