High levels of mercury in the diet might increase the risk of developing Lou Gehrig's disease.
ALS predominantly targets the neurons responsible for voluntary movements, such as eating and walking. The condition steadily worsens over time, and there is no cure.
In ALS, motor neurons that travel from the brain to the muscles via the spinal cord steadily die off. The symptoms begin with muscle weakness; this later leads to muscle twitching and, eventually, muscle wastage.
Within 3-5 years of developing the initial symptoms of ALS, the muscles necessary for respiration may no longer function, causing death.
To date, the exact reasons why some people develop ALS are not understood, although some risk factors have been discussed. For instance, age plays a part; ALS symptoms are most likely to occur between the ages of 55-75. Also, men are more likely than women to develop ALS.
Some researchers believe that ALS is more likely to develop in people who have been exposed to certain environmental toxins, such as lead and pesticides.
A recent study, which will be discussed in April at the American Academy of Neurology's 69th Annual Meeting in Boston, MA, examines one such environmental risk factor: mercury.
Mercury, fish, and ALS
The present study set out to specifically examine the relationship between ALS and mercury in the diet. The research team was headed up by Dr. Elijah Stommel, of Dartmouth College in Hanover, NH.
The particular focus of the study was mercury consumption through fish and seafood. In total, 518 participants took part in the trial, 294 with ALS and 224 without. The team collated dietary information with a particular focus on the types of seafood and fish that the participants ate. From this data, they estimated the amount of mercury each person was likely to have consumed.
Different species of fish have different levels of mercury - shark and swordfish, for instance, have higher than average levels. Because mercury is not easily excreted, animals at the top of the food chain tend to have higher quantities in their body (bioaccumulation); also, these fish also have longer lifespans, which allows levels to slowly increase over the years.
The team also paid attention to the origin of any fish that were caught and eaten; certain waterways have higher levels of mercury than others.
Additionally, the researchers took toenail samples from participants and analyzed them for their mercury content.
Links between mercury consumption and ALS
Once the data were analyzed, the team found that those who ate fish and seafood regularly and were in the top 25 percent for annual mercury had double the risk of ALS when compared with those who had lower levels.
In all, 61 percent of the individuals with ALS were in the top 25 percent for estimated mercury intake; just 44 percent of people without ALS were in the top 25 percent. Similarly, the toenail clipping results showed that higher mercury levels increased ALS risk.
Participants in the top 25 percent of mercury, based on toenail clippings or fish/seafood intake, had a twofold higher risk of developing ALS.
The study used a relatively small sample size, so larger studies will need to be conducted before firm conclusions can be drawn.
The authors are quick to remind people that eating fish and seafood carries numerous health benefits; however, they also suggest that people make more informed choices regarding the types of fish they consume. Fish with lower levels of mercury, including salmon and sardines, might be better choices than shark or swordfish.
Mercury is a known neurotoxin, so, whether the link between ALS and mercury is confirmed or not, reducing intake of this heavy metal remains a sensible choice.