Page 27 - Spring2008

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and suggested that beta-carotene from fruits and average of 30,000 IU per day. Since researchers Many
vegetables in a mixed diet has a conversion factor are still discovering new roles for vitamin A and
closer to 21. 14 still poorly understand many of those already traditional
In 2003, Tang and colleagues showed discovered, it would be prudent to assume that diets

that even the efficiency of beta-carotene dis- the ten-fold increase over the RDA found in tradi- contained
solved in oil had been grossly overestimated. tional diets may have some benefi t—providing it
The researchers gave a concentrated dose of is accompanied by a rich array of other fat-soluble much more
radio-labeled beta-carotene dissolved in oil to vitamins, especially vitamin D, which protects vitamin A

22 adult volunteers and traced its conversion to against its toxicity. than our
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vitamin A both in the intestine and after intestinal It would be virtually impossible to ob-
absorption. The mean total conversion rate for the tain this amount of vitamin A from plant foods government
oil-soluble carotene in this experiment was 9.1, without either juicing or using supplemental recommends.
and individual rates varied from 2.4 to 20.2. 13 beta-carotene. Even these methods may be insuf- In 1953, for
Figure 1 compares the vegetables richest fi cient, however, since larger doses of carotenes
in carotenes to the animal foods richest in vita- are converted less efficiently than smaller ones. 13 example,

min A. Eating liver once a week or taking a half Massive doses of beta-carotene, moreover, in- Greenland
teaspoon of high-vitamin cod liver oil per day crease levels of oxidative stress and stimulate the Inuit
provides the RDA of 3,000 IU. To obtain the production of enzymes that degrade true vitamin
same amount with plant foods, one would have A. By inducing a cellular vitamin A defi ciency, subsisting on
to consume two cups of carrots, one cup of sweet large doses of beta-carotene cause cancerous traditional

potatoes, or two cups of cooked kale every day. changes in lung tissue even worse than those foods
The presumed conversion rate, however, is just an seen from cigarette smoking. For this reason,
average—by definition, many people will convert high-dose beta-carotene supplementation led to consumed

carotenes more efficiently than the average and increases in cancer mortality and total mortality an average

many will convert them less efficiently than the in two human trials. Although no studies have of 30,000 IU
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average. People who convert carotenes poorly demonstrated this type of harm from juicing, car-
may suffer from vitamin A deficiency even if they rot juice has the potential to raise blood levels of per day.

are careful to eat large amounts of carotene-rich beta-carotene to the extremely high levels found
foods every day. in the aforementioned trials and large amounts of
Many traditional diets contained much it may theoretically pose a risk. By contrast, the
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more vitamin A than our government recom- amount of beta-carotene found in a diet rich in
mends. In 1953, for example, Greenland Inuit vegetables protects against oxidative stress and
subsisting on traditional foods consumed an cancer. 17

FIGURE 1. VITAMIN A YIELD OF PLANT AND ANIMAL FOODS.

All values are derived from the USDA National Nutrient Database for Standard Release 17, except cod liver oil, which is
derived from the information provided by commercial manufacturers. All values of vitamin A yield are expressed per 100
grams of food, except cod liver oil, which is expressed per teaspoon. Vitamin A yield values follow West et al. (2002) in
assuming that the retinol activity equivalent (RAE) figures for vegetables overestimate the true conversion by 75 percent.
These values, however, represent an average conversion factor from a mixed diet and therefore do not represent differ-
ences in bioavailability between specific foods—the carotenoids in carrots, for example, are five times more bioavailable
than those in spinach.

PLANT FOODS Vitamin A Yield IU per 100 g ANIMAL FOODS Vitamin A IU per 100 g
Sweet Potatoes 1,500 Turkey Giblets 35,800
Carrots 1,145 Beef Liver 25,800
Kale 1,295 High-Vitamin Cod Liver Oil (1 tsp) 5,750
Spinach 997 Commercial Eggs 570
Collard Greens 770 Commercial Butter 330

SPRING 2008 Wise Traditions 27