1. The plasma 25-hydroxycholecalciferol [25-(OH)D3] response to measured u.v. irradiation applied thrice weekly for 10 weeks was investigated in normal and in anticonvulsant-treated subjects.

2. Levels of plasma 25-(OH)D3 achieved after u.v. irradiation were similar in both normal and anticonvulsant-treated subjects, suggesting that hepatic microsomal enzyme induction does not lead to low plasma 25-(OH)D3 concentrations.

3. Cholecalciferol was present in plasma of normal subjects in a very low concentrations (< 5·0 nmol/l) and did not increase until plasma 25-(OH)D3 levels exceeded 62·5 nmol/1.

4. Cholecalciferol occurred in significant concentrations in plasma during whole body u.v. irradiation or during oral dosage of 62·5 nmol (1000 i.u) or more daily.

5. Plasma 25-(OH)D3 concentrations reached a steady state after 5–6 weeks of u.v. irradiation or of oral intake within the usual intake range.

6. Cholecalciferol synthesis in skin calculated from the steady-state equation was 0·0015 ± 0·0008 nmol/mJ.

7. Cholecalciferol synthesis in skin was also calculated from the oral dosage required to yield the same plasma 25-(OH)D3 concentration as u.v. irradiation and was 0.0024 ± 0.0018 nmol/mJ.

8. Rates of cholecalciferol synthesis calculated from these data suggest that many of the population of England receive insufficient u.v. irradiation to maintain vitamin D status throughout the year.

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