The holiday season is magical: we spend lots of time with friends and family
And speaking of tradition, everybody has their own supposed immune-boosting or cold-relieving "secrets:" their grandma's tonic recipe, a laundry list supplement regimen, or even an intense sweat session. But do these methods actually work?
If you're like us and want to a) emerge on the other side of the holiday season without a nose rubbed raw from tissues, and b) do so by making decisions based on science, here's the evidence to get you there.
The biomarkers (and foods) to focus onWhite blood cells (WBCs)
They're the troops of your immune system: they're the cells that fight against infection and mount your defenses for future attacks. Your WBC levels are therefore a direct reflection of the current state of your immune system and overall health.
Cortisol rises due to many different types of physiological stress, be it physical, emotional, or even mental. But this biomarker is especially relevant during the cold, dry winter months: studies show that rising levels can suppress your immune system, leaving you susceptible to all things cold & flu.1
If you do get unlucky with your wintertime wellness, you can blame invaders like viruses and bacteria for increased hsCRP levels. Infections can cause full-body inflammation as your immune system tries to fight the good fight, which manifests as heightened levels of indicators like hsCRP. That's why anti-inflammatory foods like avocado, nuts, and soybeans are so popular for maintaining immune function.
Proper electrolyte levels (like potassium and sodium) are essential to a full-force immune response. They're what directs water – a key component for countless biological processes – to the cells that need it. Potassium can be much more difficult to get in the diet than sodium, so prioritize eating K-rich foods like fatty fish, squash, and potatoes.
Iron is essential for the proper development and maturation of WBCs, and therefore optimal iron levels are critical for a fully-developed immune response.2 But like humans, bacteria need iron for important metabolic functions.3 Therefore, blood iron must be tightly regulated: too little can make the immune system weak, and too much can make invaders strong. A good way to thread this needle is to get your iron from food rather than supplements. Iron-rich foods include red meat (note: not poultry), beans, and shellfish.
Magnesium has a role in multiple different layers of your immune system, and therefore even a temporary disturbance or depletion can leave you susceptible to infection.4 But you're in luck: magnesium-rich foods include whole grains, nuts, and seeds.
Evidence-based immune-boosting tipsHere are some actionable tips to help keep you from being bed-ridden in the first place.
Work the sweet spot with your workouts
Studies show that moderate-intensity workouts, like a bike ride or flow yoga, can actually help to give your immune system a slight boost, bolstering your defenses from illness-inducing invaders.5,6,7
But there are two sides to this coin. These same studies show that overexertion – like from an especially tough workout – can actually muffle your immune response to such invaders (perhaps because it temporarily depletes magnesium).4
So what's the takeaway? If you feel a tickle in your throat or have been reaching for tissues more often than usual, don't push it in the gym. Get a slight sweat on (sweat is part of the front lines of your immune defense, after all), but save the PRs for another day.
Hydrate, hydrate, hydrate
While conclusive evidence on the role of hydration in the development of chilly-weather illnesses is still TBD, evidence does show that proper hydration can help to ward off certain risk factors – like high cortisol levels – known to raise your chances of getting sick.8 In fact, proper hydration can actually soften cortisol spikes associated with intense exercise.9
So, while a tall glass of water won't exactly warm your bones on a chilly day, it can help keep the chills at bay. If you find yourself struggling to hit your daily water goals (.5-1 oz per pound of body weight), try things like herbal tea or seltzer water to switch things up. Stay away from juices and other sweetened drinks, though – their high sugar content make them great fuel for unhealthy bacteria in the gut.
Prep the system with probiotics
While we're on the subject of the gut, let's talk about the helpful bacteria that live there. One out of every two cells in your body is actually a bacteria cell that helps to digest food, regulate cellular processes, and, yes, fight their harmful counterparts. The strains Lactobacillus and Bifidobacterium are especially effective at the latter; they have both been shown to block pathogen invasion.10
Unfortunately, many probiotic supplement brands make products that are not effective; you're better off getting these two strains from fermented foods like sauerkraut, kimchi, yogurt, tempeh, and kombucha.
Feel like a million when the wind chill is zeroSo, even if you're someone who "always" gets sick this time of year, there are steps you can take to reverse this trend. And next time you get some suspicious immune-boosting advice, know your good ol' friend Science will always be there for you.
 Tønnesen E, Christensen NJ, Brinkløv MM. (1987). Natural killer cell activity during cortisol and adrenaline infusion in healthy volunteers. Eur. J. Clin. Invest., 17(6):497–503.
 Soyano, A., & Gómez, M. (1999). Role of iron in immunity and its relation with infections. Archivos
 Beard, J. L. (2001). Iron biology in immune function, muscle metabolism
 Laires, M. J., & Monteiro, C. (2008). Exercise, magnesium and immune function. Magnesium research, 21(2), 92-96.
 Pedersen, B. K., & Hoffman-Goetz, L. (2000). Exercise and the immune system: regulation, integration, and adaptation. Physiological reviews, 80(3), 1055-1081.
 Gleeson M, Nieman DC, Pedersen BK. (2004). Exercise, nutrition and immune function. J Sports Sci. 22(1):115–125. 3.
 Nieman DC. Exercise, infection, and immunity. (1994). Int J Sports Med. 15 Suppl 3
 Maresh CM, Whittlesey MJ, Armstrong LE, et al. Effect of hydration state on testosterone and cortisol responses to training-intensity exercise in collegiate runners. (2006). Int J Sports Med. 27(10):765–770
 Adams, W., & Casa, D. (n.d.). Immune Function: Basic Considerations of Exercise and Hydration. Storrs, CT: Korey Stringer Institute, University of Connecticut.
 Yan, F., & Polk, D. B. (2011). Probiotics and immune health. Current opinion in gastroenterology, 27(6), 496.