Spring, summer, fall, winter: four seasons to every year. According to a fascinating new study, our bodies and its unique population of gut and nasal bacteria recognize only 2 distinct seasons, and these bi-seasonal fluctuations affect everything from blood pressure, inflammation, and insulin sensitivity to tumor development.
Published in the journal Nature Communications, this new evidence of human, bacterial, and fungal seasonality has illuminated ground-breaking information about how our bodies adapt to changes in our environments. By studying the microbiome populations found within the nasal linings and the guts in 100 individuals living in the same region in California for a 4-year period, scientists have made astute observations which define our calendar years and changing seasons into just two significant periods.
The scientists tracked over 1,000 physiological variations which correspond to two seasonal patterns, dividing the year into (1) late spring and (2) late fall and early winter. Aside from measuring and identifying bacteria and fungi present in these study participants, they also monitored weather changes and the changes associated with the pollens and corresponding meteorological data. The first seasonal pattern is dubbed “omics seasonal pattern one” and peaks in the last part of April, while pattern two peaks in December.
Pattern 1: Late Spring
The characteristics of pattern one include the highest expression of molecules which are related to cardiovascular disease, as well as schizophrenia, seizures, sleep patterns, and even down to development of collagen. During late spring, blood pressure is higher, often due to factors from the previous winter where physical activity is lower.
Joint inflammation, especially knee inflammation, osteoarthritis, and rheumatoid arthritis are elevated in this seasonal pattern due to a higher expression of inflammation pathways. Not surprisingly, allergy gene expression also peaks in late spring and early summer.
The researchers also identified that a gene responsible for circadian rhythms peaks during the late spring—but what’s interesting is that this particular gene, when deregulated, is one of the key players in cancer development in mammals. When they looked further into this seasonal expression, they discovered that there is a seasonal variation in tumor stages, along with more cancer diagnoses occurring in the spring. The highest concentration of localized tumors are also discovered most in the springtime.
The microbes that live in the nasal passages change composition depending on the bi-seasonal patterns as well: during the late spring, the diversity of the nasal microbes are at their highest. The researchers found that these changes in nasal microbiota correlated to changes in airborne pollen and fungi counts, suggesting that the changes in the airborne environment directly affect the population of bacteria found in our respirator systems.
The blood marker, A1c, which is a measure of blood glucose levels over the past 100 days, is also at its highest during seasonal pattern 1, which has been noted in previous studies. Blood glucose tends to be higher in the winter in diabetic patients. Since the A1c test is a snapshot of the past 100 days, it makes sense that it would be higher in the spring, as it represents a picture of the blood sugar over the winter time period when physical activity is reduced and holiday foods may be consumed.
The researchers also studied individuals diagnosed with insulin sensitivity and insulin resistance, and they found distinct variations in bacterial diversity. These bacteria also exhibited seasonal preferences. The differences in the microbiota diversity were apparent throughout the whole year, except during seasonal pattern one, when the gap between the microbial population became narrower.
Diversity in gut microbiome population is significantly higher during this pattern, while nasal bacteria is less diverse at this time.
Pattern 2: Late Fall/Early Winter
In omics pattern two, the gut microbiome reaches its highest level of diversification. Immune responses and interleukin pathways are highest, coinciding for the time of year when viral illnesses are most common. Other families of genes responsible for helping fight infections also exhibit their highest expression during this time.
Inflammation in the myocardium of the heart, and esterification of cholesterol (a process necessary to help “good” HDL cholesterol particles remove free cholesterol from the body and transport it to the liver for processing and elimination) are also highest in late fall/early winter.
Acne is found to worsen in winter, which researchers believe is due to a combination of decreased humidity and the increased permeability of skin because of cold air temperature. Other factors include a thickening of the epidermis and the stimulation of inflammatory mediators produced in the skin during this time.
The researchers also noted that the volume of urine produced is also highest during omics pattern two.
This study used some of the best resources we have for understanding the complex and dynamic changes our bodies go through: its bacterial population. The importance of the microbiome in the gut has received a lot of attention lately, but studying the nasal microbiome gives us a much more precise glimpse into what’s occurring in our immediate and shared environments. Gut microbiome composition is extremely individualized and influenced by foods and genetic predispositions, so it’s not useful for finding correlations with all people.
The study of how the microbes inside our respiratory system are affected by changes in the air in a localized region offers a unique examination of how we are reacting to the air around us. This study was done on a localized and small population in California, but it does open up some interesting avenues for tracing the health of residents through the changes in their ambient environment.
Sailani, M.R., Metwally, A.A., Zhou, W. et al. Deep longitudinal multiomics profiling reveals two biological seasonal patterns in California. Nat Commun 11, 4933 (2020). https://doi.org/10.1038/s41467-020-18758-1
Bagheri, Babak et al. “The Ratio of Unesterified/esterified Cholesterol is the Major Determinant of Atherogenicity of Lipoprotein Fractions.” Medical archives (Sarajevo, Bosnia and Herzegovina) vol. 72,2 (2018): 103-107. doi:10.5455/medarh.2018.72.103-107