Recently, much attention has been given to the carotenoids lutein and zeaxanthin. While zeaxanthin is not yet available as an individual carotenoid, it can be found in supplements containing natural beta-carotene. Lutein and zeaxanthin are the only two carotenoids found in the macula of the eye and may play a significant role in the prevention of age-related macular degeneration (AMD). People who consume diets rich in lutein and zeaxanthin- containing foods (spinach, and other greens) have a reduced risk for developing AMD.

Vitamin C

Ascorbic acid is probably the most well-known water soluble antioxidant. Unlike vitamin E and beta-carotene, vitamin C is not transported in lipoprotein particles but rather circulates freely in plasma. In addition to many of vitamin C's functions as a coenzyme, it is a strikingly powerful reducing agent. In other words, it gives up electrons exceedingly well, and therefore serves as an excellent antioxidant. Vitamin C is crucial for the role it plays in the recycling of vitamin E and other antioxidants. It is particularly important for immune function, eye health, cancer and cardiovascular disease prevention, as well as other diseases in which free radicals are suspected to play a role. Smoking is particularly detrimental to vitamin C metabolism, which is why smokers have lower plasma vitamin C levels as well higher levels of oxidative stress. Therefore, vitamin C supplementation appears to be of some benefit to smokers.

Glutathione

Glutathione (GSH) is a tripeptide consisting of the amino acids glutamic acid, cysteine, and glycine. GSH acts as a substrate for a number of enzymes including the peroxide-removing enzyme glutathione peroxidase. In addition, GSH has the ability to scavenge free radicals directly. Cellular GSH levels are high in many tissues, including the eye and the liver where GSH acts to prevent damage from radicals and detoxify dangerous compounds. Decreased GSH levels are found in numerous disease states, which may have either resulted from or predisposed the individual to the condition. Supplementation with GSH and/or its precursor cysteine is beneficial to maintaining optimal antioxidant status. GSH also helps recycle vitamin C as well as other antioxidants.

Lipoic Acid

Alpha-lipoic acid, (also known as thioctic acid) is a unique antioxidant because it possesses both water and fat-soluble characteristics. Lipoic acid functions as a cofactor in dehydrogenase enzyme complexes, such as the mitochondrial pyruvate dehydrogenase system. Consequently, lipoic acid plays an important role in the conversion of glucose to energy. More recently, it has been observed that lipoic acid can function independently from its role as a cofactor. Lipoic acid is able to directly scavenge free radicals and, like vitamin E, can help prevent lipid peroxidation. However, due to its dual solubility, it can also scavenge free radicals not located in lipid membranes. This also allows lipoic acid to help regenerate other antioxidants in the complex web of antioxidant interactions. Research demonstrates that lipoic acid can play an important role in diabetes. Like many other diseases, diabetes is associated with increased oxidative stress, which may contribute to the progression of pathologies associated with the disease. Supplementation with at least 600 mg/d of lipoic acid has been shown to significantly improve glucose metabolism. Research currently underway may help to determine the effect of lipoic acid on long term diabetic complications.

Bioflavonoids

Bioflavonoids, also known as flavonoids, are phenolic compounds (many of which have phytoestrogenic effects) that occur naturally in many plants. Flavonoids are an extremely complex group of compounds. Briefly, they are generally divided into six categories:

Isoflavones - found predominantly in soy,
Flavonols - found in onions, kale and broccoli,
Flavones - found in greens, including thyme and parsley,
Flavonones - found in citrus fruits,
Catechins - found in tea and apples,
Anthocyanidins/Proanthocyanidins - found in grapes, cherries, strawberry and other colored fruits.

Many of these flavonoids exhibit potent antioxidant activity and interact with other antioxidants to help fight free radicals. Interestingly, cultures that consume diets high in flavonoid-containing foods have significantly lower incidences of certain diseases. Recent studies indicate that flavonoids have the ability to inhibit tumor growth in vitro, increase capillary function, improve venous insufficiency, reduce the susceptibility of LDL to oxidation, as well improve other parameters associated with free radical damage. Given the complexity of this family of compounds, scientists still have a great deal to learn in terms of the exact mechanisms by which these compounds may offer protection against disease.