Clyde, what amount of fluid loss is to be expected as the temperature varies? I observed 1.5 liter/hour fluid loss on an 81°F day (78 minute workout). The next day, with temps ranging from 56-60°F, I observed 0.9 liters/hour (110 minute workout). My scale is accurate to 0.1 lb and I weighed the water to the gram. :) Lloyd
ANSWER: It depends on your surface-to-volume ratio (thinner people have more exposed surface area per pound of body weight), the humidity (humidity slows perspiration, which elevates core temperature in the body), and wind (moving air over the surface of the skin removes the "insulation" layer of air trapped on the laminar boundary of the skin). Moving faster, such as when you are on the bike versus running, equates to a faster relative movement of air over the skin (the equivalent to a faster wind and a greater 'wind chill'). Since it is not reasonable to acccount for all these changes on an ongoing basis, my recommendation is to do enough weigh ins before and after workouts that you have a sense of what your perspiration losses are like on a sunny day and on a foggy/cloudy/humid day for both a medium intensity and higher intensity workout during your coldest and hottest season of training. That would mean four workout sessions with weigh ins during the Winter and four more during the Summer, where each of these seasons you are doing medum intensity sunny, medium intensity humid, high intensity sunny, high intensity humid. Doing 2 or 3 of EACH of these would give you a better number since then you take into account day-to-day variation. Alternatively, to simplify all this, just do a weigh for a relatively intense training session once in the cold and once in the hot season, extrapolate linearly between the seasons. Add or subtract 10-50% relative to your baseline measurement if you think the environment calls for it. Perspiration can change by a factor of 2 (i.e. 100%) from one day to the next, which is almost what you saw happen in your training (described in your question above), but when changes are that large it is best to do more measurements rather than just guessing, since guessing could throw your fluid and salt need estimates off so far that you end up either chronically dehydrated (not enough salt intake) or bloating from excess fluid retention (too much salt intake), one of the reasons that high salt intake results in high blood pressure.
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