The Quest for the Environment and Dietary Risk Factor for Alzheimer’s and Parkinson’s Disease: Time to Face the Truth!

Alzheimer’s disease (AD) and Parkinson’s disease (PD) are the first and second most common neurodegenerative diseases, respectively.[1] Alzheimer’s disease has an incidence of 10% in the population above 65 years.[2] Alzheimer’s disease affects 1% of the population above 60 years, affecting 1–2 per 1000 population at any time.[3]

The etiology is complex and multifactorial in AD and PD. It includes environmental toxin exposure, dietary factors associated with genetic predisposition, and unknown factors that trigger the process of neurodegeneration and result in characteristic pathology. Gene–environment interplay has a significant role in the development of idiopathic PD.[4] Recent evidence points toward a potential role of dysfunction of the gut–brain axis due to alteration in the gut microbiome in PD. This could have a vital role in the pathogenesis of PD.[5] This alteration of the gut microbiome is partially due to the dietary intake of food products. A large meta-analysis showed that macro-nutrient intake might be associated independently with PD risk except for carbohydrates (RR-1.24), whereas polyunsaturated fatty acid intake could be protective against PD.[6]

Two associations had class 1 evidence in the development of PD—1. Physical activity (protective against PD), and 2. Constipation (positive association).[1] Milk and dairy product consumption, pesticide exposure, and methamphetamine enhance the risk of PD, whereas tobacco smoking, caffeine, green and black tea, and Non steroidal antiinflammatory drugs (NSAID) use were all protective against the development of PD.[3] An Indian study by Behari et al.[7] reported that drinking well water for more than ten years had an increased risk of developing PD. In contrast, exposure to pets and tobacco smoking for up to 20 years (and not beyond 20 years) had an inverse correlation to the development of the disease.

Mediterranean-DASH Intervention for Neurodegenerative Delay diets is shown to have an inverse association with early onset dementia risk (Filippinin et al., 2020). In AD, a mediterranean-styled diet was associated with reduced AD biomarkers. A multicountry observational study showed that meat consumption, eggs, and high-fat dairy contribute to the risk of AD, while diets high in grains, fruits, vegetables, and fish, higher vitamin D levels appear to protect against AD.[8] Air pollution, aluminum, silicon, selenium, pesticides, and vitamin D deficiency are some environmental risk factors that may contribute to the development of dementia.[9] Similar findings were observed in an Italian cohort regarding the outcome of early onset AD.[10]

In this issue, Roy et al.[11] present the findings of the study where they assess the risk of PD and AD due to dietary and environmental factors using a well-validated questionnaire. Among the food items, premorbid consumption of macronutrients—carbohydrate and fat, milk and meat products—were significantly higher in PD compared to AD and healthy controls. Weekly fruit intake was lower in both PD and AD cohorts. The authors report past consumption of fruits and high dietary fiber intake is protective against PD. Among the environmental factors, living near a water body and rural living predicts the incidence of PD, with Nagelkerke R Square value for these factors being 0.435. Rural living positively correlated with the incidence of AD as well.

Even though this study had a relatively small size, the study was able to identify important risk factors, which may be used to plan early intervention in the premorbid phase of these diseases. This study endorses the previous observations and also is important as it comes from a large representative ethnic south Asian population.