Plants produce a wide range of chemical compounds (phytonutrients) that can have a beneficial effects on human health. Of these the carotenoids are one of the most widely researched. Carotenoids demonstrate a large variety of biological effects, and have been shown to have a potentially protective effect against cancer and cardiovascular disease. The health effects of carotenoids in general are dependent on the compounds being effectively absorbed and then efficiently passed to the circulation where they can exert biological effects. However, plant cells possess a cellulose cell wall which is not able to be digested by human enzymes in the gut. In order for the absorption of contained carotenoids to proceed, the cells containing the compounds must be physically broken open. Chewing will break open a small number of the cells in the oral cavity.
Research supports the contention that plant cell wall digestibility is a limiting factor in human carotenoid absorption, because studies have demonstrated that cooking, pre-digesting or mincing fruit and vegetables increases their bioavailability. For example, a study published in the Journal of Nutrition in 19991 demonstrated that the bioavailability of β-carotene was almost doubled when the whole leaf spinach was enzymatically pre-digested before consumption, and that simply mincing the spinach increased the bioavailability. Processing of tomatoes into tomato paste includes both mechanical homogenization and heat treatment, and there is good evidence that this processing increases the bioavailability of the carotenoid lycopene considerably2. The presence of dietary fat can also increase carotenoid absorption, because fat is necessary for the formation of micelles (along with bile salts) which carry the carotenoids to the enterocytes for absorption.
Supplementation may be a good way to increase levels of individual carotenoids because there is not the problem of the plant cell wall to hinder absorption and reduce bioavailability. In fact, research published in the American Journal of Nutrition in 19893 comparing the β-carotene absorption from supplements and vegetables showed that supplements were more efficient at raising serum levels than carrots, broccoli or tomato juice. However, taking high levels of a single nutrient can inhibit the biological effects of other nutrients with similar metabolic fates. Vegetables and fruits contain a wide range of carotenoids, whereas supplements tend to contain a single carotenoid, usually β-carotene. For this reason, whole foods are a better source of carotenoids than supplements. Cooking and mincing the plant material increases the absorption of carotenoids and is therefore recommended where applicable.
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