Are We Over-Sanitizing? The Hygiene Hypothesis

While many new parents believe that it’s best to keep their children in a pristine, clean environment, some research suggests that being exposed to a variety of microorganisms early in life is good for a child’s immune system. This research indicates that children who are raised in very sterile environments are more likely to develop hay fever, asthma, and certain food allergies. The has been dubbed the "hygiene hypothesis."

The hygiene hypothesis was first proposed more than 30 years ago by David P. Strachan, a professor of epidemiology, in the British Medical Journal. Since then, it has been challenged by new scientific developments, adapted, expanded, and fine-tuned. Among the changes, additional research has found that some stress-related psychiatric disorders, such as depression and anxiety, may also be on the rise in developed countries, like the U.S., perhaps in part, due to a changing microbial environment.

Although it may seem strange in the age of Covid-19 to suggest that not using antibacterial soap and hand sanitizer is actually better for your health, evidence suggests that changes to the microbial environment brought on by such products can disrupt our immuno-regulatory circuitry and lead to ongoing inflammation in the body, which can impact our health in ways we’re only now discovering.

The Developing Immune System

The hygiene hypothesis is a very rich and complex argument that can be broken down into simple terms. When a fetus is inside the womb, it has a relatively weak immune system because it is protected by the mother’s antibodies. Once the baby is born, the immune system must start working for itself and it takes time to develop to full capacity. The hygiene hypothesis suggests that this development requires being exposed to the “right” germs at the right time.

If an infant’s immune system is not exposed to certain contaminants found in everyday life, the infant may have an underdeveloped immune system. The child would then have a harder time fighting off infections later in life when it inevitably comes into contact with these contaminants. For example, one study found that exposure to germs triggered an internal inflammatory response in children raised in cleaner environments, leading to ailments such as asthma and auto-immune conditions (e.g., inflammatory bowel disease, type 1 diabetes).

So where do psychiatric disorders come into the picture? Just as incidents of allergies, asthma, and auto-immune conditions are on the rise around the world, especially in developed countries, so too are incidents of major depressive disorder. The rate of increase is too rapid to be attributed to genetic changes.

One possible explanation for the increase in both physical and mental conditions is that our obsession with cleanliness in modern society has disrupted our immune systems’ ability to shut down inflammation in the body. The loss of this ability, according to the hypothesis, has led to an increase of both autoimmune and allergic diseases and also plausibly contributes to an increase in the incidence of mood disorders because of the link between these conditions and bodily inflammation.

Further, patients suffering from affective and anxiety disorders present with features that mirror inflammatory conditions such as:

• Pro-inflammatory cytokines (proteins secreted by cells to communicate with other cells) in the blood and central nervous system (CNS).
• Elevated levels of circulating C-reactive protein (CRP) (an indication of inflammation in the body)
• Activation of lymphocytes (a type of white blood cell that fights infection and the main type found in lymph)
• Inflammatory cellular signaling pathways.

The Hygiene Hypothesis and Gut Microbiota

In a previous article, I outlined the gut-brain connection and discussed how the treatment of gastrointestinal issues may also improve mood disorder symptoms. Combining gut microbiota studies with the above research into the hygiene hypothesis directs clinicians to new areas of research. The hygiene hypothesis itself touches on immunological, microbiological, and evolutionary science. Thus, examining the conclusions requires a holistic approach explaining the broader impact of our modern lifestyle on human beings.

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For individuals who are genetically susceptible to developing depression and anxiety, for example, it’s possible that disruption to the microbiota or the lack of exposure to vital microorganisms in infancy may contribute to symptoms of depression later in life. Although most scientists agree that more research needs to be done into both the hygiene hypothesis and gut microbiota, a future where clinicians can use microbiota profiling as a diagnostic tool for psychiatric patients seems to be just around the corner.

So the next time you worry about your toddler eating something off of the floor, let the hygiene hypothesis offer you some comfort.