skin · Outcome
Hair density, growth, and breakage resistance.
How to measure:
Three levers, one stack. Each works through a different mechanism. They compound.
Major
2000-5000 IU daily
Effect size
Corpus
5 papers
0 small · 2 moderate · 3 large · 4 unclear
“Patients with alopecia areata have lower serum vitamin D levels compared to healthy controls.”
No corpus-backed pick yet
We require at least 3 corpus papers for a habit to surface here. The literature on this outcome × pillar is still being extracted.
No corpus-backed pick yet
We require at least 3 corpus papers for a food to surface here. The literature on this outcome × pillar is still being extracted.
Everything we found, ranked by how strongly we recommend it. Tap any to see the studies.
Top pick
The one we recommend first
Also recommended
Evidence-backed alternatives or additions
Worth considering
Only in specific contexts (see card)
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Commonly suggested, not supported
Every pick has two signals. They answer different questions and we never bundle them into one number.
Effect size
How big is the change on hair quality, in the units that matter for this outcome.
Confidence
How sure are we the effect is real for hair quality, given the studies we have.
For every pick, the “papers” number is how many studies we've cataloged that test this entity for hair quality. “Favorable” means the study reported the direction you want.
The tier badge (S, A, B…) on the entity itself is its overall research confidence across all outcomes. It's the same letter wherever you see this entity on the site, not specific to hair quality.
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