Hydration is one of the most overlooked levers in endurance performance — and one of the most punishing when you get it wrong. In this episode, Zoë and TJ unpack why dehydration is so much more than feeling thirsty, walking through the cascading downstream effects on your gut, your blood, your muscles, and ultimately your race result. They cover gut osmolality and why a too-concentrated drink mix actually pulls water the wrong way, the link between plasma volume drop and cardiac drift, and why dead legs late in an ultra often trace back to a sodium problem rather than a fitness one.

The conversation then turns to the practical side: how to DIY your sweat rate test at home, why your sweat sodium concentration is the number that changes everything, and which lab tests Zoë and dietician Kylee Van Horn actually recommend after testing six different options. They also break down why generic 300–800 mg per hour sodium guidance fails most athletes, with real roster examples ranging from 200 mg to over 2,300 mg per hour.

Before the hydration deep-dive, Zoë and TJ tackle a thoughtful listener question about the asides they sometimes make regarding endurance training and sexual health. They walk through the RED-S framework, what suppressed libido and menstrual dysfunction actually signal in athletes of all genders, and why these conversations belong in the coaching toolkit rather than as punchlines. Coach Kylee Van Horn at Fly Nutrition is mentioned as a go-to sports dietician for clinical questions.

If you've ever had a mystery DNF, persistent GI distress, or fallen apart late in a race for reasons you couldn't pin down, this is the episode to listen to twice.

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