A couple of years back a great deal of media attention was focused on a local Aussie runner who raced a 10K fun run during the height of the summer heat. The night before the race, he and a few mates had had a big night out on the drink, then he fronted up the next morning at the 10K to sweat it out.
His pace became slower and slower over the course of the event, but still he pushed on until at 9K he started weaving. With 400 meters left to go he collapsed and lapsed into a coma and was rushed to the hospital.
They managed to revive him, but unfortunately efforts to save one of his legs failed. His body temperature had soared so high and he was so badly dehydrated that doctors described his leg and buttock muscles as "having the consistency of cooked meat." Additionally, he had to be placed on dialysis for four weeks, suffering from an ailment known as exercise-induced rhabdomyolysis and resultant acute renal failure.
Overloading the System
Rhabdomyolysis (the incidence of which has been linked to dehydration and heat stress, which, as the above scenario indicates, can produce muscle damage) is defined as a degeneration of muscle cells and is characterized by muscle pain, tenderness, weakness and swelling plus myoglobinuria, the presence of myoglobin--a protein found in muscle tissue--in the urine.
When blood myoglobin concentrations rise due to muscle damage and the renal threshold for filtering this protein is reached, myoglobin can spill over into the urine, coloring it with a light tinge of brown. Overloaded by the elevated levels of myoglobin, the kidneys can struggle, leading to renal failure in some severe circumstances. In fact, 2004 Olympian Sheila Taormina missed much of the 1999 triathlon season after developing rhabdomyolysis.
Note, however, that a less-dangerous condition, known as hemolysis, or the break down of red blood cells, can also result in a darkening of the urine, as can bleeding of the bladder tissue. In these cases, the dark color is due to the presence of hemoglobin in the urine (hemoglobinuria) and/or intact red blood cells (hematuria). Exercise can cause this hemolysis of red blood cells through impact (i.e. foot-strike hemolysis in runners) and exercise can also cause damage to the bladder that results in bleeding into the urine.
Given this, it is important, although difficult outside the lab, to distinguish whether dark urine is due to the presence of hemoglobin or myoglobin. This is because hemoglobin in the urine generally denotes a less serious, self-limiting condition, whereas the presence of myoglobin in the urine can produce and reflect damage to the kidneys.
There are a number of ways to differentiate the causes of dark urine. Hemoglobin is normally retained in the serum by binding to a specific protein called haptoglobin. When hemoglobin levels exceed the binding capacity of haptoglobin, the serum will become red or brownish and hemoglobin will also be released into the urine.
An increase in myoglobin in the blood (and consequently in the urine) does not cause a staining of the serum. This difference in clinical signs allows for a quick and simple differentiation of myoglobinuria from hemoglobinuria for a physician.
The Heat Connection
As the opening example demonstrates, there does appear to be a close link between the muscle damage that can occur due to heat illnesses, such as heat exhaustion and heat stroke, and rhabdomyolysis and acute renal failure. Given that heat stress can be aggravated by dehydration, the importance of developing and following an effective nutrition strategy during racing and training, particularly in hot/humid conditions, cannot be over-emphasized.
Still, many triathletes race in challenging conditions every season and few suffer anything worse than mild dehydration, so why are some people more susceptible to rhabdomyolysis? The specificity of training holds one of the keys.