Introduction

The colors of many fruits and vegetables are due to a class of compounds known as carotenoids. Over 600 carotenoids have been identified in nature. Humans are unique in that they can assimilate carot-enoids from the foods that they eat whereas many other animals do not. Thus, carotenoids are an important class of phytochemicals. Phytochemicals are compounds derived from plants that may or may not have nutritional value. While many carotenoids circulate in humans, the most commonly studied ones are ¡-carotene, a-carotene, ¡-cryptoxanthin, lycopene, lutein, and zeaxanthin (Figure 1). The nutritional significance of carotenoids is that some are used by the body to make vitamin A. Indeed, approximately 50 carotenoids can be converted by the body into vitamin A and are known as provitamin A carotenoids. The three most abundant provitamin A carotenoids in foods are ¡-carotene, a-carotene, and ¡-cryptoxanthin. Provitamin A carot-enoids, especially ¡-carotene, provide less than one-half of the vitamin A supply in North America but provide more than one-half in Africa and Asia.

Dietary recommendations for the intake of specific carotenoids have not been established due to lack of an adequate evidence base. To date, carotenoids are not considered essential nutrients. Dietary recommendations for vitamin A exist: 900 retinol activity equivalents (RAE) for men and 700 RAE for women. An

RAE is equivalent to 1 mg of retinol. The recommendations for infants and children are less and range from 300 to 600 RAE depending on age. Consumers need to eat sufficient amounts of carotenoid-rich fruits and vegetables to meet their daily vitamin A requirement, and to achieve optimal dietary carotenoid intake to lower the risk of certain chronic diseases. In 2001, the Institute of Medicine revised the amount of caro-tenoids needed to provide vitamin A from foods as being approximately 12 mg of ¡-carotene or 24 mg of other provitamin A carotenoids to yield 1 RAE. Currently, high-dose pharmacological supplementation with carotenoids is not advised. Despite this, a tolerable upper intake level, the maximum daily amount of a nutrient that appears to be safe, has not been established for any individual carotenoid; however, supplemental ¡-carotene at 20mgday_1 or more is contraindicated for use in current heavy smokers by the European Commission.

Because many factors affect the assimilation of carotenoids from foods (Figure 2), conversion factors need to be considered. This is especially important when most sources of vitamin A are from provitamin A carotenoids in the population. Bio-availability of preformed vitamin A, i.e., retinol and retinyl esters, is not a major concern because 80-95% of them are absorbed. However, foods that are high in preformed retinol (liver, eggs, and fortified milk) are not necessarily consumed by everybody. When discussing carotenoids from food, four terms need to be defined (see Table 1):

• bioaccessibility refers to how much carotenoid can be extracted from the food and is available for absorption;

• bioavailability is how much carotenoid is absorbed from the food and is available for physiological function;

• bioconversion relates to the provitamin A carot-enoids and is defined as the amount of retinol that is formed from absorbed provitamin A carot-enoids; and

• bioefficacy encompasses all of the biological processing of provitamin A carotenoids and is the amount of retinol formed from the amount of carotenoid contained in the food.

The study of carotenoid bioefficacy from foods is important in international health as the most frequently consumed sources of vitamin A are fruit and vegetables. A 100% bioefficacy means that 1 mmol of dietary ¡-carotene provides 2 mmol of reti-nol in the body; however, 100% bioefficacy does not actually occur in the process of digestion and carotenoid uptake by the body.

^-Carotene

^-Carotene a-Carotene a-Carotene

Lycopene

Lycopene

,3-Cryptoxanthin

,3-Cryptoxanthin

Zeaxanthin

Zeaxanthin

Lutein

Figure 1 The structures of the most common carotenoids found in the human body. Three of them, ^-carotene, a-carotene and ß-cryptoxanthin, can be used by the body to make vitamin A. All carotenoids are antioxidants found in fruits and vegetables.

Lutein

Figure 1 The structures of the most common carotenoids found in the human body. Three of them, ^-carotene, a-carotene and ß-cryptoxanthin, can be used by the body to make vitamin A. All carotenoids are antioxidants found in fruits and vegetables.

Once in the body, carotenoids can act as potent antioxidants, which are substances that neutralize free radicals formed from the natural metabolic processes of cells. Free radicals damage tissues and cells through oxidative processes. While free radical formation is a natural process in the body, environmental factors such as smoking and pollution can increase free radical load and thus disease risk. Carotenoids may counter these influences by functioning as an antioxidant and quenching oxygen-containing free radicals. In high- and low-density lipoproteins and cell membranes, carotenoids may also regenerate the antioxidant form of vitamin E as well as protect vitamin E from oxidation.

At the whole-body level, some population studies have indicated that certain carotenoids from either dietary intake or blood concentration data are associated with better immune response, lower rates of age-related macular degeneration (AMD) and cataract, as well as lower risk for certain cancers and cardiovascular disease. ft-Carotene may increase immunological functions by enhancing lymphocyte proliferation independent of its provitamin A functions. The associations between specific carotenoids

Food

Bioaccessibility = ß-Cfreed/ß-Ctotal

Bioavailability = ß-Cabsorbed/ß-Ctotal

Bioconversion = Retinal /ß-Ca

Bioavailability = ß-Cabsorbed/ß-Ctotal

Bioefficacy = Retinal /3-Ctotal

Figure 2 A schematic outlining the path of ^-carotene (,3-C) as it moves out from the food into the intestinal wall. The definition of terms associated with understanding ^-carotene release, absorption, and conversion to retinol are illustrated: bioaccessibility, bioavailability, bionconversion, and bioefficacy. (Reproduced with permission from Tanumihardjo SA (2002) Factors influencing the conversion of carotenoids to retinol: Bioavailability to bioconversion to bioefficacy. International Journal of Vitamin and Nutrition Research 72: 40-45.)

and decreased risk of various diseases are summarized in Table 2.

Blood levels of specific carotenoids are often used as biomarkers of fruit and vegetable intake to strengthen or replace dietary intake data. A wide variation in analytical methods exists and standardization between laboratories does not routinely occur. Nonetheless, higher blood concentrations have been favorably correlated with certain disease states. For example, vitamin A and carotenoid concentrations in serum were measured in middle-aged women who later developed breast cancer. Median concentrations of ^-carotene, lycopene, lutein, and total carotenoids were significantly lower in women with breast cancer compared with case-control women who had not developed breast cancer. In contrast, vitamin A concentrations were either not different or showed a mixed response between cohorts, suggesting that carotenoids may be protective against breast cancer. Furthermore, the Nurses' Health Study, which included a cohort of over 83 000 women, also showed a significant inverse association between dietary ^-carotene intake and breast cancer risk. This was especially strong for premenopausal women with a family history of breast cancer or high alcohol consumption. However, other prospective studies have had mixed results.

Table 1 Terms that are associated with the ^-carotene vitamin A value of foods and subsequent utilization as retinol

Term

Definition

100%

Bioaccessibility

^-Carotene freed

1 mmol freed

^-Carotene in food

1 mmol in food

Bioavailability

^-Carotene absorbed

1 mmol absorbed

^-Carotene in food

1 mmol in food

Bioconversion

Retinol formed

2 mmol formed

^-Carotene absorbed

1 mmol absorbed

Bioefficacy

Retinol formed

2 mmol formed

^-Carotene in food

1 mmol in food

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Breaking Bulimia

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