Health Benefits of Green Leafy Vegetables
Author: Winston Craig, MPH, PhD, RD.
Greens - A Neglected Gold Mine
Mother was right all along. Grandma also told you they were good for you. So why do Americans eat green leafy vegetables only about once or twice a week? Why are cabbage, broccoli, turnip greens, and spinach rarely seen at the American dinner table? Why is lettuce the only green vegetable that most Americans ever use, when green vegetables are recognized by nutritionists as one of the most inexpensive sources of so many important nutrients?
Leafy vegetables are ideal for weight management as they are typically low in calories. They are useful in reducing the risk of cancer and heart disease since they are low in fat, high in dietary fiber, and rich in folic acid, vitamin C, potassium and magnesium, as well as containing a host of phytochemicals, such as lutein, beta-cryptoxanthin, zeaxanthin, and beta-carotene. One study showed that an increment of one daily serving of green leafy vegetables, lowered the risk of cardiovascular disease by 11 percent. In the Adventist health study, the frequent consumption of green salads by African-Americans was associated with a substantially lower risk of mortality.
Because of their high magnesium content and low glycemic index, green leafy vegetables are also valuable for persons with type 2 diabetes. An increase of 1 serving/day of green leafy vegetables was associated with a 9 percent lower risk of diabetes. The high level of vitamin K in greens makes them important for the production of osteocalcin, a protein essential for bone health. The risk of hip fracture in middle-aged women was decreased 45% for one or more servings/day of green, leafy vegetables compared to fewer servings.
Green vegetables are also a major source of iron and calcium for any diet. Swiss chard and spinach are not considered good sources of calcium, due to their high content of oxalic acid. Green leafy vegetables are rich in beta-carotene, which can also be converted into vitamin A, and also improve immune function. Millions of children around the world have an increased risk of blindness, and other illnesses because of inadequate dietary vitamin A from green leafy vegetables.
Lutein and zeaxanthin, carotenoids found in dark-green leafy vegetables, are concentrated in the eye lens and macular region of the retina, and play a protective role in the eye. They protect against both cataract and age-related macular degeneration, the major cause of blindness in the elderly. Some studies suggest that lutein and zeaxanthin may help reduce the risk of certain types of cancer, such as breast and lung cancer, and may contribute to the prevention of heart disease and stroke.
Green veggies contain a variety of carotenoids, flavonoids and other powerful antioxidants that have cancer-protective properties. In a Swedish study, it was reported that eating 3 or more servings a week of green leafy vegetables significantly reduced the risk of stomach cancer, the fourth most frequent cancer in the world. Cabbage, cauliflower, brussels sprouts, and broccoli are rich in indoles and isothiocyanates, which protect us against colon and other cancers. Broccoli sprouts have been reported to contain 10 or more times as much sulforaphane, a cancer-protective substance, than does mature broccoli. A higher consumption of green leafy vegetables has been shown to significantly decrease the risk of breast cancer and skin cancer.
Studies have identified a gene, connexin 43, whose expression is upregulated by chemopreventive carotenoids and which allows direct intercellular gap junctional communication. In many human tumors gap junctional communication is deficient and its upregulation is associated with decreased proliferation. Hence, the cancer-preventive properties of carotenoids are partly explained by their impact on gene regulation.
Quercetin is a bioflavonoid found in leafy green vegetables. Quercetin has an antioxidant and anti-inflammatory activity and displays unique anticancer properties. Quercetin is a natural compound that blocks substances involved in allergies and acts as an inhibitor of mast cell secretion, and causes a decrease in the release of interleukin-6.
There was considerable concern associated with the E.coli O157:H7 outbreak in the fall of 2006 (26 states were affected) that was associated with contaminated bagged baby spinach. The outbreak resulted in 205 confirmed illnesses and three deaths, with the infection causing bloody diarrhea and dehydration. FDA investigators suggested that the outbreak was possibly caused by the presence of wild pigs on the ranch, or that the irrigation water had been contaminated with cattle feces or grazing deer.
Green, leafy vegetables provide a great variety of colors from the bluish-green of kale to the bright kelly green of spinach. Leafy greens run the whole gamut of flavors, from sweet to bitter, from peppery to earthy. Young plants generally have small, tender leaves and a mild flavor. Many mature plants have tougher leaves and stronger flavors. Collards, Swiss chard, bok choy, and spinach provide a mild flavor while arugula, mizuna and mustard greens provide a peppery flavor. Bok choy is best known for use in stir-fries, since it remains crisp, even when cooked to a tender stage. One should always choose crisp leaves with a fresh vibrant green color. Yellowing is a sign of age and indicates that the greens may have an off flavor. Salad greens provide a whole range of important nutrients and phytochemicals to keep us healthy.
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Americans typically eat <1 serving/day of whole grain products. The recommendation is for at least 3 servings/day of whole grains. Four large epidemiological studies all reveal a 25 to 35 percent reduction in risk of coronary heart disease with a regular intake of whole grain products.