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As the seasons change, so do certain diseases. For example, during the winter, individuals might get their annual cold. While during the spring, allergies may go haywire. Perhaps the cold months of winter make arthritis worse, or the summer months exacerbate multiple sclerosis (MS). There are many conditions impacted by the changing seasons, including autoimmune diseases, cardiovascular events, acute gout, type 1 diabetes, type 2 diabetes, hip fractures, mental health disorders, migraines, and emergency surgery. Even mortality is affected by the seasons, with summer and winter associated with more deaths.

There are many reasons for the seasonal variance in health and disease, and they most likely work in tandem to influence an individual’s susceptibility to disease during certain times of the year. Let’s look closely at some of the more influential factors.

Infectious Diseases

Infectious diseases are well-known to correlate with changing weather. Several viruses have a seasonal pattern, including human rhinovirus (the culprit behind the common cold), respiratory syncytial virus (a virus with symptoms similar to a cold), COVID-19, and influenza (the flu). Another disease that has seasonal variance is Staphylococcal aureus. S. aureus is one of the more common infections patients can get in hospitals and is also associated with food poisoning, skin disorders, pneumonia, meningitis, and sinusitis. In a systematic review of studies on the seasonality of S. aureus, researchers found evidence pointing to seasonal variance, with more infections occurring in the summer and fall, especially hospital-acquired methicillin-resistant Staphylococcus aureus (MRSA). A more recent study confirmed the seasonality of MRSA but found peaks in the spring in nursing homes. Additionally, vancomycin-resistant enterococci, Klebsiella pneumoniae, and Escherichia coli were found to follow seasonal incidence patterns.

The seasonal changes in infectious diseases impact more than just the number of people who contract an acute illness; it can also affect long-term health. For example, seasonal patterns of infectious diseases, such as influenza, are linked to increased cardiovascular mortality. Additionally, people who deal with long-term disability directly related to a disease, such as postural orthostatic tachycardia syndrome (POTS), which may be caused by infectious Epstein-Barr Virus (EBV), are more likely to experience worse symptoms in hotter seasons. The seasonal outbreak of certain viruses might lead to a similar association between seasons and chronic illnesses due to the infections acting as triggers.

Although genetic predisposition plays an important role, many autoimmune disorders are triggered by infections. Several pathogens have a link to autoimmune disorders, such as the connection between EBV and lupus. In some cases, autoimmunity arises due to molecular mimicry, in which the virus’s proteins are similar to that of human cells or tissue. The antibodies created to target the virus or bacteria end up mistaking the self as the enemy and attack, leading to an autoimmune condition.

There might also be a link between infections and other chronic illnesses. Infection can lead to inflammation and other immune responses that trigger imbalance, which may be implicated in the pathophysiology of chronic diseases. For example, several cancers are linked to communicable diseases, the most well-known being the link between cervical cancer and HPV (human papillomavirus). Furthermore, H. pylori and hepatitis B and C viruses are implicated in cancer. Emerging evidence connects Chlamydia pneumoniae to cardiovascular diseases and  periodontitis infections to stroke and atherosclerosis. Acute diarrhea might lead to gastrointestinal conditions, while getting the flu while in the womb might increase one’s chance of psychiatric conditions. Even obesity has some links to infection, with a few studies finding a link between adenovirus 36, a virus that causes tonsillitis, upper respiratory infections, and other acute illnesses.

Genetic Expression

Another reason for seasonal variance in health might be due to changes in gene expression at certain times of the year. DNA codes for all processes in the body, and what proteins are synthesized determines whether the gene is expressed. Although actual genes play a role in disease susceptibility, the expression of the genes is what matters. The environment, including diet, exercise, and the seasons, affects the expression of the genes.

In one older study, researchers found 94 DNA transcripts with significant seasonal variability. Many of these play a role in protein production, immune function, and coding for immune cells in the blood. This seasonal variation impacts regulatory functions, including the immune system. During the seasons when gene expressions for immune cells are upregulated, disease susceptibility may increase.

Another study found that 23 percent of the human genome had some significant difference in the expressions in a seasonal pattern. Researchers used data from several locations in different latitudes, including Germany, the U.K., Australia, the U.S., and Iceland. There were two distinct patterns of immune system response expressions: 2,311 genes were upregulated in the summer, while 2,826 genes were upregulated during the winter. Based on these findings, it appears the immune system is more pro-inflammatory during the winter months for those living in temperate climates such as the U.K. This genetic variance was observed with the summer and winter months of the study populations, meaning those in the southern hemisphere also showed the upregulation of immune response genes in corresponding seasons. People in Iceland had a unique set of gene expressions, most likely due to its proximity to the poles and 24 hours of sunlight in the summer and near darkness in the winter.

These studies point to something happening on a genetic level that impacts the immune system from season to season and is potentially correlated to the climate in which a person lives.

Vitamin D Levels

Vitamin D is considered the “sunshine vitamin” because the body synthesizes it from UV rays absorbed in the skin. As such, it should come as no surprise that there is a seasonal variance in vitamin D status. Winter and spring often have the lowest levels of vitamin D, while summer months tend to have the highest.

Vitamin D deficiency increases the risk of contracting infectious diseases, several autoimmune conditions, and other illnesses. There are also connections between vitamin D deficiency and metabolic syndrome. In addition to its role in immunity and chronic diseases, vitamin D deficiency might also increase the risk of having a stroke, according to one study.

A possible reason behind this is vitamin D’s important role in immunity and health as a critical regulator of adaptive and innate immune systems. Vitamin D assists in fighting pathogens, supporting innate immune responses, and suppressing the immune responses that can lead to inflammation and its association with many autoimmune conditions. Thus, seasonal changes in vitamin D status could affect the immune system’s efficacy, putting a person at more risk of disease when vitamin D levels are lower. The seasonality of vitamin D status modifies its ability to regulate immunity, leaving one susceptible to diseases.

Additionally, vitamin D plays a role in bone health because it regulates calcium levels in the body. During winter, not only is there a decrease in vitamin D levels, but there is also an increase in serum parathyroid hormone levels and an increase in bone resorption, which weaken the bones. This effect is associated with a higher risk of injury from falling, as well as hip and wrist fractures, based on a study of ambulatory women in Australia. Although having a fracture can lead to problematic consequences for anyone, it can predict early mortality for the older population, which is already at risk of vitamin D deficiency and broken bones. There is a high mortality among those who have a hip fracture, typically due to complications of the fracture.

One older study found the postoperative mortality at one year was 27.3 percent, and at the end of the 9-year follow-up period, it was 79 percent. After adjusting for age, men had a higher risk with a hazard ratio of 1.55 (95% CI: 1.21-2.00). These findings point to a three-fold higher risk of death in patients with hip fractures than in the general population. Other factors that have been found to increase the risk of mortality after hip fractures are the following: age, American Score of Anesthesiologists (ASA) classification, institutional residence, cancer, dementia, diabetes, chronic illness, cardiovascular problems, and overall poor health. Vitamin D deficiency may exacerbate or advance these outcomes.

Due to vitamin D’s important role in the body, monitoring vitamin D status has become routine. However, seasonal variance should be considered to ensure sufficient levels throughout the year, and more frequent assessments of vitamin D status are recommended.

Melatonin

Melatonin plays several critical functions in the body, including regulating the immune system and glucose and providing antioxidant defenses against excessive oxidative stress. It is well known for being a significant regulator of circadian rhythms, including the sleep-wake cycle. As such, it is highly impacted by the length of days and nights, so it also has a seasonal variance, with higher serum levels during the winter compared to summer.

Alterations to melatonin levels could impact susceptibility to various diseases, including autoimmune disorders, neurodegenerative disorders, gastrointestinal imbalances, and bone loss. Melatonin also has cardioprotective properties. It is anti-inflammatory, acts as an antioxidant, and is anti-hypertensive. Disruption in melatonin production and the subsequent circadian rhythms have been implicated in several chronic illnesses, including obesity and metabolic syndrome. Melatonin also has a role in innate immunity. It might act as a buffer, which means it stimulates the immune system when needed or provides anti-inflammatory responses when the immune system is already overly stimulated.

With the seasonal variance of melatonin, many of its health benefits could be affected, leaving one more susceptible during certain times of the year. For example, the seasonality associated with MS has been linked to changes in melatonin levels rather than solely environmental factors. According to one study, there is a negative correlation between MS activity and night length, most likely due to interruptions in the expression of T-cells.

Temperature

Extremely cold and hot weather can negatively impact health, especially in older adults, children, and vulnerable populations. The impact of the weather and ambient temperature depends on the climate in which a person lives. However, in a multi-country study reviewing data that spanned around a decade from a total of 384 locations in thirteen countries with diverse climates, including Australia, Canada, China, Italy, Spain, Sweden, the U.K., and the U.S., researchers found ambient temperature might play a role in mortality risk. Deaths attributed to temperature were 7.71 percent of the mortality for the countries studied. Interestingly, moderately-cold and moderately-hot weather patterns were associated with more mortality risk. Additionally, cold temperatures (versus hot) were linked to increased mortality. Nonetheless, temperature is an essential factor when considering health risks.

These and similar studies focus on the association between ambient temperature and mortality. However, temperature can also affect other health conditions, such as heat stroke or hypothermia, apart from temperature-specific illnesses. For example, both hot and cold weather variability are associated with increased mortality from cardiovascular and respiratory diseases. One study found that for each 1°C rise in temperature related to climate change, the mortality risk increases by 6.12%. Heat is well-known for exacerbating MS. Temperature also impacts environmental factors, increasing or decreasing the spread of pathogenic bacteria and viruses. Therefore, temperature has the potential to act as a trigger for a wide range of acute and chronic illnesses, leading to a seasonal pattern as the temperature changes throughout the year.

The mechanisms behind the seasonal variance in human diseases are complex. Several factors are involved beyond the above, including dietary changes and pollution levels. Some of these can be modulated to reduce the impact of seasonality on health status, such as ensuring vitamin D sufficiency and adequate melatonin levels throughout the year. Recognizing how the changing seasons affect each individual—and looking at possible underlying reasons for it—might help the individual find ways to mitigate the impact.

Ways to Prevent Infectious Disease, No Matter What the Season

• Be aware of what diseases are in season.
• Wash hands thoroughly.
• Avoid rubbing eyes.
• Cultivate a healthy microbiome.
• Nourish the immune system by eating a nutrient-rich diet to avoid deficiencies.
• Supplement with immune-boosting nutrients and herbs as appropriate, according to a practitioner’s recommendations (e.g., vitamin C, probiotics, iron, zinc, vitamin A, vitamin E, B-vitamins, especially B6 and B12, selenium, quercetin, and other antioxidants).

Support Sufficient Vitamin D Levels

• Depending on skin type and sensitivity to sun exposure, spend between 5 and 30 minutes outside between 10 AM and 3 PM twice a week without sunscreen with at least an arm and face exposed to the sun.
• Consume vitamin D-rich foods, including cod liver oil, salmon, fortified orange juice, and fortified alternative milks.
• Supplement vitamin D at a dosage recommended by a healthcare practitioner, especially in the winter.

Maintain Healthy Levels of Melatonin  

• Keep melatonin levels at the appropriate levels, especially in the winter.
• Spend some time outdoors in the morning and maintain a solid sleep hygiene practice to support circadian rhythms.
• Support melatonin production by consuming sufficient levels of tryptophan, folate, vitamin B6, zinc, and magnesium.
• Supplement with melatonin when appropriate under the guidance of a health practitioner.

Maintain a Moderate Temperature 

• Use caution in extreme temperatures, especially extreme cold and extreme heat.

As always, seek individualized guidance from a healthcare practitioner during this time to better prepare for seasonal changes.