"Nathan® Insulated Soft Flask with Bite Top 18 ounce: The Nathan® Insulated Soft Flask with Bite Top 18 ounce is a hydration bottle designed for runners and endurance athletes who need reliable fluid access during training runs, trail outings, and long-distance efforts. The insulated construction keeps fluids cooler 38% longer than non-insulated soft flasks, which means your drink stays refreshing well into a run rather than warming up in the first few miles. At 18 ounces (535 mL), it holds enough fluid to support extended efforts without adding unnecessary bulk to a vest or hand carry setup.The bite valve keeps liquid from escaping between sips, so the flask can be stowed and retrieved without worrying about leaks during daily training or more demanding outings. It works with all Nathan hydration vests, making it a straightforward addition to an existing hydration system. The soft flask construction compresses as fluid is consumed, reducing bulk as the run progresses, so you're not carrying dead weight in the final miles.Why Our Fit Experts Love It""I brought this flask on a longer weekend run tucked into my hydration vest - the bite valve held up the whole time with no leaking, and my drink was noticeably cooler at mile eight than I expected. It's a simple, dependable option for anyone logging serious miles."" - Sullivan, Road Runner Sports Fit Expert, Naperville ILWho It's Best ForThe Nathan® Insulated Soft Flask with Bite Top 18 ounce is best for runners and endurance athletes who use hydration vests during daily training, long runs, or trail outings and want a flask that keeps fluids cooler for longer. It suits anyone already in the Nathan hydration ecosystem looking for a compatible, leak-resistant flask option. VIP Family members can take the Insulated Soft Flask with Bite Top 18 ounce for a 90-Day test drive with our Perfect Fit Guarantee."