06/03/2026
Have you ever noticed how some days a run feels effortless and other days it feels like you’re dragging an anchor behind you?
You check your watch. The pace is slower. Your heart rate is higher. The effort feels harder than it should. Sometimes your legs feel heavy before you’ve even gotten very far into the workout.
Most people assume they’re losing fitness.
More often than not, they’re simply under-fueled, under-hydrated, or not replacing the electrolytes they’re losing through sweat.
As the temperatures rise this summer, your body has one primary goal: keep you cool. The way it does that is by producing sweat. When that sweat evaporates, it carries heat away from your body.
The problem is that sweat doesn’t just contain water.
It contains electrolytes too, particularly sodium.
Every athlete loses a different amount. Some people lose very little. Others can lose well over 1,500 mg of sodium every hour. That’s why one runner can get through a long run with a bottle of water while another starts cramping, slowing down, or feeling completely depleted despite drinking plenty.
Research has consistently shown that performance begins to suffer when dehydration reaches around 2% of body weight. For a 150-pound athlete, that’s only about 3 pounds of fluid loss. It doesn’t take very long to reach that point during a summer run, soccer tournament, baseball game, football practice, or even a day spent outside working in the yard.
The effects are often subtle at first.
Your pace slips a little.
Your heart rate creeps up.
The workout feels harder than normal.
You lose focus.
Your energy drops.
What makes this tricky is that many athletes try to solve the problem by simply drinking more water.
Water is important, but it isn’t always the whole solution.
Think of it this way. If your truck was leaking oil, adding more gasoline wouldn’t fix the problem. Your body works similarly. When you lose large amounts of sodium through sweat, replacing only water may not fully restore what your body needs to perform well.
This becomes especially important for endurance athletes, but it also applies to kids playing sports all summer.
One of the most common things I see i