Dr. Ted Schettler
Environmental Threats to Healthy Aging: With a Closer Look at Alzheimer's and Parkinson's Diseases. Stein J, Schettler T, Valenti M, Rohrer B (2008). Released by Greater Boston Physicians for Social Responsibility and the Science and Environmental Health Network. Available at www.agehealthy.org.
For many years, we at SEHN, along with countless others, have focused considerable attention on children's environmental health. In 2000, with colleagues from Greater Boston Physicians for Social Responsibility, I co-authored "In Harm’s Way: Toxic Threats to Child Development," describing the impacts of a variety of environmental agents on children’s brain development and function. Then we began to wonder about the impacts of environmental factors on brain function later in life. What role might they play in the risk of developing Alzheimer's or Parkinson's disease? These are the two most common neurodegenerative diseases in the US, with enormous impacts on the quality of life. Since the number of people over age 65 in the US is expected to nearly double in the next 20 years, this question seemed both timely and urgent.
Soon after embarking on a lengthy and extensive review of hundreds of scientific studies from diverse fields of medicine, public health, nutrition, toxicology, evolutionary biology, environmental health, and ecology, we realized that there is rarely a single explanation for these diseases. In fact, many chronic diseases of industrialized society are the consequence of multiple, interacting factors, entangled in causal webs that create the conditions out of which disease patterns emerge.
We call this an ecological health framework. Others prefer to call it a complexity model, but whatever its name, it explicitly embeds the individual in the context of family, community, society, and ecosystem. In this framework, context, relationships, and history matter—even at the cellular and sub-cellular levels.
It became clear that both a top-down and bottom-up approach would help us to better understand these multi-factorial diseases. Instead of looking exclusively through a microscope at DNA and sub-cellular mechanisms, we also had to look at large, sweeping trends of the 20th century to begin to understand the complex sea of conditions that give rise to changing patterns of disease. We had to consider the "environment" as the entire physical, biologic, social, and cultural context in which we are conceived, grow, age, and ultimately die.
We looked at public health advances, changes in the built environment, the emergence of industrial agriculture, changing dietary patterns, the rapid growth of the chemical industry, and trends in income distribution. These changes were accompanied by an epidemiologic shift from disease patterns of the early 20th century to the now-familiar upward trends in obesity, diabetes, hypertension, cardiovascular disease, cancer, and asthma. We explored connections to Alzheimer’s and Parkinson’s diseases, also working our way down into cellular and sub-cellular mechanisms.
What did we find?
First, genetic factors play a relatively minor role in the vast majority of cases of Alzheimer's and Parkinson's diseases. Even when a particular genetic marker is present, it is unlikely to determine a health outcome without interacting with environmental factors.
Second, we identified a cluster of diseases or conditions including diabetes, cardiovascular disease, obesity, and metabolic syndrome that tend to co-occur in individuals and in populations. We call this the "Western disease cluster" because of connections to many features of industrial society dominant in the US, and increasingly exported and widely adopted around the world. It turns out that cognitive decline, dementia, Alzheimer’s disease, and to some extent Parkinson's disease are also linked to this cluster.
Third, environmental factors aggregate in individuals or populations and then feed into final common biologic pathways -- such as inflammation, oxidative stress, and disrupted insulin signaling -- that increase the risk of diseases in the Western disease cluster. These pathways are also drivers of the risks of Alzheimer's and Parkinson's disease.
This means that there is likely to be considerable variability in the distribution of risk factors within a group of people with these diseases. Put another way, many roads lead to the clinical condition, and we should not expect to discover a single cause. Ultimately, diverse factors, distributed very differently from one person to another, feed into fewer and fewer final common biologic pathways on the way to clinical disease.
Further, we found ample epidemiologic and laboratory animal evidence that the risk of degenerative diseases of aging can begin to be established early in life—including during fetal development. For example, in animal studies, prenatal exposure to certain pesticides primes the brain so that it becomes much more susceptible to damage from post-natal exposures to the same or other pesticides. One area of the brain involved is the substantia nigra, which ultimately fails to produce sufficient amounts of the neurotransmitter dopamine in people with Parkinson's disease.
Pesticide exposures in the work setting are also linked with cognitive decline, including impaired memory and attention. And, in the general population, exposure to some pesticides has been linked to dramatically increased risks for diabetes, pre-diabetes, and metabolic syndrome. Air pollution and lead exposures also increase the risks of dementia and pathologic changes in the brain associated with Alzheimer's disease, beginning in childhood.
We also discuss potential risks of Parkinson's disease associated with high manganese levels in infant soy formula or formulas with high amounts of added iron. These are foods consumed decades before clinical Parkinson's disease is likely to become apparent. In addition, links between many features of a typical Western diet and neurodegenerative diseases, as well as obesity, diabetes, cardiovascular disease, and metabolic syndrome, became clear. Saturated fats, trans-fats, inadequate omega 3 fatty acid consumption, refined carbohydrates, and inadequate dietary anti-oxidants make these diseases more likely. A Mediterranean-like diet seems to be protective. Inadequate exercise and chronic socioeconomic stressors add to the risk.
We concluded that the biologic mechanisms and pathways involved in many of today’s chronic diseases of aging are influenced by an entire collection of factors—nutrition, exposures to toxic chemicals, air pollution, exercise, and socioeconomic stressors—at play throughout the lifespan. We can, therefore, clearly see links between our industrial food system, chemical use and regulation, energy policy, and socioeconomic inequities and the risks of Alzheimer's and Parkinson's diseases, among others. As we contemplate measures to prevent or slow the onset of these diseases, we can begin by thinking of ways to disrupt their increasingly obvious connections to many features of industrialized society.
What to do:
The rich interactions of numerous environmental factors at multiple levels suggest many opportunities for interventions. Here are some possibilities.
Individuals and families can:
But individuals cannot do it all. Many risk factors are beyond our control acting alone. Policy-makers and government officials also have opportunities and responsibilities to act. They can, for example:
- Eat healthy food. Watch the nutritional and caloric value of what you eat; eat lots of fruits, green and orange vegetables, legumes, whole grains, and foods high in omega-3 fatty acids (like fish, canola oil, and walnuts); avoid saturated, hydrogenated, and trans-fats, as well as sugars, and limit consumption of refined carbohydrates (especially cookies, cakes, corn chips, crackers and refined cereals). As a general rule of thumb, eat mostly fresh plant-based foods, avoid fast or processed foods, and don’t eat too much.
- Incorporate exercise into daily life wherever possible. Aim for at least an hour a day of vigorous walking or equivalent activity; if unable, do what you can.
- Reduce or avoid exposure to toxic chemicals. Exposures to hazardous chemicals—including pesticides; metals such as lead, mercury, and excessive iron; toxic solvents in glues, paints, and varnishes; and endocrine disruptors like bisphenol A, which leaches into food and beverages from the lining of tin cans and polycarbonate plastic bottles—are linked with diseases of the Western disease cluster and related neurodegenerative conditions.
- Be socially engaged. Stay active with family and friends; avoid social isolation—a risk factor for dementia.
- Promote healthier food policies. Shifting towards more local, sustainable food systems can improve nutrition, food access, and food security and reduce environmental degradation and greenhouse gas production associated with industrial agricultural systems. A variety of programs are already making healthier food available to schools, hospitals, businesses, and communities. Posting calorie counts on menu items and prohibiting trans-fats in restaurants are proven strategies for improved nutrition. Breast feeding should be strongly encouraged and accommodated in workplaces and communities.
- Reform chemical regulations. Require pre-market safety testing and safer substitutes for hazardous chemicals in the production of consumer products, thereby reducing risks in homes, schools, workplaces, and communities.
- Improve the health care system. Take advantage of numerous, multi-sector opportunities for primary disease prevention and create a system for universal, comprehensive, and equitable health care access.
- Transition to renewable, clean energy. Prioritize energy conservation and efficiency. Expedite the transition to clean, renewable energy, thereby reducing air pollution, greenhouse gas emissions, and dependence on fossil fuels. Linking energy-efficient public transportation with bikeways and sidewalk networks saves energy and encourages physical activity.
The 20th century witnessed unprecedented changes in the natural, built, and social environments. These changes improved many aspects of daily life and contributed to a remarkable increase in lifespan. But new patterns of living have given rise to new patterns of chronic disease that increasingly undermine health and the quality of life in an aging population. This will become particularly apparent as the number of people over 65 in the US steadily increases. The implications are profound. Cross-cutting strategies aimed at preventing chronic human diseases also hold promise for restoring degraded ecosystems, here and abroad. These solutions are within our reach, and their implementation is urgently needed.