Re: The air temperature for an event is 25 degress with humidity at 60%

There are many things that you need to think about when answering your question. Let me try to answer simply first, then get into more detail. As we run or exercise our blood glucose level drops, we loose water and salt. Now for more detail….

When we exercise, we burn both fat and carbohydrate (including muscle carbohydrate, called glycogen). When we eat or drink things that contain carbohydrate, our blood glucose level increases (this is also important for diabetics, and why they need to not only watch for foods with pure sugars but also foods loaded with carbohydrates). After running for a time, say 1 to 2 hours, the blood glucose level drops significantly as we use up these stores. Taking carbohydrates into our bodies by eating an energy bar or drinking a sports drink before and during the run will help to keep glucose levels even. Now, there are many other factors to consider, did you eat a large meal of carbohydrates the night before the race (marathon runners carbo-load on pasta), what’s the runner’s starting level (beginner, run everyday?), are you aerobically fit? In fact professional runners “train” their bodies through repeated practice to work with less carbs and lower blood sugar.

You also ask about salt and water loss while running. Simply, when we exercise hard, we sweat. When we sweat, we loose water and salts through our skin and it needs to be replaced. Drinking plain water will help. There have been several studies that find that your body takes the water in better (absorbs it) if there is some carbohydrate in the water, like a sports drink provides. Sports drinks are also said to provide the salts that need to be replaced. But more is not better! Too much salt can result in dehydration. Too much water can result in many problems, most commonly cramping but sometimes a condition called hyponatremia. This is problem happens when the salt level drops too low from excessive sweating combined with too much fluid, which serves to effectively “dilute” the salt that is present in the body. This is very dangerous. There are also many factors to consider here which includes weather conditions which you mention, the hotter it is, the more you’ll sweat. The more you sweat, the more fluid and salt you loose. If the humidity is very low, your sweat evaporates quickly and you might not realize how much fluid you have lost, if its very humid, your body fluid doesn’t evaporate well enough and therefore you retain heat.

I hope this answers your question.

Keith Anderson

Brigham and Women’s Hospital, Boston