A new study has uncovered a biological link between pain and fatigue that may help to explain why more women than men suffer from conditions such as fibromyalgia and chronic fatigue syndrome.
Researchers at the University of Iowa found that in mice a protein involved in muscle pain works in conjunction with the sex hormone testosterone to protect against muscle fatigue.
Pain and fatigue frequently occur together in various diseases. For example, 94% of chronic fatigue syndrome (CFS) patients suffer from muscle pain along with the primary symptom of fatigue. Equally, fatigue is common amongst people suffering from chronic musculoskeletal pain conditions such as fibromyalgia. Up to 75% of these people report suffering from fatigue.
In CFS there is thought to be mitochondrial dysfunction which almost certainly contributes to both fatigue and muscle pain as metabolic waste products accumulate. In conditions where fatigue is secondary to pain however, the link with fatigue is not well understood.
The researchers led by Kathleen Sluka, Ph.D., professor in the Graduate Program in Physical Therapy and Rehabilitation Science in the UI Roy J. and Lucille A. Carver College of Medicine, set out to identify biological links between pain and fatigue.
They also sought to explain why the majority of people suffering from pain and fatigue disorders are women. According to National Fibromyalgia Partnership, Inc women account for around 80% of fibromyalgia patients.
During the study Professor Sluka and colleagues compared exercise-induced muscle fatigue in male and female mice. Some of the mice produced a protein (ASIC3) known to be involved in musculoskeletal pain and some did not.
The researchers used a series of three one-hour running sessions to produce different levels of fatigue in the different groups of mice. Level of fatigue was measured by the temporary loss of muscle strength caused by the exercise.
When comparing the different groups of mice it was found that male mice who produced the protein were less fatigued by the exercise than the female mice were. Impoirtantly however, the results showed that male mice without the protein demonstrated greater levels of fatigue which were in fact comparable to that seen in the female mice.
The key finding of the study was revealed when female mice with the protein were given testosterone. When primed with the male hormone their muscles became as resistant to fatigue as the male mice with the protein. In contrast, muscle fatigue in female mice without the protein that were given testosterone was unchanged.
"The differences in fatigue between males and females depends on both the presence of testosterone and the activation of ASIC3 channels, which suggests that they are interacting somehow to protect against fatigue," Professor Sluka said.
"These differences may help explain some of the underlying differences we see in chronic pain conditions that include fatigue with respect to the predominance of women over men."
The findings provide compelling evidence that muscle pain and fatigue are not seperate conditions but tend to go hand in hand. They also offer an explanation as to why women are affected to a greater degree by poorly explained conditions which include fibromyalgia CFS, and irritable bowel syndrome (IBS).
Professor Sluka said: "Our long-term goal is to come up with better treatments for chronic musculoskeletal pain," Sluka said.
"But the fatigue that is typically associated with chronic, widespread pain is also a big clinical problem -- it leaves people unable to work or engage in social activities. If we could find a way to reduce fatigue, we could really improve quality of life for these patients."
The research team intends to continue their studies to find out whether pain enhances fatigue more in men than women.
Source: University of Iowa
Another recent study also proposed a mechanism to explain why women seem to be more susceptible to chronic pain and fatigue conditions than men. That research found differences in the serotonin system between the sexes. Serotonin is an important neurotransmitter which is strongly associated with mood and pain.
To read about this study click here.