“Exercise performance is impaired when an individual is dehydrated by as little as 2% of body weight. Losses in excess of 5% of body weight can decrease the capacity for work by about 30% (Armstrong et al. 1985)”
It’s vitally important to stay hydrated and replace electrolytes and salts on long runs if you don’t want to impair your performance. These running packs and hydration vests let you carry ample liquids, comfortably.