Nobody has tested vitamin D on ordinary tiredness
The trials recruited people with multiple sclerosis, post-COVID fatigue or cancer, or with levels below the deficient line. None enrolled someone simply tired with a normal level.
What the study found
The fatigue trials all recruited people who were ill or deficient
A four-week trial in 120 tired, deficient young adults found fatigue scores falling further on vitamin D3 than on placebo, though half on placebo improved too. The others recruited multiple sclerosis, post-COVID and cancer patients. None enrolled someone simply tired with a normal level.
The caveat · Self-rated tiredness, easily swayed
The four-week trial scored fatigue on a self-reported questionnaire, a measure that moves easily on expectation, in people averaging 29 years old. The multiple sclerosis pooling covered five trials and 345 people, and its benefit runs from small down to almost nothing. The palliative trial lost 94 of its 244 participants, mostly to death from cancer.
What we don't know
The four-week trial scored fatigue on a self-reported questionnaire, a measure that moves easily on expectation, in people averaging 29 years old. The multiple sclerosis pooling covered five trials and 345 people, and its benefit runs from small down to almost nothing. The palliative trial lost 94 of its 244 participants, mostly to death from cancer.