Function of Carotenoids

Antioxidant action
The beneficial effects of some carotenoids are partly due to their conversion to vitamin A. They also have potent activity of their own due to their ability to act as antioxidants and protect against free radical damage. This type of damage may lead to several medical problems, such as inflammatory damage and tissue injury after trauma; and chronic conditions, such as cardiovascular disease, eye disorders, autoimmune diseases and cancer. Carotenoids also affect cell growth regulation and gene expression.
 

Protection against cancer
High levels of dietary carotenoids have been linked to decreases in the risks of several types of cancer.

Colon cancer
In a small study of cancer patients done in 1997, Italian researchers assessed carotenoid levels in four healthy patients, seven patients with pre-cancerous lesions and seven patients with colon cancer. They found significantly lower carotenoid levels in the cancer patients.

Breast cancer
In a study published in the Journal of the National Cancer Institute in 1996, researchers examined the links between dietary intake of carotenoids (including nonfood supplements) and premenopausal breast cancer risk. The study involved 297 premenopausal women 40 years of age or older who were diagnosed with breast cancer from November 1986 to April 1991. These were compared with 311 women without cancer. The results showed a reduction in risk associated with high intake of several nutrients including beta carotene, lutein and zeaxanthin.

In a study published in 1998, researchers in Missouri examined blood levels of various nutrients in women who developed breast cancer after donating blood to a bank over a ten-year period. They then compared these levels to women who were free of cancer. They found lower levels of the carotenoids beta cryptoxanthin, lycopene, lutein and zeaxanthin in patients who developed breast cancer.

In a 1997 study reported in the American Journal of Clinical Nutrition, researchers at Harvard School of Public Health compared carotenoid concentrations in the breast fat tissue from 46 cancer patients and 63 women with benign breast lumps. They found an increased risk in those with low levels of beta carotene, lycopene, lutein and zeaxanthin.

Lung cancer
Several population studies have shown lower levels of carotenoids in lung cancer cases. In a study published in 1998, researchers at Johns Hopkins University measured nutrient levels in blood samples from 258 patients with lung cancer and compared these with those in samples from 515 people free of cancer. Blood concentrations of cryptoxanthin, beta carotene, lutein and zeaxanthin were significantly lower among the cancer patients. Small differences were noticed for alpha carotene and lycopene.

Protection against cardiovascular disease
Population studies have shown that diets high in carotenoids can protect against cardiovascular diseases. They may do this by preventing the oxidation of LDL cholesterol and reducing free radical damage at sites of atherosclerotic plaque formation.

Researchers involved in the Massachusetts Health Care Panel Study examined the links between consumption of carotene containing fruits and vegetables and death from cardiovascular disease among 1299 elderly people. The results of the study, which were published in the Annals of Epidemiology in 1995 showed that during the follow-up period of almost five years, there were 161 deaths from cardiovascular disease. The risk of death in the group who ate the most carotene containing foods was almost half that of those people whose carotene consumption was low.

In a 1996 study UK researchers compared blood levels of antioxidant vitamins in Belfast, Northern Ireland with those of people in Toulouse, France where the incidence of coronary heart disease is much lower. The results showed that levels of carotenoids such as lutein, cryptoxanthin and alpha carotene were much higher in people from Toulouse.

Protection against eye disorders
Oxidative damage is also implicated in the development of eye disorders such as cataracts and macular degeneration. Researchers at Harvard Medical School examined the link between cataract development and intake of various foods and antioxidant vitamins in over 50 000 women. The results of their studies showed that those with high beta carotene and vitamin A intakes were less likely to develop cataracts. Those whose diets contained spinach also seemed to have a lower risk. The researchers concluded that dietary carotenoids, although not necessarily beta carotene, can decrease the risk of cataracts severe enough to require extraction.

Absorption and metabolism
Carotenoids need bile acids for absorption and unless they are converted to vitamin A in the wall of the small intestine, they are absorbed unchanged. Conversion appears to depend on several factors including protein, thyroid hormone action, zinc and vitamin C. Around 40 to 60 per cent of dietary beta carotene is absorbed, although this appears to be reduced in the presence of low stomach acid. Beta carotene can be stored in the lung, liver, kidneys, skin and fat.

Sources
Good sources of beta carotene include carrots, sweet potatoes, pumpkin and other orange winter squashes, cantaloupe, pink grapefruit, spinach, apricots, broccoli, and most dark green leafy vegetables. The more intense the green, yellow or orange color the more beta carotene the vegetable or fruit contains. These foods are also good sources of alpha carotene. Beta carotene is not destroyed by cooking which, in fact, may make it easier to absorb.

Good sources of lycopene include tomatoes, carrots, green peppers and apricots. Spinach, paprika, corn and fruits are high in zeaxanthin; and green plants, corn, potatoes, spinach, carrots and tomatoes are high in lutein.

Beta carotene
As an antioxidant, beta carotene has beneficial effects in protecting against oxidative damage including that caused by ultraviolet light. Beta carotene has been shown to stimulate and enhance many immune system processes. It increases the numbers of immune cells such as B and T lymphocytes and natural killer cells. T cells play a very important role in determining immune status and are produced by the thymus gland, which is particularly sensitive to free radical and oxidative damage. Beta carotene protect macrophages, white blood cells which engulf and destroy foreign substances. It also facilitates communication between immune cells and makes the stimulatory action of interferon on the immune system more powerful.

Protection against cancer
As with other carotenoids, research suggests that low levels of beta carotene increase the risk of certain types of cancer, including those of the lung, stomach, breast, prostate, colon, ovary and cervix. Several population studies have shown that cancer victims have lower dietary and/or blood beta carotene levels than healthy people. The evidence from these studies is strongest for lung cancer.

In a study done in Wellington, New Zealand researchers investigated the links between beta carotene and cancer. They compared levels in 389 people with cancer to those in 391 hospital patients without cancer. They also assessed the family members of the study participants to compensate for the fact that changes in beta carotene levels may have occurred after the cancer developed. Low levels of beta carotene were found in people with a number of cancers, including those of the lung, stomach, esophagus, small intestine, cervix, and uterus. Low levels were also found in the relatives of these cancer patients and the links were strongest in those with lung cancer. In this study patients with cancers of the breast, colon, prostate, and skin did not have lower levels of beta carotene and neither did their families. These results suggest that the cancer sites associated with beta carotene levels are, in general, sites for which smoking is a strong risk factor.

Lung cancer
Researchers at Yale University School of Medicine compared diets in 413 nonsmokers suffering from lung cancer and compared these with 413 people without cancer. The results of the study, which were published in 1994, showed that high dietary intake of fruit and vegetables and beta carotene was linked to a decreased risk of lung cancer in both men and women.

Breast cancer
In a study published in 1996, Italian researchers investigated the relationship between selected nutrients and breast cancer risk in 2569 women with the disease and 2588 women with no history of cancer. The results showed significantly less risk in women with high beta carotene intakes.

In another recent study published in the British Journal of Cancer, West Australian researchers investigated the effect of increased intake of beta carotene on survival in breast cancer patients. Over a six-year period only one death occurred in the group with the highest consumption of beta carotene, while there were eight and 12 deaths in the intermediate and lowest groups of consumption respectively.

Prostate cancer
Results from the Chicago Western Electric Study published in 1996, suggest that surviving prostate cancer is more likely in men with higher beta carotene intakes. The study involved 1899 middle-aged men who were followed for a total of 30 years. During that time 132 men developed prostate cancer and survival was found to be less likely in those with low beta carotene intakes.

Cervical cancer
Research suggests that women with low beta carotene levels in their cervical tissues may be at increased risk of cervical cancer. Laboratory studies show that beta carotene can slow the growth of cervical cancer cells.15 Increasing intake of beta carotene may help to overcome this tissue-specific deficiency.

Protection against heart disease and stroke
Several studies suggest high dietary beta carotene intake can protect against cardiovascular disease. As an antioxidant, beta carotene has been shown to inhibit oxidative damage to cholesterol and protect against atherosclerotic plaque formation.

The relationship between intake of dietary antioxidants and risk of stroke was investigated as part of the Chicago Western Electric Study. The researchers found a moderately reduced risk in those with high beta carotene intakes.

In a 1997 study, researchers in Italy investigated the relationship between non-fatal heart attacks and dietary intake of beta carotene. The study involved 433 heart attack patients and 869 women without cardiovascular disease. The results showed that women with high beta carotene intakes had around half the risk of heart attack of those with low intakes.

Eyes
Free radical damage is implicated in the formation of cataracts and as an antioxidant, beta carotene may exert protective effects by reducing this damage. It may also act as a filter and protect against light-induced damage to the fiber portion of the eye lens. Beta carotene may also protect against macular degeneration, a disease of the retina to which older people are particularly susceptible.

Finnish researchers recently compared the differences between beta carotene levels in patients admitted to eye wards for senile cataract with those without eye disorders. The results showed that those with low concentrations of beta carotene were 1.7 times as likely to suffer from cataract.
 

Recommended dietary allowances
There is no RDA for beta carotene. An intake of 6 mg beta carotene is needed in order to meet the vitamin A RDA of 1000 mcg RE. 1 RE is equivalent to 6 mcg beta carotene. Some experts recommend a daily intake of 10 to 30 mg.

The RDA for vitamin A for women who are breastfeeding increases from 800 mcg RE to 1300 mcg RE. This can be met by increasing the intake of beta carotene-rich foods.

Supplements
Beta carotene supplements are available in various forms, including synthetic forms and those extracted from algae and palm oil. Some studies suggest that those extracted from palm oil are absorbed more efficiently. Natural beta carotene may have greater beneficial effects than synthetic forms.

Toxic effects of excess intake
Unlike vitamin A, beta carotene is not toxic in large amounts although it may cause the skin of the hands, feet and face to become yellow. This disappears when large intakes are stopped and does not appear to have any ill effects. Research in animals suggests the possibility of menstrual problems with long-term excessive intake.

Therapeutic uses of supplements
Beta carotene supplements have been used in cancer and cardiovascular disease prevention trials, including the Finnish Alpha Tocopherol Beta Carotene Cancer (ATBC) Prevention Study, the US Carotene and Retinol Efficacy Trial (CARET) and the US Physicians Health Study. In 1996, these studies reported results which received wide publicity.
 

Interactions with other nutrients
The conversion of beta carotene to vitamin A depends on vitamin C, zinc and thyroid hormones. The function of beta carotene is enhanced by the levels of the other antioxidants, vitamin C, vitamin E and selenium. Large doses of beta carotene may increase the requirements for vitamin E. Beta carotene improves iron absorption.

Interactions with drugs
Oral contraceptive use may decrease beta carotene levels. Drinking large amounts of alcohol and smoking lowers blood levels of carotenoids.

Cautions
People with hypothyroidism or liver disease have trouble converting beta carotene to vitamin A and should not rely solely on beta carotene to meet their vitamin A requirements. Diabetics may also have trouble converting beta carotene to vitamin A, although recent research suggests that this may not be the case.

Large doses of beta carotene may increase the risk of cancer in those who drink alcohol and smoke heavily. Vitamin C supplements may be useful in protecting against the damaging effects of large doses of beta carotene.