‘The human body responds to two seasonal shifts — exposure, exercise and diet determine how’

Michael Snyder is the Stanford W. Ascherman Professor of Genetics at Stanford University. He speaks with Srijana Mitra Das at TE about ways —and whys — seasons impact our physiology:

Forget festive sales, syrupy sweets or silken sweaters — the very molecules inside your body, those tiny collections of atoms bonded together, indicate when the seasons are changing and how you are going to like the shift. Further, your body may not always react to the widely understood ‘spring/summer/ autumn/winter’ quartet — you may simply respond to transitional periods, the seasonal portals of late spring-early summer and late autumn-early winter. So says Stanford genomicist Michael Snyder, who led a team studying molecular data spread over four years from 105 participants in California, aged 25 to 75, seeing how their systems corresponded to weather transitions.

Snyder, who gestures, gets up and moves energetically several times as we speak, describes his research. ‘We use big data to study human health. We do deep profiling on people and track them over time, so we can catch disease before it appears. A lot of my work is to do with wearables.’ He holds up his arms, covered in biosensors that look chunky, yet slim, black and Matrix-like, tracking variables from blood pressure to glucose. You might imagine someone loaded under a bunch of those might look a bit serious and ponderous about life but Snyder doesn’t — instead, he smiles broadly as he discusses his work.

Why does the human body react to two seasons, instead of four? Snyder explains, ‘Well, first, this study was based in California. So, the seasonal extremes are a little less. Second, if you think about it, ‘four seasons’ is just an arbitrary designation that humans made up, right? It’s not set in stone, so we went into this study agnostically, saying, let the biology tell us how many seasons there are. As it turned out, we found the body responding to two bursts when a lot of stuff happens. There are different exposures in these two phases, from temperature shifts to microbes and even your gut microbiome, which changes possibly because of new foods people eat. Alongside, a lot of metabolic differences occur while people’s activity level also differs significantly between warmer and cooler days. All these factors trigger changes in the body.’

Thus, Snyder’s study found the latespring period came with increases in inflammatory biomarkers involving allergies — there is also a rise in molecules involved in rheumatoid arthritis and osteoarthritis and spikes in HbA1c, a form of hemoglobin and a protein that signals risk for Type 2 diabetes rise. Snyder’s team linked the latter to indulgent eating through winter — put down that pie and just eat an apple — and a waning of vigorous exercise in colder months. Further, the gene PER1, which works to regulate the circadian sleep-wake cycle, was also at its highest then.

Following the data into winter, the study found an increase in immune molecules which fight viral infection alongside rises in molecules that contribute to acne. Further, molecular developments leading to hypertension or high blood pressure intensified in winter. The study also found differences in the microbiomes of people who processed glucose normally versus those who were insulin-resistant, which were around half the participants. Veillonella, a bacteria involved in lactic acid fermentation and glucose processing, was higher in insulin-resistant people through the year, except in mid-March through late June.

Why do pathways change in spring, causing all these impacts? Snyder replies, ‘Well, there are several immune burst changes that occur then. It’s caused by factors like getting exposed to more pollen outside. I think one reason why your glucose and hemoglobin A1C, a measure of your glucose levels, are worse then is exercise-related. People don’t exercise as much in the winter — not even in California. So, basically, their hemoglobin A1C goes up over the winter — then, spring comes and it starts going down.’ Snyder gets up, walks around a bit and sits down again as he waves his hands, emphasising, ‘I think this is a big deal because knowing this is going to happen, you can make more effort to exercise during the winter and avoid it — exercise is fundamental to everything here.’ Diet also plays a role. ‘There’s a tendency for people to eat less well, less mindfully, in winter because there is less produce and fresh fruit around, so we should pay more attention to eating better. Also, we saw hypertension start up in the late fall-early winter duration — blood pressure rising is linked to slowing down on exercise.’

Snyder muses and adds, ‘We really did expect the late fall-early winter phase to show more biological changes — it was surprising to find these instead in spring but it’s because people are coming out of a long, relatively inactive winter and they suddenly go out and find sun, pollen, etc., and many immunological things happen.’ Another interesting finding comes from winter when Snyder says higher depression can be experienced owing to reduced sunlight. As he says, ‘Mental conditions are deeply influenced by seasons.’

It’s quite likely, says Snyder, that these findings could vary in India which has several seasonal shifts and possibly more biological responses. What he does highlight is climate change now altering all these reactions — ‘Climate change is really going to affect our exposures, the amount of time we’re out in the sun, etc. It is concerning how this could impact the human system, which has evolved over millions of years to respond to seasonal transitions. We have to be on the alert for this.’