07-ING Marathon Training, Miami 2010: The Sweat Rate: a Crucial Measure of Dehydration During Exercise & 4 Mile Run

Today’s preparation for the 2010 ING Miami Marathon consisted of a short, easy four mile run. Nothing big, just enough to get the weekly mileage up. On another note, today as promised, we will be discussing loss of body weight during exercise or as nutritionists call it: The Sweat Rate. The sweat rate is an important indicator and gives us very important information about the body’s state of dehydration during exercise. The sweat rate also allows us to calculate optimum fluid replacement. Of course optimum fluid replacement is extremely important not only for recovery, but also to restore glycogen homeostasis and prepare us for the next bout of exercise. But firs today’s stats, followed by an explanation of  “The Sweat Rate.”

Today’s Stats: October 1, 2009

• Temperature: F 48
• Distance: 4 miles
• Time: 33:20
• Pace: Target 08:58
• Pace: Actual 08:20
• Running Goal: Easy run
• 15 minute post workout pulse: 72ppm
• Calories expanded: 680

First things first: never start an exercises session dehydrated! Its easy, just have a couple of glasses of water before hand, or if you are going on a long run, consider a carbohydrate drink. It’ll get you going.

You have heard it over and over. After hard exercise or a race it is important to replace all fluids lost, thus eliminating weight loss due to exercises. The preferred option is to have a meal up to four hours after exercises, but who wants to eat after a 13 mile run. Not many people, so the next best thing is to replace fluids with a carbohydrate drink. There are several options out there, I am using two products and I will elaborate why, and how in another post. Today, let’s just cover the basics.

What Is Dehydration: (ADA, 2009). Dehydration is a water deficit in excess of 2% to 3% in body mass! Using my run as an example: The model predicts a water loss of 4.72lb (236lb*.02%) on the lower end and 7.08lb on the upper end. The sweat rate of the 13 miler was right the middle, establishing the fact, that yes, I was dehydrated. The ideal thing would have been to avoid this dehydration by drinking fluids throughout the run, but unless in a race, it is very impractical. A runner should also take care never to consume more fluid than is indicated by the sweat rate. Doing so may cause hyponatremia, and prove potentially fatal. Hyponatremia is a result of sodium levels dropping too rapidly due to water intake. The body’s cells start to swell and eventually will burst. David Rogers, who collapsed and later died as a result of hyponatremia after completing the 2007 London Marathon is just one example of the hidden dangers of too much water (BBC News, 2007).

General Rules: Fluids: (Mitchell, 2003). Athletes should drink 2 to 3 cups of fluids for each pound of weight lost during exercises . This works well for short runs. Let’s use my long run from yesterday as an example. I lost 5.8lb during exercises. For ease of calculation let’s make it an even 6lb. Using this rule I should have drank 18 cups of fluid. That my friends is 144oz or one gallon and one pint of water! Almost impossible... But remember, it is over the course of 4 hours.

General Rules: Carbohydrates: (ADA, 2009). The optimum fuel your body runs on during endurance sports. Athletes should consume anywhere between 6 and 10 g/kg of carbohydrates per day. (2.7 to 4.5g/lb). For a runner weighing 150lb that would mean that they should consume somewhere between 405g and 675g of carbohydrates per day. I’ll cover more specific guidelines in a later post.

Post exercise regimen should consist of carbohydrate intake of 1 to 1.5 g/kg (0.5 to 0.7g/lb) of body weight up to six hours post run. Again, I’ll cover the reasoning behind this recommendation in a later post. But it has to do with optimum absorption rate of nutrients post run. There is a certain window for optimum rehydration that presents itself. Here is a post I composed recently on the subject of "Factors Influencing the Restoration of Fluid and Electrolyte Balance After Exercises in the Heat."

Next run we’ll go in the hills again for a 9 miler. We continue covering the antioxidant network.

Beste Gesundheit,

Werner

References:

ADA. (2009). Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the American Dietetic Association: 109:3 pgs 509-27

BBC News. (2007). A father’s tribute to marathon son. Retrieved from http://news.bbc.co.uk/2/hi/uk_news/england/london/6587093.stm

Mitchell M. (2003). Nutrition across the lifespan. (2nd ed.). Philadelphia, Pennsylvania: Saunders

Comments

[Frank]

I would agree with most of the comments in this post, especially the 2nd paragraph. It did remind me a little bit about a similar medicine article I noticed here http://theinstituteofgoodtaste.blogspot.com/2010/01/experimental-medicine.html