I had a genetic test done (not 23andme or other retail genetic tests) and it turns out I have genes that essentially cause me to have low absorption for vitamins d, b6 and b12 and vitamin e. According to my bloodwork I also had extremely low Vit d in the blood (way below average). I started megadosing and honestly I feel like a different person, when I got a cold before it would last for 7 days, and twice it led to bronchitis and then pneumonia this year since megadosing I’ve had two colds and both lasted no more than 2 days. So I’m not superman, I still get sick but the severity of the colds have been significantly lower than before megadosing.