Author: Winston Craig, MPH, PhD, RD.
During the winter many of us in the Northern states are indebted to Florida for the tasty citrus that comes our way. When we think of citrus, many people think of vitamin C. However, an orange does not top the list of fruits for vitamin C content. Guava, kiwi fruit, or a cup of strawberries have more vitamin C than an orange.
Vitamin C is one of the beneficial antioxidants found in our food that may provide protection against heart disease and cancer. Vitamin C can protect LDL cholesterol from oxidation and can also block the formation of nitrosamines, cancer causing chemicals. In addition, vitamin C enhances cellular immunity by promoting the killing ability of white blood cells.
Winter time is quite often flu season. Some people wonder whether citrus has anything to offer in the way of protection against flu or relief from the common cold? Many Americans take a vitamin C (ascorbic acid) supplement to protect themselves against the common cold and flu. The results of many clinical trials have shown that while vitamin C does not prevent a cold, its use can slightly reduce the severity of cold symptoms and may somewhat reduce the duration of the cold. The mild benefit obtained from the use of vitamin C can be achieved from as little as 250 mg/day (the equivalent of about 4 oranges). Higher doses of vitamin C do not provide any extra benefit.
Oranges, in addition to containing vitamin C, also provides substantial amounts of vitamin B1, dietary fiber, potassium and folic acid. Pectin, a fiber found in citrus, is known to significantly lower blood cholesterol levels. Potassium protects against sodium-induced elevation of blood pressure. Orange juice helps replenish electrolytes in children with diarrhea. Folic acid, in addition to protecting against neural tube defects, has been shown to have antioxidant properties. Adequate amounts of folic acid can decrease the risk of rectal, cervical and other cancers. Folate deficiency is associated with an impairment in the number and lifetime of white blood cells. While marginal folic acid intake is very common in the U.S., orange juice can provide 100 micrograms of folic acid per 8 oz. serving. Other good sources of folic acid include fresh green leafy vegetables, oatmeal and legumes.
In addition to the vitamins and minerals mentioned above, citrus contains a host of active phytochemicals that protect our health. In fact, there are over 170 phytochemicals in an orange. These compounds include carotenoids, flavonoids, terpenoids, limonoids, glucarates.
Over 60 flavonoids have already been characterized in citrus. Their properties include antitumor and antiviral activity, anticarcinogenic and anti-inflammatory activity, and ability to inhibit platelet aggregation and therefore reduce the risk of coronary thrombosis. The flavonol quercetin has a greater antioxidant activity than beta-carotene and vitamin-E. The flavonoids, tangeretin and nobiletin, are potent inhibitors of tumor cell growth and can activate the detoxifying P-450 enzyme system. Dr Bracke of Belgium has shown that tangeretin can block the invasion of normal tissue by malignant tumor cells.
There are at least 38 limonoids in citrus, with limonin and nomilin being the two major ones. These triterpenoid compounds partly provide the bitter taste in citrus. They occur in high concentrations in grapefruit and orange juice. Limonoids also possess the ability to inhibit tumor formation by stimulating the major detoxifying enzyme, glutathione S-transferase.
Orange and lemon oil contains substantial amounts of limonene, a terpenoid that also possesses anticancer activity. Citrus pulp and the albedo (the white of the orange) is rich in glucarates. These substances are being studied for their potential use to prevent breast cancer and lower the risk of PMS symptoms. Glucarates have the ability to modify estrogen metabolism.
There are about 20 carotenoids in an orange. Only pink grapefruit has a high content of beta-carotene. However, other citrus (tangerines, oranges) contain high levels of other carotenoids (lutein, zeaxanthin, beta-cryptoxanthin) that have significant antioxidant activity and protect against age-related macular degeneration, the leading cause of blindness after age 65. Pink gapefruit also contains a high level of lycopene, the red pigment in tomatoes and guava that has significant antitumor activity.
The National Cancer Institute recommends that we eat 5 or more (Strive for Five) servings per day of fruit and vegetables, especially green and yellow vegetables and citrus fruit. Clearly, with so many beneficial elements in citrus, it makes good sense to follow the adage “an orange a day, keeps the doctor at bay”.