Vitamin A, an essential nutrient from food, supplements or topical creams, powers dim-light vision and maintains the eye, gut and airway linings. The strongest benefits appear when a shortfall is corrected; retinol creams reliably soften fine wrinkles. Long-term excess can scar the liver and may weaken bones and harm a developing baby, though studies disagree. High-dose beta-carotene raised lung cancer in smokers. Evidence links benefit to sufficiency, not high intake. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Serum retinol | 50–70 µg/dL | Detects deficiency and excess |
| Fasting retinyl esters | Below 10% of total serum vitamin A | Early sign of excess |
| hs-CRP | Below 1.0 mg/L | Flags inflammation that lowers retinol |
| ALT | 10–25 U/L | Screens for liver injury |
| Serum calcium | 9.0–10.0 mg/dL | Detects high blood calcium |
| 25-hydroxyvitamin D | 40–60 ng/mL | Offsets vitamin A's blunting of vitamin D |
| Serum zinc | 90–120 µg/dL | Zinc needed to mobilize stores |
| Triglycerides | Below 100 mg/dL | Retinoids can raise blood fats |
| Bone density (DXA T-score) | −1.0 or higher | Tracks fracture risk |
| Pregnancy test | Negative | Avoids exposure that can cause birth defects |
Cadence: Baseline before supplementing above the dietary allowance; serum retinol, retinyl esters and liver enzymes at 8–12 weeks after starting or changing the dose, then every 6–12 months; bone density every two years for long-term higher intakes. Diet or topical retinol only: no routine vitamin A testing.