Here is a really well-written and researched article from the Food and Agriculture Organization (FAO) on the nutritional and health benefits of citrus fruits.
Nutritional and health benefits of citrus fruits
Christine Economos is Assistant Professor, Tufts University School of Nutrition Science and Policy, Medford, Massachusetts, United States. William D. Clay is Chief, Nutrition Programmes Service, Food and Nutrition Division, FAO, Rome.
Citrus fruits have long been valued as part of a nutritious and tasty diet. The flavours 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 fibre (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 fibre), 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 (see Table), 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).
Nutritional facts about citrus fruit
Orange | Grapefruit | Tangerine | |
Weight (g) | 131 | 236 | 84 |
Energy (kcal) | 62 | 78 | 37 |
Fibre content (g) | 3.1 | 2.5 | 1.7 |
Ascorbic acid (mg) | 70 | 79 | 26 |
Folate (mcg) | 40 | 24 | 17 |
Potassium (mg) | 237 | 350 | 132 |
Source: Gutherie and Picciano, 1995.
Carbohydrate
The main energy-yielding nutrient in citrus is carbohydrate; citrus contains the simple carbohydrates (sugars) fructose, glucose and sucrose, as well as citric acid which can also provide a small amount of energy. Citrus fruits also contain non-starch polysaccharides (NSP), commonly known as dietary fibre, which is a complex carbohydrate with important health benefits. The predominant type of fibre in citrus is pectin, making up 65 to 70 percent of the total fibre. The remaining fibre is in the form of cellulose, hemicellulose and trace amounts of gums. Citrus also contains lignin, a fibre-like component. In the body, NSP holds water-soluble nutrients in a gel matrix which delays gastric emptying and slows digestion and absorption. This tends to promote satiety, and may reduce the rate of glucose uptake following consumption of glycaemic (available) carbohydrate, thus helping to prevent a surge in blood glucose levels. Improper regulation of blood glucose results in either hyperglycaemia (high blood glucose) or hypoglycaemia (low blood glucose). NSP can also interfere with the reabsorption of bile acids which may help in lowering plasma cholesterol levels.
A reasonable goal for dietary NSP/fibre intake is 25 to 30 g/day, but in many developed countries the actual average intake is closer to 15 g (USDA, 1996; United States National Academy of Sciences, Food and Nutrition Board, 1989; Cleveland, Goldman and Borrud, 1996). With one medium orange containing approximately 3.0 g of NSP, citrus fruit can make a valuable contribution to meeting the daily fibre goal (Whitney and Rolfes, 1999).
Vitamin C
Vitamin C (ascorbic acid), an essential water-soluble vitamin, plays a key role in the formation of collagen, a primary component of much of the connective tissue in the body. Adequate collagen synthesis is essential for strong ligaments, tendons, dentin, skin, blood vessels and bones, and for wound healing and tissue repair. The weakening of these tissues is a symptom of vitamin C deficiency. Vitamin C is an important aid in the absorption of inorganic iron; it has also been shown to aid in the treatment of anaemia and stress. Contrary to popular belief, vitamin C does not seem to prevent the onset of the common cold, but in some studies it has been reported to reduce the length and severity of the symptoms.
Contemporary interest in vitamin C centres on its ability to perform antioxidant functions. As an antioxidant, it can help prevent the cell damage done by "free radical" molecules as they oxidize protein, fatty acids and deoxyribonucleic acid (DNA) in the body. Free radical damage has been implicated in the progression of several diverse and important disease states including cancer, cardiovascular disease and cataract formation (Gershoff, 1993; Harats et al.,et al., 1997). Being a good source of antioxidants, if regularly consumed, citrus can be an important part of a diet aimed at reducing the risk of such chronic disease. 1998; Jacques
Only 10 mg of vitamin C per day are required to prevent vitamin C deficiency and the devastating disease scurvy (United States National Academy of Sciences, Food and Nutrition Board, 1990). However, for good health and sufficient body storage of vitamin C, 30 to 100 mg/day is generally recommended, although some recent studies have provided evidence that more than 200 mg/day may be optimal for the prevention of chronic disease. Too much vitamin C (above 500 mg), generally seen with very high levels of supplementation, may be dangerous, especially for those at risk of iron overload (Fleming et al., 1998). Consuming five servings of fruits and vegetables each day can result in an intake of about 200 mg of vitamin C. Citrus fruits are a particularly good source of vitamin C, with one medium orange or grapefruit providing approximately 70 mg and 56 mg, respectively. A 225 ml glass of orange juice contains approximately 125 mg of vitamin C (Whitney and Rolfes, 1999).
Folate
Folate is a water-soluble vitamin essential for new cell production and growth. It helps in the production of DNA and ribonucleic acid (RNA) and mature red blood cells, which ultimately prevent anaemia. In the United States, the recommended daily intake of folate is 180 mcg for females and 200 mcg for males. Over the past decade, however, it has become clear that higher levels of folic, 400 mcg, are associated with the prevention of neural tube defects, a severe birth defect (Centers for Disease Control and Prevention, 1992). A 225 ml glass of orange juice provides 75 mcg of folic acid (Whitney and Rolfes, 1999).
Potassium
Potassium is an essential mineral that works to maintain the body's water and acid balance. As an important electrolyte, it plays a role in transmitting nerve impulses to muscles, in muscle contraction and in the maintenance of normal blood pressure. The daily requirement of potassium is approximately 2 000 mg and, while frank deficiency of potassium is rare, there is some concern that a high sodium-to-potassium intake ratio may be a risk factor for chronic disease. Increased consumption of citrus fruits and juices is a good means of increasing potassium intake. One medium orange and one 225 ml glass of orange juice provide approximately 235 mg and 500 mg of potassium, respectively (Whitney and Rolfes, 1999).
Phytochemicals
These naturally occurring compounds found in plants have a wide range of physiological effects and may help to protect against various chronic diseases, including cancer and heart disease. The wide variety and number of known phytochemicals continue to grow, as does understanding of their role and importance in the diet (Steinmetz and Potter, 1991). Several classes of phytochemicals, including monoterpenes, limonoids (triterpenes), flavanoids, carotenoids and hydroxycinnamic acid, have been isolated from citrus (see Figure 1).
