| CITRUS FRUITS Nutritional and health benefits of citrus fruits
Citrus fruits have long been valued as part of a nutritious and tasty diet. The favors provided by citrus are among the most preferred in the world, and it is increasingly evident that citrus not only tastes good, but is also good for people. It is well established that citrus and citrus products are a rich source of vitamins, minerals and dietary fiber (non-starch polysaccharides) that are essential for normal growth and development and overall nutritional well-being. However, it is now beginning to be appreciated that these and other biologically active, non-nutrient compounds found in citrus and other plants (phytochemicals) can also help to reduce the risk of many chronic diseases. Where appropriate, dietary guidelines and recommendations that encourage the consumption of citrus fruit and their products can lead to widespread nutritional benefits across the population.
MORE THAN VITAMIN C: THE NUTRIENT CONTENT AND FUNCTIONS OF CITRUS Citrus is most commonly thought of as a good source of vitamin C. However, like most other whole foods, citrus fruits also contain an impressive list of other essential nutrients, including both glycaemic and non-glycaemic carbohydrate (sugars and fiber), potassium, folate, calcium, thiamin, niacin, vitamin B6, phosphorus, magnesium, copper, riboflavin, pantothenic acid and a variety of phytochemicals. In addition, citrus contains no fat or sodium and, being a plant food, no cholesterol. The average energy value of fresh citrus is also low, which can be very important for consumers concerned about putting on excess body weight. For example a medium orange contains 60 to 80 kcal, a grapefruit 90 kcal and a tablespoon (15 ml) of lemon juice only 4 kcal (Whitney and Rolfes, 1999).
PREVENTION POTENTIAL OF CITRUS There is considerable evidence that citrus foods may help reduce the risk, or retard the progression, of several serious diseases and disorders.
- Cardiovascular disease It is well accepted that a diet low in saturated fat and cholesterol and rich in fruits and vegetables reduces the risk of heart disease. Epidemiological studies have also shown a significant association between vitamin C intake and protection against cardiovascular mortality, but the precise mechanism of protection is still unclear. One major culprit in the development of heart disease appears to be a high level of oxidized low-density lipoprotein (LDL), the so-called bad cholesterol. Significantly, a recent study has shown that high intakes of vitamin C (500 mg/day) obtained from the juice of freshly squeezed oranges, prevented a rise in the levels of oxidized LDL, even in the presence of a high-saturated fat diet (Harats et al., 1998).
A low dietary intake of folate contributes to the decrease of plasma folate and the raising of plasma homocysteine levels (Bloom, 1998; Tucker et al., 1996). Homocysteine is a toxic agent for the vascular wall and, when plasma levels rise above normal, there is an increased risk of cardiovascular disease. An inverse dose-response relationship has been identified for fruit and vegetable intake and plasma homocysteine levels. Frequent consumption of folate-rich foods, such as oranges and orange juice, tends to increase plasma folate levels and, thus, lower homocysteine levels.
- Cancer After numerous studies of fruit and vegetable intake and cancer development, there is a consensus that consuming these foods has a protective effect (Block, Patterson and Subat, 1992). However, it is unlikely that one anticarcinogenic substance in particular is responsible for the benefit. There is reasonable scientific support for vitamin C's protective role in cancer. Many of the animal, cell culture and human studies have suggested it has a positive effect. However, epidemiological studies provide good evidence that protective effects are more closely associated with the consumption of fruits and vegetables rather than with the enormous levels of vitamin C often used in cell culture and animal studies.
- Neural tube defects During the first stage of pregnancy, adequate folate intake is critical for reducing the risk of severe birth defects, namely spina bifida and anencephaly. Public health recommendations in the United States include the consumption of 400 mcg of folate per day for women of child-bearing age (Centers for Disease Control and Prevention, 1992). Regular consumption of citrus foods can help supply adequate folate and thus reduce the risk of these birth defects.
- Anaemia Vitamin C can increase the absorption of non-haem iron (the inorganic iron form found in plant foods) two- to fourfold (Fleming et al., 1998). The bioavailability of non-haem iron is much lower than that of haem iron, which is found in foods of animal origin (Whitney and Rolfes, 1999). Vegetarians and individuals who consume little meat and animal products are at an increased risk of iron-deficiency, which can progress to anaemia over time. Worldwide, anaemia is one of the most serious nutrient-related public health problems, resulting in poor growth, impaired psychomotor development, reduced physical performance and decreased cognitive function. Consuming citrus fruits rich in vitamin C can help prevent anaemia and its devastating consequences.
- Cataracts Oxidation of the eye's lens plays a central role in the formation of age-related cataracts. The role of dietary antioxidants, such as vitamin C, in the aetiology of cataracts has been a recent focus of research (Jacques et al., 1997). Lower cataract risk has been shown in individuals with high blood concentrations or intakes of vitamin C and carotenoids. There is now evidence to show that a high level of vitamin C intake over the long term decreases the risk of cataract development. Although epidemiological studies that measure past nutrient intake and status suggest a protective effect from citrus, further studies are needed to examine the long-term benefits of citrus fruit consumption and cataract protection.
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