Hot-Weather 70.3: Weight Gain, Not Dehydration
An athlete you coach is a 41-year-old female age-grouper, 68 kg, preparing for a 70.3 in forecast 32°C heat and 75% humidity. In a 3-hour race-simulation ride she drank 2.4 L of water with one low-sodium electrolyte tablet, gained 1.2 kg, and finished with a headache, nausea, bloating, and swollen fingers. She has decided she was dehydrated and plans to drink more on race day. Her sweat rate from a cool-weather test was 1.0 L/h, and she takes an NSAID for knee pain. Her race goal is 5:30, and she has no history of kidney disease. She is 5 weeks out, trains 10 hours/week, and has one previous hot-race DNF with similar symptoms. She wants a simple hydration plan she can follow without weighing herself during the race.
1. Interpret the 3-hour simulation. What is the likely problem, and what makes it dangerous?
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The 1.2 kg weight gain is the key. She took in more fluid than she lost, so this is not dehydration. A 3-hour ride with 2.4 L is 0.8 L/h; her cool-weather sweat rate was 1.0 L/h, and heat and humidity would normally raise it, yet she still gained weight. That means intake exceeded even a likely hot-weather sweat loss, or her actual sweat rate was lower than assumed. The low-sodium tablet adds little sodium. Headache, nausea, bloating and swollen fingers after exercise in heat are classic exercise-associated hyponatremia symptoms, not simply dehydration. Risk is higher because she takes an NSAID, which can reduce renal free-water excretion, and because she plans to drink more. Severe hyponatremia can cause vomiting, confusion, seizures and cerebral edema. Do not tell her to drink more. If symptoms are mild and she is mentally clear, stop fluids, consume salty food if tolerated, and seek medical assessment; if confusion, repeated vomiting or seizure occur, arrange emergency care. Follow up with a heat sweat-rate test and sodium review.
2. Write the race-day fluid and sodium plan you would give her.
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Start from measured losses, not a fixed rule. Do a 2-hour hot ride: weigh nude before and after, record fluid intake, and calculate sweat rate as (pre-weight minus post-weight plus fluid intake) divided by hours. If, for example, she loses 1.2 L/h, aim to replace roughly 50-70% of that (about 0.6-0.8 L/h), not 100%, and never finish heavier than she started. Use a sports drink with 400-700 mg sodium/L or add sodium to water, targeting about 300-600 mg sodium/h in heat; individual sweat sodium varies, so a sweat patch test is useful if available. Pre-race: 5-7 mL/kg (about 350-500 mL) 2-4 hours before, then small sips. On the bike: two 750 mL bottles with electrolyte, sip 150-250 mL every 15-20 minutes, total 0.6-0.8 L/h; on the run: cups and a flask, 100-200 mL every 15-20 minutes. Avoid NSAIDs. Use thirst as a guide plus scheduled checkpoints, and stop drinking if she feels bloated or is gaining weight. Carry salt tablets only if the concentration is known and practised.
3. She asks whether she should follow the rule 'drink before you are thirsty.' How do you answer?
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Explain that the slogan is not a safe universal rule. Thirst is a reasonable guide for most age-groupers, especially when sodium is included and the event is under 4-5 hours. Forced drinking to a schedule can produce dilutional hyponatremia, as her simulation showed. In heat, thirst may lag early, so a flexible plan based on measured sweat rate and sodium is better than either extreme. Teach her to use body-weight checks before and after key sessions, urine colour, and symptoms rather than a fixed 1 L/h target. The goal is to avoid more than 2% body-mass loss without gaining weight. If she finishes a session heavier, she drank too much; if she loses more than 2%, she needs more fluid or sodium. This is a skill to practise in training, not a race-day experiment.