> I mean, maybe the human body really does have such a tight tolerance, it really needs 75% of what would be dangerous to maximize health.
Since your body produces Vitamin D from sunlight - and that process is self-limiting - it's certainly biologically possible that the margins really are that small. Historically most people spent far more time outside than we do now.
Unfortunately if that is true, the only reliable way to get the dosing right without expensive testing would probably be sun exposure, which many people simply aren't going to get with modern lifestyles. There's also no evolutionary reason why there couldn't be an inherent trade-off between Vitamin D and UV-related skin damage/skin cancer: evolution tends towards "good enough" rather than "perfect".
Since your body produces Vitamin D from sunlight - and that process is self-limiting - it's certainly biologically possible that the margins really are that small. Historically most people spent far more time outside than we do now.
Unfortunately if that is true, the only reliable way to get the dosing right without expensive testing would probably be sun exposure, which many people simply aren't going to get with modern lifestyles. There's also no evolutionary reason why there couldn't be an inherent trade-off between Vitamin D and UV-related skin damage/skin cancer: evolution tends towards "good enough" rather than "perfect".