Why Runners Get Ill More Often in Winter (And What Actually Helps)
Ask around any club or training group in January, and you'll hear the same pattern: someone's just come down with a cold in the middle of their best training block of the winter. It's a well-known frustration in endurance sport, and it's not just bad luck or coincidence — there's a genuine, well-discussed overlap between heavy winter training and a higher rate of minor illness.
Why heavy training and winter illness overlap
Sports nutrition circles commonly discuss a temporary dip in immune resilience in the hours after a genuinely hard or long session — sometimes referred to as an "open window" effect — combined with shorter daylight hours, colder temperatures, and more time spent in enclosed spaces (gyms, changing rooms, indoor training sessions, crowded commutes to early winter runs) where illness spreads more easily. Put those factors together across a demanding winter marathon block and it's easy to see why club runners report picking up more colds in December and January than at any other point in the year.
The training-load balance most runners get wrong
The instinct when marathon training feels behind schedule is often to add more volume or intensity to catch up — exactly the wrong move if you're already run down. Recognising early fatigue and mild illness signs and backing off for a few days, rather than pushing through a demanding session while already under the weather, tends to cost far less overall training time than training through illness and ending up properly unwell for a week or two.
What actually helps
Sensible basics matter more than anything exotic here: enough sleep, not stacking too many hard sessions back-to-back without recovery, and covering the nutritional gaps that are specifically worse in a UK winter — vitamin D being the clearest example, as NHS vitamin D guidance lays out for the general population. Layering up appropriately for cold-weather sessions, and not lingering in wet kit after a run in freezing temperatures, are both simple, unglamorous habits that reduce the general burden on your immune system through a hard winter block.
Covering the basics daily
RunnerVits' Morning Runner Essentials combines Vitamin C (500mg) for immune support with Vitamin D3, K2 and B12 — a simple daily foundation rather than a reactive "cold and flu" product you only reach for once symptoms have already started, which by then is really just managing symptoms rather than supporting resilience in the first place.
Quick answers
Should I keep training if I already feel a cold coming on?
A widely used rule of thumb in sports medicine circles is the "neck check" — symptoms above the neck (mild congestion, sore throat) are often fine to train through at reduced intensity, while symptoms below the neck (chest, fever, body aches) are a clearer signal to rest.
Does cold weather itself cause colds?
Not directly — colds are caused by viruses, not temperature — but cold weather changes behaviour (more time indoors in close proximity to others, drier air) in ways that make transmission more likely, which is the more accurate version of the "cold weather makes you ill" idea.