A Comparison of Mercury Exposure from Seafood Consumption and Dental Amalgam Fillings in People with and without Amyotrophic Lateral Sclerosis (ALS): An International Online Case-Control Study

Abstract

Exposures to toxic metals such as mercury have been suggested to be risk factors for amyotrophic lateral sclerosis (ALS). Human intake of mercury commonly occurs via consumption of seafood or from mercury-containing amalgam dental restorations ('mercury fillings'). We therefore compared mercury exposures from these sources in 401 ALS and 452 non-ALS respondents, using an internationally-available online questionnaire that asked respondents how often they ate seafood and what their favourite types of seafoods were. Respondents were also asked to record numbers of current or former mercury fillings. ALS and non-ALS respondents did not differ in their frequency of seafood consumption or in monthly mercury intake from favourite seafoods. Both groups had similar numbers of current, as well as former, mercury fillings. In conclusion, this study found no evidence that mercury exposure from eating seafood, or from mercury dental fillings, was associated with the risk of developing ALS. Therefore, if mercury does play a role in the pathogenesis of ALS, other sources of exposure to mercury in the environment or workplace need to be considered. Alternatively, a susceptibility to mercury toxicity in ALS, such as genetic or epigenetic variations, multiple toxic metal interactions, or selenium deficiency, may be present.

Keywords: ALS; amyotrophic lateral sclerosis; case-control study; dental amalgam filling; fish consumption; international study; mercury; motor neuron disease; online questionnaire; seafood.

Figure 1

Estimating mercury exposure from seafood consumption. The average mercury concentration in µg/g from up to three favourite seafoods was multiplied by a standard portion size of seafood (227 g), then multiplied by the monthly frequency of seafood consumption to estimate monthly mercury exposure from seafood in micrograms.

Figure 2

Selection of respondents for analysis. The final datasets of respondents used for analyses was achieved after exclusion criteria were applied for younger age, not answering questions on seafood consumption or dental fillings, and removal of outlier values. CON: control; current: current dental fillings; former: former-only dental fillings; frequency: frequency of seafood consumption; mercury: mercury content of favourite fish; never: replied ‘never had any current or former mercury fillings’; outlier: extreme outliers (more than three times the inter-quartile range); M: male; F: Female.

Figure 3

Proportion of respondents eating seafood at different frequencies. ALS and control respondents ate seafood at similar frequencies, ranging from never to daily. None of the slight differences in frequency proportions between ALS and controls was statistically significant.

Figure 4

Distribution of mercury exposure from seafood. No difference is seen in the distribution of monthly mercury exposure from seafoods in micrograms between ALS and control respondents. Bar: median monthly exposure (µg).

Figure 5

Distribution of numbers of current and former-only mercury fillings. The distribution of numbers of mercury fillings are similar between ALS and control groups for both (A) current fillings, and (B) former-only fillings. Bar: median number of fillings.