The deficiencies with the strongest peer-reviewed evidence for producing measurable skin effects: vitamin D, vitamin B12, zinc, iron/ferritin, and omega-3 status.
Each shows up on skin differently.
Vitamin D (25-OH) — inadequacy is associated with barrier dysfunction, increased inflammatory skin conditions, and slower wound healing (Bikle, 2011). The clinical normal range (30 ng/mL+) is inadequate for skin function; 40-60 is a better target.
Vitamin B12 — deficiency produces skin pallor, glossitis, angular cheilitis (mouth-corner cracks), and hyperpigmentation, particularly on knuckles and joints. Common in vegetarians, older adults, and women on long-term acid reducers (Kannan, 2008).
Zinc — deficiency shows up as slow wound healing, acne severity, white nail spots, and increased skin infections. Zinc is a required cofactor for collagen synthesis (Ogawa, 2018).
Iron/ferritin — low ferritin (below ~40 ng/mL) is associated with pallor, dry skin, brittle nails, and hair thinning. Common in menstruating women (Trost, 2006).
Omega-3 fatty acids — low omega-3 index (below ~5%) is associated with barrier dysfunction, dryness that moisturizer doesn't fix, and increased inflammatory reactivity (Pilkington, 2011).
Notably, the clinical reference ranges for all five sit well below what dermatology-supportive levels look like. Standard blood work will call you fine at levels that skin still struggles with.
Four of these five markers are on the JenSkin panel.