This happened to us too, until I heard the podcast about Vitamin D. Nipped that in the bud almost immediately.
I still was sick all the time. That combination of not enough sleep (kids crying in the night etc.), work stress and insane amount of viruses from nursery is killer.
Not well known but there are people who simply can not absorb D thru the gut. A high school friend is one. Shouldn’t discourage the 90%+ who can benefit because of exceptions.
I raised vitamin D intake in winter, I take betaglucans and if I feel like I am starting to get under the weather I take 2-3g of vitamin C per day.
Since we started that, no colds or flus for me or my wife and we have a kindergarten kid.
Result: not been sick more than a day or two for fifteen years. And rarely, once or twice a winter.
One exception, during covid I had it and it lasted five full days at medium intensity, then went away permanently.
This is not a “placebo effect” at all, was like turning on a light switch, immediate results. During the winter I heard about it, I’d been sick for almost three weeks straight, couldn’t shake it. Naysayers look silly in my opinion, or are perhaps an exception I mentioned above.