Vitamin D reduces incidence and duration of colds in those with low levels

Deficiency vs. supplementation

• Many comments stress the study only applies to adults with low baseline vitamin D; results should not be generalized to people with normal levels.
• Several note that vitamin D deficiency is common, especially in winter or high latitudes, and that correcting a deficiency of any essential nutrient will usually improve health and resilience to infections.

Anecdotes, placebo, and onset of effect

• Multiple people report fewer or milder colds after starting daily vitamin D, often at 2,000–5,000 IU.
• Others challenge self-assessment (“how do you know it helped?”), pointing out colds are self-limiting and placebo effects and regression to the mean are strong.
• Some note that vitamin D levels change over weeks, so “loading” for a few days when already sick may have limited physiological impact unless very deficient.

Dosage, safety, and toxicity

• Suggested doses range from 600 IU to 10,000+ IU daily; there is large disagreement on what is “safe”.
• Several cite conventional guidance of 4,000 IU/day as an upper limit without supervision and warn about hypercalcemia, kidney issues, and very slow washout after overdose.
• Others argue historical and recent data suggest much higher intakes can be safe for many people, but emphasize wide individual variation and the need for blood tests.
• Co-supplementation with magnesium and vitamin K2 is frequently recommended; some mention fat intake and timing affect absorption.

Sunlight, geography, and lifestyle

• Commenters in northern regions (PNW, Canada, UK, etc.) say winter UV-B is too weak or sun angle too low to make meaningful vitamin D, even with significant skin exposure.
• There’s discussion of heliotherapy and the broader health benefits of time outdoors vs. modern indoor lifestyles.

Evidence quality and broader vitamin debate

• A Lancet meta-analysis is cited suggesting no overall effect of vitamin D on respiratory infections, with debate about subgroups (deficient vs. non-deficient, dose, outcome type).
• Several criticize the trial’s journal, rapid peer review, near-perfect retention, sparse author info, and minimal control of confounders; some call it “shady”.
• Others argue that, despite noisy literature and unclear “optimal” levels, vitamin D is cheap, generally safe at moderate doses, and plausible enough that trying it—ideally guided by lab tests—can be rational.