Quercetin belongs to a group of plant pigments called flavonoids that give many fruits and vegetables their color. Flavonoids are also antioxidants. Their antioxidant properties can reduce the risk of heart disease in men [1]. They exhibit their protective effects by inhibiting lipid peroxidation in early atherosclerosis [2]. In particular, they prevent the free radical oxidation of the polyunsaturated fatty-acid-like segments of low-density lipoprotein (LDL) molecules. Should LDL become oxidized, it could be easily absorbed by macrophages. This would be toxic to the vascular endothelium [2].

It stands to reason that if atherosclerosis leads to heart disease and ischemic stroke, and, if antioxidants prevent heart disease, then they should also be beneficial in reducing the incidence of ischemic stroke. This was clearly established by a study reported in the Archives of Internal Medicine. This study investigated the effect of diet on the prevention of stroke and revealed a significant inverse relationship between the long-term intake of bioflavonoid antioxidants and stroke incidence [3]. Specifically, men with a high intake of bioflavonoids (mainly quercetin) had a 73% lower incidence of stroke. In this population of men, they found that tea contributed approximately 70% of bioflavonoid intake and apples contributed approximately 10%. This article clearly shows that the habitual consumption of bioflavonoids protects against stroke.

Quercitin and Alzheimer’s Disease

Quercetin may protect brain cells from Alzheimer’s disease [4]. In this study, brain cells were exposed to varying amounts of quercetin, vitamin C, or no agent. The brain cells were then exposed to hydrogen peroxide which can mimic cell damage that occurs in the brains of Alzheimer’s disease patients. The cells that had been treated with quercetin showed significantly less damage than the vitamin C treated cells or brain cells that did not receive any antioxidant protection [4].

Although the exact mechanism by which quercetin is neuroprotective is unknown, many believe due to its antioxidant effects it can neutralize cell-damaging compounds called free radicals. This is intriguing since Alzheimer’s disease (AD) is an

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oxidative-stress-related neurodegenerative disease. In fact, beta amyloid peptides (found in Alzheimer’s disease) are associated with oxidative stress and neurotoxicity. Quecetin has been shown to reduce oxidative stress and neurotoxicity associated with these proteins [5].


  1. Hertog MGL, et al. Dietary antioxidant flavonoids and risk of coronary heart disease: the Zutphen Elderly Study. Lancet 342:1007-1011,1993.
  2. Steinberg D, et al. Beyond cholesterol: modifications of low-density lipoprotein that increase its atherogenicity. N Engl J Med 915-924,1989.
  3. Keli SO, et al. Dietary flavonoids, antioxidant vitamins, and the incidence of stroke: the Zutphen study. Arch Intern Med (United States) 156(6): 637-4,1996.
  4. Chang Y. Lee, et al: Journal of Agricultural and Food Chemistry, December 2004 (Volume 52, Number 24).
  5. Ansari MA, Abdul HM, Joshi G, Opii WO, Butterfield DA. Protective effect of quercetin in primary neurons against Abeta(1-42): relevance to Alzheimer’s disease. J Nutr Biochem. 2009 Apr;20(4):269-75.