Not all carotenoid have been the subjects of studies isolating their effects from those of the others. But of those that have been the subjects of such studies, among the most promising are lycopene (the dominant carotenoid in tomatoes) and lutein and zeaxanthin (which researchers often list together because they often work together; in fact, scientists were unable to isolate one from the other until recently).
Studies link higher blood levels of lycopene to a reduced risk of prostate, cervical, digestive tract and other cancers, as well as to a lower incidence of heart attack. Since the human body doesn't produce lycopene, we must take it in by eating lycopene-rich foods and/or taking lycopene supplements. Lycopene is found most abundantly in tomatoes, tomato products, scallions, red grapefruits, guava juice, apricots and watermelon.
The results of studies so far indicate that people who eat the most servings per week of leafy green vegetables have a lower risk of macular degeneration. Macular degeneration, the leading cause of blindness in people over age 65, is the age-related degeneration of a small part of the retina. Evidence linking the intake of greens to a lower incidence of this condition has opened new paths for research into a condition that otherwise has no known cause or cure. The primary carotenoids in leafy green vegetables are lutein and zeaxanthin. They are also present in the macula and the lens of the eye. So researchers figure that it's at least worth investigating whether these carotenoids are behind the eye protection afforded to those eating leafy greens.
Leafy greens are the richest lutein and zeaxanthin sources, but significant amounts are found in other vegetables, including corn, pumpkin, celery, okra and red peppers. Eating a wide range of vegetables is your best bet. Canthaxanthin, a red carotenoid, is found in red, orange and yellow fruits and vegetables. Although canthaxanthin hasn't been the subject of extensive research, recent laboratory and animal studies show that it may play an important role in inhibiting the growth of cancer cells and that it can shrink skin tumors in mice. Alpha carotene and cryptoxanthin, too, are found in red, orange and yellow fruits and vegetables. Some research suggests that alpha carotene and cryptoxanthin help protect the body from cellular damage.
Foods that naturally contain retinol are also high in fat, so you could boost your fat consumption too high if you ate, say, lots of egg yolks and butter. And if you were to take a high-dose retinol supplement, you might be at risk of reaching a blood level of retinol higher than the range considered healthy. In contrast, eating fruits and vegetables gives you plenty of provitamin A (carotenoids), which your body converts to vitamin A as needed. The advice of most experts is to eat a balanced diet. Then you will get your vitamin A from a combination of animal and plant foods (unless you are a vegetarian who eats no dairy or eggs), along with many other nutrients.
Polar bear and seal livers are extremely rich in vitamin A because both animals eat so much fish and vitamin A is stored mostly in the liver. Arctic explorers have developed fatigue, vomiting, headache and irritability within a few hours of eating these foods!
Recent research suggests that beta carotene and lycopene may be best absorbed when eaten with a bit of fat, such as the oil-and-vinegar dressing on a salad. But the foods that naturally contain retinol originate in animals and are high in fat already. So you won't need to add extra fat to satisfy the requirement.