There were some trials in the past that showed increased lung-cancer incidence with "Vitamin A" supplementation in smokers. That also led to questions regarding the safety of retinol. Since beta-carotine supplementation was central in those trials the following study seems interesting:
Naturally occurring eccentric cleavage products of provitamin A β-carotene function as antagonists of retinoic acid receptors. - PubMed - NCBI
... These findings suggest that β-apocarotenoids function as naturally occurring retinoid antagonists. The antagonism of retinoid signaling by these metabolites may have implications for the activities of dietary β-carotene as a provitamin A and as a modulator of risk for cardiovascular disease and cancer.
Although the mechanisms responsible for the formation of the eccentric cleavage products of β-carotene in mammals are not fully known, it is clear that some of the long chain β-apocarotenals (e.g. 8′, 10′, 12′, 14′) are found in the plasma of humans (12) and experimental animals (13) and that these are increased under conditions of oxidative stress and high dietary doses of β-carotene (24).
Our analyses of both β-carotene-containing animal diets and fruits containing β-carotene suggest that any dietary source of β-carotene also contains β-apocarotenoids. It may also be useful to consider these findings in attempts to alleviate vitamin A deficiency in humans through the biofortification of crops with high levels of β-carotene.
Naturally occurring eccentric cleavage products of provitamin A β-carotene function as antagonists of retinoic acid receptors. - PubMed - NCBI
... These findings suggest that β-apocarotenoids function as naturally occurring retinoid antagonists. The antagonism of retinoid signaling by these metabolites may have implications for the activities of dietary β-carotene as a provitamin A and as a modulator of risk for cardiovascular disease and cancer.
Although the mechanisms responsible for the formation of the eccentric cleavage products of β-carotene in mammals are not fully known, it is clear that some of the long chain β-apocarotenals (e.g. 8′, 10′, 12′, 14′) are found in the plasma of humans (12) and experimental animals (13) and that these are increased under conditions of oxidative stress and high dietary doses of β-carotene (24).
Our analyses of both β-carotene-containing animal diets and fruits containing β-carotene suggest that any dietary source of β-carotene also contains β-apocarotenoids. It may also be useful to consider these findings in attempts to alleviate vitamin A deficiency in humans through the biofortification of crops with high levels of β-carotene.
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