A daily intake of water soluble vitamins in needed by humans. However, a regular intake of fat soluble vitamins is not required because humans have a capacity for storage. The liver is able to store some fat soluble vitamins such as vitamin A, but the largest store comprises the adipose tissue. Adipose tissue is metabolically active and a continual turnover of its contents occurs. This releases vitamins stored within the tissue to circulation, providing a plasma level representative of long term intake, irrespective of short term ingestion. Two vitamins important to humans that are known to be stored extensively in adipose tissue are vitamin E and the carotenoids. Vitamin E is generic name for a group of 8 isomers which all share the activity of α-tocopherol and the carotenoids are a group of plant compounds, some of which possess vitamin A activity.
Both the carotenoids and vitamin E are fat soluble antioxidants in vitro, and their storage in adipose tissue plays an important role in their metabolic fate. Data for the vitamin E and carotenoid content of adipose tissue comes from studies that have used tissue samples from hospitalised subects1. The vitamins were extracted using biopsies and identified using high performance liquid chromatography. Interestingly, there was a large variation in the carotenoid and vitamin E concentrations within the 19 subjects. In fact, the total carotenoid concentration varied 40-fold from and the total vitamin E concentration varied 11-fold. This may reflect dietary intake or may result from variation in absorption, metabolism and excretion rates. Researchers are increasingly showing that variation in vitamin status in humans is highly variable between subjects and this may have implications for the susceptibility of some individuals to disease.
Of the carotenoids present in adipose tissue, lycopene and β-carotene predominated as expected. It is known that these carotenoids are present in the diets of humans at levels higher than other carotenoids, which explains their adipose concentrations. Lycopene intakes can be particularly high in those consuming products containing tomatoes. Lycopene and β-carotene comprised 18.5 and 20.2 % of the total carotenoids in the samples taken. Of the vitamin E isomers, α-tocopherol was the most predominate form. Because the human tocopherol transfer protein (TTP) in the liver preferentially recognises α-tocopherol, this form predominates in the plasma and is therefore likely to be the most concentrated in adipose tissue. In fact, high intakes of γ-tocopherol as may be found in the typical United States diet still result in accumulation of α-tocopherol in plasma and adipose tissue. In the subjects tested, α-tocopherol comprised of up to 80 % of the total vitamin E in adipose tissue.
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