Fluid Loss, part 2

As highlighted the other day, monitoring fluid loss is a critical part of training, especially in the warmer months. The reality is that as athletes we often shirk our responsibly to develop a comprehensive nutrition plan. Instead, we remain short-term focused and only think through the next workout. It is no wonder why we have a bad Sunday ride when we lost pounds of fluid the day before.

Thus it becomes even more important to regularly monitor your fluid loss so that you can adjust your nutrition plans. Most of us know what basic dehydration feels like. The concepts can probably be extrapolated into a basic understanding that dehydration affects overall performance. Take for instance the following study:

• A group of cyclists rode in hot conditions for 2 hours then undertook a simulated cycling hill-climb to exhaustion on an inclined treadmill. On one occasion they were well hydrated, and the other time allowed to lose 2.5% of their body mass prior to the hill climb. Despite having a lighter body weight (by nearly 2 kg), the dehydration made them stop riding 29% earlier and reduced their power output for a given speed. (Ebert)
  

Think about your last long ride and imagine you stopped 29% earlier. Was it a good workout? Probably not. The key here is that dehydration begins to significantly impact performance at a mere 2.5% change in body weight. Think about that for another minute.

In my last post, I conducted a basic fluid loss test. In just a 67 minute run, I managed to shed 3 pounds of body weight...or 1.8%. Another half a pound and I begin approaching the level of the cyclists referenced above. Now consider the following chart demonstrating the severity of fluid loss on a 150 pound athlete:

Weight Loss (150 lb athlete)	% body weight	Physiological Effect
3 lbs	2%	Impaired performance
6 lbs	4%	Capacity for muscular work declines
7.5 lbs	5%	Heat exhaustion
10.5 lbs	7%	Hallucinations
15 lbs	10%	Circulatory collapse and heat stroke

Fluid loss is inevitable on a warm day but minimizing the loss helps you maintain performance. As you can see, performance begins to decline at just 2% of body weight. For me that is roughly 3.5 pounds. But just as important, is how the weight loss impacts recovery.

I contend that recovery should never be limited to post workout. Instead, recovery is every bit integral to pre workout and workout nutrition strategies. Understanding the impact of fluid loss highlights the importance of being properly hydrated going INTO the workout. Endurance athletes can ill afford to be under hydrated on a warm day. It is like playing with fire.




Footnotes:

Ebert TR, Martin DT, Bullock N, Mujika I, Quod MJ, Farthing LE, Burke LM, Withers RT. 2007. Influence of hydration status on thermoregulation and cycling hill climbing. Med. Sci. Sports Exerc. 39: 323-329.

"The Maintenance of Fluid Balance during Exercise", International Journal of Sports Medicine, vol. 15(3), pp. 122-125, 1994--chart

Fluid Loss and Heat Training

Summer is here. For those of us in the South, we were met with 90 degree temperatures a number of times over the past week. Just as predictable as heat and humidity to Atlanta, it is the time of year where athletes begin feeling less energetic and more fatigued. The reality is the heat beats you down.

To understand why, you need to look no further than a basic science book. The body is made up of water. Sweat is 99% water, blood is 90% water, muscle mass is 75%. This easily demonstrates why we weigh ourselves and find our weight often +/- a few pounds. Most of it is water loss or retention.

This becomes crucial for athletes who train in warm and humid conditions. As heat and humidity rise, the body begins to sweat as a cooling mechanism. In order to sweat, there needs to be an ample supply of water. As the body loses fluids in the form of sweat it becomes even more critical to replace the fluids to keep the body hydrated. Most of this is common sense. But have you really ever thought, beyond basic dehydration, about how fluid loss impacts your training?

Over the past year of training, it has come to my attention that I am a VERY heavy sweater. This seems to stand in face of conventional wisdom given my general fitness level and lean build. However, I often walk back into the house with a heartrate strap that will produce a small river beneath. I think this problem has the potential to become a major detriment in training...both during the workout and in the recovery process. My goal is to devote time this summer to determining my fluid loss rate and developing a better hydration strategy.

Your nutrition that worked in April might need a slight modification to meet your needs on a hot day. However, most of us still stick to the one bottle per hour rule of the thumb when training. Instead, we need to think past this general concept and begin understanding how much fluid our body actually NEEDS during the workout.

Calculating your fluid loss or need is a simple process:

1. Weigh yourself prior to the workout
2. Workout, 60 minutes at a moderate intensity is most ideal
3. Weigh yourself after the workout
4. Subtract the difference between the weights
5. Repeat often


The difference in weight can then be converted to fluid loss if you assume that 1 pound of body weight equals 16 ounces of fluid. Thus, the key becomes determining the proper level of hydrating to prevent significant weight loss. For example, I conducted this test on a humid morning run:

1. Pre workout weight: 168.4
2. Workout: 67 minutes, 7.5 miles, moderate intensity
3. Post workout weight 165.4
4. Total weight loss: 3 pounds or 1.8% of body weight.

So in theory, I needed 48 ounces (3 x 16 oz) of fluid to maintain my weight. Over the course of the workout, I likely consumed about 14 ounces of water....so I really needed 62 ounces (48 + 14) to offset my weight loss. Of course, consuming 62 ounces over the hour is improbable. Generally, the body is only able to absorb about 34 ounces (1 liter) per hour but the deficit illustrates the importance of maintaining proper body hydration, both before and during training.

So as the training and the heat begins to increase, I encourage you to give more thought to your hydration plans. The old rule of one bottle per hour may work for you...it may not. And unless you determine your fluid loss need, then you might be putting your body in jeopardy and setting yourself up for performance failure. Therefore, the question becomes at what point does weight loss become detrimental to training and recovery?

In the next few days, I will go into more detail about the detriments of fluid weight loss and what you can do about it.