PHYS-35 · Principle
Do hard races and big blocks make me ill?
They may add risk. Infections in athletes go with heavy load, competition, travel and poor sleep; whether one hard session opens a window is disputed. Regular exercise goes with fewer colds.
How the body works, as far as the reviews below go: they disagree, and the entry gives both readings. A study entry shows what one experiment measured.
Practical Application
Do hard races and big blocks make me ill? They may add risk. Infections in athletes go with heavy load, competition, travel and poor sleep; whether one hard session opens a window is disputed. Regular exercise goes with fewer colds.
What it does not mean: that hard sessions are dangerous on their own, or that a dip in blood immune cells after a session is proof of suppression.
Mechanism
Blood lymphocyte numbers and function fall for 1-2 h after hard exercise; one review reads this as cells redeploying to tissues for surveillance (Campbell & Turner 2018), another as part of the transient immune change after heavy exertion (Nieman & Wentz 2019).
After the 1987 Los Angeles Marathon, 12.9% of finishers reported an infection within a week against 2.2% of runners who did not start; few such reported episodes were confirmed in the laboratory, 11 of 37 in one study (Campbell & Turner 2018).
Athletes average about 1.8 respiratory infections a year against about 2.3 in the general population (REF-1233); in 1002 adults, aerobic exercise on 5 or more days a week went with 43% fewer days of upper-respiratory illness than near-sedentary living (Nieman & Wentz 2019).
After a 3-day intensified block, runners had far more muscle damage and inflammation than cyclists but the same fall in innate immune function and the same 12-week symptoms (REF-235; reading this as an "open window" is the base's inference: the abstract credits the fall to the 3-day block).
Common misreading
"After a hard session there is a 72-hour danger window"; "every sore throat after a race is an infection".
Safety (base practice): fever, chest symptoms or breathlessness out of proportion mean no training (SYNTH-36).