title: Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design.
abstract: BACKGROUND
  The vitamin A activity of beta-carotene is variable and surprisingly low in women. The reasons for this are not well understood. The vitamin A activity of beta-carotene in men is still uncertain. Contributions of dietary factors compared with individual traits are largely unknown.
  
  
  OBJECTIVE
  Our objective was to measure the intrinsic variability in the vitamin A activity of beta-carotene among healthy, well-fed men living in a controlled environment.
  
  
  METHODS
  We used a double-tracer test-retest design. We dosed 11 healthy men orally with 30 micromol hexadeuterated (D6) retinyl acetate (all-trans-19,19,19,20,20,20-[2H6]retinyl acetate) and then with 37 micromol D6 beta-carotene (19,19,19,19',19',19'-[2H6]beta-carotene) 1 wk later. Doses were taken with breakfasts containing 16 g fat. We measured D6 retinol, D6 beta-carotene, and trideuterated (D3) retinol (derived from D6 beta-carotene) concentrations in plasma. Areas under the plasma concentration x time since dosing curves (AUCs) were determined for D6 retinol, D6 beta-carotene, and D3 retinol.
  
  
  RESULTS
  All men had detectable D6 retinol concentrations in plasma. The mean (+/-SE) absorption of D6 beta-carotene in all subjects was 2.235 +/- 0.925%, and the mean conversion ratio was 0.0296 +/- 0.0108 mol retinol to 1 mol beta-carotene. Only 6 of 11 men had sufficient plasma concentrations of D6 beta-carotene and D3 retinol that we could measure. The mean absorption of D6 beta-carotene in these 6 subjects was 4.097 +/- 1.208%, and the mean conversion ratio was 0.0540 +/- 0.0128 mol retinol to 1 mol beta-carotene.
  
  
  CONCLUSIONS
  The vitamin A activity of beta-carotene, even when measured under controlled conditions, can be surprisingly low and variable.
authors.affiliation: Department of Nutrition, University of California, Davis, CA 95616, USA.
authors.forename: Sabrina J
authors.initials: SJ
authors.lastname: Hickenbottom
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Follett
authors.forename: Yumei
authors.initials: Y
authors.lastname: Lin
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Dueker
authors.forename: Betty J
authors.initials: BJ
authors.lastname: Burri
authors.forename: Terry R
authors.initials: TR
authors.lastname: Neidlinger
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Clifford
mesh_terms: D000042:Absorption; D000328:Adult; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D012016:Reference Values; D014801:Vitamin A / Q000096:biosynthesis* / Q000097:blood; D019207:beta Carotene / Q000097:blood / Q000378:metabolism* / Q000493:pharmacokinetics
chemical_list: D019207:beta Carotene; D014801:Vitamin A