carotenoid conversion to vitamin A

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In a message dated 9/30/02 5:03:09 AM,

writes:

>hi liz, sorry to bug you for sources again, but i *thought* this

>

>was the case (re possible inefficient conversion) and am interested in

>

>reading primary researh on this...do you recall where you read it?

Can't remember where I originally read it but here are a couple of references

that talk about the revised conversion factor:

J Nutr 2002 Sep;132(9):2920S-6S

 

Consequences of revised estimates of carotenoid bioefficacy for dietary

control of vitamin a deficiency in developing countries.

West CE, Eilander A, Van Lieshout M.

Division of Human Nutrition and Epidemiology, Wageningen University,

Wageningen, The Netherlands and. Department of Gastroenterology, University

Medical Centre Nijmegen, Nijmegen, The Netherlands.

According to existing recommendations of the Food and Agriculture

Organization (FAO)/World Health Organization (WHO), the amount of provitamin

A in a mixed diet having the same vitamin A activity as 1 micro g of retinol

is 6 micro g of beta-carotene or 12 micro g of other provitamin A

carotenoids. The efficiency of this conversion is referred to as bioefficacy.

Recently, using data from healthy people in developed countries and based on

a two-step process, the U.S. Institute of Medicine (IOM) derived new

conversion factors. The first step established the bioefficacy of

beta-carotene in oil at 2 micro g having the same vitamin A activity as 1

micro g of retinol; the second step established the bioavailability of

beta-carotene in foods relative to that of beta-carotene in oil at 1:6. Thus,

2 micro g of beta-carotene in oil or 12 micro g of beta-carotene in mixed

foods has the same vitamin A activity as 1 micro g of retinol. Based on

existing FAO food balance sheets and the FAO/WHO conversion rates, all

populations should be able to meet their vitamin A requirements from existing

dietary sources. However, using the new IOM conversion rates, populations in

developing countries could not achieve adequacy. Additionally, field studies

suggest that, instead of 12 micro g, 21 micro g of beta-carotene has the same

vitamin A activity as 1 micro g of retinol, which implies that effective

vitamin A intake is even lower. Therefore, controlling vitamin A deficiency

in developing countries requires not only vitamin A supplementation but also

food-based approaches, including food fortification, and possibly the

introduction of new strains of plants with enhanced vitamin A activity.

and:

Am J Clin Nutr 2002 May;75(5):900-7 Related Articles, Links

 

Variability in conversion of beta-carotene to vitamin A in men as measured by

using a double-tracer study design.

Hickenbottom SJ, Follett JR, Lin Y, Dueker SR, Burri BJ, Neidlinger TR,

Clifford AJ.

Department of Nutrition, University of California, , CA 95616, USA.

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. DESIGN: 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. CONCLUSION: The vitamin A activity of

beta-carotene, even when measured under controlled conditions, can be

surprisingly low and variable.

Namaste, Liz

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