A recent meta-analysis published in BMC Geriatrics analyzed 60 human studies of environmental risk factors for dementia. The authors of the paper applied statistical analysis based on the overall quality of each study (i.e., the highest quality papers had a larger sample size and robust measures of exposure and outcome) in order to synthesize their results into broader conclusions. Based on these calculations, they assigned each risk factor a score based on the strength of the evidence for its involvement in dementia. A “strong” score indicates solid evidence linking a risk factor to dementia, while “moderate” or “weak” scores suggest that more research needs to be done before a conclusion can be drawn.
Air pollution was concluded to be a significant risk factor for dementia based on the results of eight independent studies. Sources of air pollution with strong scores included nitrogen oxides, particulate matter (such as dust, smoke, and soot), and ozone, while carbon monoxide and environmental tobacco smoke received a moderate score. Major sources of air pollution include cars, construction sites, industrial smokestacks, and power plants. Individuals living in urban areas may want to consider investing in home air filters in order to reduce their exposure to air pollution.
Metals and Micronutrients
If you’ve read my article on the role of metals in Alzheimer’s disease, you know that certain metals are correlated with an increased risk of Alzheimer’s, though the evidence is still not conclusive enough to say for sure whether they contribute directly to the development of dementia. The meta-analysis confirmed this interpretation. Most of the metals in the analysis (including lead, copper, iron, and zinc) were given a weak score. The only metals to earn a moderate score were aluminum and arsenic. My article mentioned above includes some easy tips for reducing your exposure to aluminum and other metals.
In addition, two micronutrients were identified as potential dementia risk factors when consumed above recommended dosages. Selenium, which is found in meat, seafood, dairy products, and mushrooms, scored moderate on the risk scale. Silica, a mineral found at varying concentrations in drinking water, scored high risk. Though small amounts of both these minerals are important for health, it may be best to avoid taking them in supplement form (unless instructed by a physician) in order to avoid over-consumption.
The review also analyzed studies on exposure to potentially-harmful substances in the workplace. Pesticides and fertilizers were identified as high risk factors, a worrying find for those in the agricultural industry. This may explain why children living in rural areas are at higher risk for dementia as adults, which was concluded in a previous meta-analysis by the same authors. Exposure to industrial solvents or degreasers was also a high risk factor. The occupational factors rated at moderate risk included metals, diesel motor exhaust, and (strangely enough) electromagnetic fields. The latter finding remains highly controversial.
The evidence analyzed in this study identified vitamin D deficiency as a strong risk factor for dementia. This finding was supported by three independent studies of more than 15,000 subjects in total, though one small study involving 40 subjects did not find an association. Vitamin D is not easily obtained through food. However, our body can synthesize vitamin D when our bare skin (without sunscreen) is exposed to sunlight. The time required to meet our daily dose of vitamin D depends on your skin tone, the amount of sunlight available, and how much skin you have exposed. On a sunny summer day, 10-15 minutes is often enough. You may want to consider taking vitamin D supplements during the winter, if you live in an often-cloudy area, or if you do not go outside every day.
The authors of this paper noted several sources of bias in their meta-analysis. An important consideration is that the studies have been conducted almost exclusively in high-income countries, despite the highest rates of dementia occurring in low- to middle-income countries. Additionally, since none of these studies measured exposure at more than one time point, the data can’t predict whether any critical periods exist for exposure to these risk factors. In other words, exposure may be more harmful during old age than childhood, or vice versa. Finally, as with any observational study, it’s important to keep in mind that these risk factors may not all be causal (see How to Be a Smart Consumer of Science News).
Despite these sources of error or bias, the exposures identified as high or moderate risk factors for dementia should not be taken lightly. Some of them are easier to avoid than others, but it makes sense that we should all take whatever steps we can to reduce our risk of dementia.