Preventing chronic diseases whose secondary effect is often visual impairment is a priority for the US Department of Health and Human Services (HHS).1 In particular, diabetes and cardiovascular diseases are systemic in nature and can cause secondary ocular complications by inducing angiogenesis, or new blood vessel formation from pre-existing capillaries in front of or behind the retina, which subsequently can lead to partial or total blindness.
The World Health Organization (WHO) estimates there are 150 million diabetics worldwide, making it the world's most prevalent metabolic disease. This year, an estimated 24,000 Americans will become blind and 40,000 will develop ocular complications from progressive or undetected diabetic retinopathy.
The Diabetes Prevention Program, which began in 2001 at HHS, includes guidelines for a healthy diet to sharply lower the risk of diabetes onset.2 The guidelines include effective prevention strategies that target high-risk children for type 2 diabetes and cardiovascular disease, as well as educational programmes to teach older people how to adopt healthier lifestyles to delay or prevent diabetes and cardiovascular disease onset.1
Diabetes eye complications
The retina is a light-sensitive tissue at the back of the eye. When light enters the eye, the retina changes the light into nerve signals and then sends these signals along the optic nerve to the brain. Without a retina, the eye cannot communicate with the brain, making vision impossible.
Diabetic retinopathy occurs when diabetes damages the tiny blood vessels in the retina. At this point, most people do not notice any changes in their vision, but the condition is a potential complication leading to blindness. Prolonged type 1 or 2 diabetes causes diabetic retinopathy within 10 years of disease onset in 75 per cent of all diabetics. For those from 20 to 65 years old, it is the leading cause of blindness.2
When the damaged blood vessels of diabetic retinopathy leak fluid and lipids onto the macula, which lets us see detail, the fluid makes the macula swell and blurs vision. This is called macular edema. As the disease progresses, it enters its proliferative stage. Here, fragile new blood vessels grow along the retina and in the clear, gel-like vitreous that fills the inside of the eye. Without timely treatment, these new blood vessels can bleed, cloud vision and destroy the retina.
The frequency of diabetic retinopathy varies widely amongst different ethnographic groups. For instance, African-Americans experience 40 to 50 per cent higher rates than Caucasian-Americans, according to the HHS' National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Some 23 per cent of newly diagnosed diabetic Mexican-Americans have mild to moderate diabetic retinopathy, while an additional nine per cent already have advanced stages of this retinopathy and are in immediate danger of losing some vision.3 Diabetes is the main cause of new onset of blindness in adults, and increasingly in children because of the rising rate of type 2 diabetes in obese children.
But even small preventive steps can have a large beneficial effect. For example, a major NIDDK clinical trial of 3,234 people with impaired glucose tolerance was halted one year early because researchers found that those who incorporated dietary prevention techniques, such as lowering dietary fat, experienced success against onset and progression of diabetes.
Improving blood circulation
Because the small ocular arteries and veins and their branches carry excess glucose from diabetes, it impedes passage of healthy nutrients from circulating. Thus, when these vessels hemorrhage into the retinal macular area and anteriorly into the vitreous gel, they cause degrees of vision loss.
Vitamin E helps prevent diabetic retinopathy damage from glycosylated proteins that oxidise and convert to toxic substances, often leaking onto the macula.4 Glycosylated proteins can produce free radicals, which antioxidants such as vitamin E are capable of neutralising. Hyperglycaemia in diabetes mellitus has been shown to activate the enzymatic pathway diacylglycerol (DAG)-protein kinase C (PKC) in the vascular tissues, possibly altering vascular function. In cases of diabetes mellitus, the potentially damaging enzymatic pathway interferes with normal retinal blood circulation, an interference that vitamin E has been shown to help ameliorate in rat models.5
Alpha-lipoic acid, when combined with vitamin E, inhibits the level of this degenerative disease as well as in heart disease.6 This antioxidant is both fat and water soluble, unlike vitamin E which is only fat soluble. Due to its antioxidant activities and its ability to penetrate eye tissue, alpha-lipoic acid helps prevent age-related macular degeneration as well as helps protect both the lens and retina from degeneration. It has also been shown to reduce the risk of cataracts in diabetes mellitus patients.7,8
Pycnogenol (Pinus pinaster), an extract from the French maritime pine, was shown in a recent study to help slow progression of diabetic retinopathy by sealing the leaky capillaries that cause the condition. During the two-month double-blind, placebo-controlled study of 40 patients, the group taking 50mg Pycnogenol three times daily showed no deterioration of retinal function and some showed a significant recovery of visual acuity compared to the placebo group, whose members' condition progressively worsened.9
Bilberry (Vaccinium myrtillus), as a preventive treatment in diabetic complications, was found to lower blood sugar by 26 per cent in a rat study.10 Its flavonoid component of anthocyanosides strengthens retinal connective tissue and reduces both the leakiness and fragility of the ocular blood vessels.11
Chromium, an essential trace mineral found in whole wheat, brown rice, potatoes and brewer's yeast, helped lower glycosylated haemoglobin and controlled blood sugar in one study.12 By maintaining glucose levels, chromium enables the ocular microvasculature to maintain open channels of blood flow and prevents diabetic retinopathy from occurring.
When the major organ and body systems are functioning synergistically, the fragile vision system can work in harmony for necessary energy. The proper amount of energy from glucose intake can be obtained for good vision, helping to head off diabetic retinopathy, diabetic cataracts and increased diabetes-associated myopia.
Whole grains lower glucose levels and reduce the risk of ocular complications from diabetes, primarily retinopathy, but also cataracts and posterior vitreous detachments. Diets rich in soluble fibre from fruit, beans, peas and oats are beneficial against blood-sugar variations. Diets low in fibre intake and high in glycaemic foods, including highly processed grains, instant and processed foods, and sweets, pose a 2.5 times greater risk for the development of diabetes when compared with the effects of eating evolutionary foods. Many factors, such as particle size, help reduce the glycaemic index rating of whole grains, thus reducing the likelihood of an onset of diabetes, whereas processed grains do not appear to provide protection against diabetes.13
Ginkgo biloba's possible effects on the insulin resistance syndrome is the focus of a National Center for Complementary and Alternative Medicine (NCCAM) study scheduled for completion in May 2004. Non-insulin-dependent diabetics who ingest ginkgo have been able to significantly increase their pancreatic beta-cell function. When the pancreas becomes dysfunctional and glucose metabolism is altered, it will affect the visual system, primarily with proliferative diabetic retinopathy and posterior vitreous detachments, and secondarily with diabetic cataracts. The study's purpose is to evaluate the efficacy of three hypoglycaemic pharmaceutical agents and to determine the mechanism through which ginkgo accelerates pancreatic function and alters or improves glucose metabolism.14
In addition, ginkgo reduces cellular adhesion to endothelial cells, protects endothelial cells from hypoxia, improves vasodilation qualities and has the ability to increase nitric oxide. Endothelial cells and nitric oxide are especially connected to diabetes-induced glaucoma.15,16
Cinnamon's most active compound, methylhydroxy chalcone polymer (MHCP), increased glucose metabolism roughly 20-fold in a test tube assay of fat cells in a US Department of Agriculture's Human Nutrition clinical trial.17 In the late 1980s, researchers assayed 50 plant extracts and spices used in folk medicine. MHCP, a type of polyphenol flavonoid that prevents formation of oxygen radicals, mimicked insulin and demonstrated synergy with it for up-regulation of glucose by cells.18
This cinnamon compound, like insulin, affects protein phosphorylation-dephosphorylation reactions in the intact adipocyte (fat cell). Bioflavonoids are known to inhibit protein kinases; therefore, it is possible that a cinnamon compound related to the bioflavonoids could inhibit the insulin response signaling and overcome insulin resistance.19
The link between diabetes and cardiovascular disease is not fully understood. Yet, when eye-care practitioners observe vasculature in patients with early hypertension, they can see numerous arteriolar vessel changes, including how they cross one another, are narrowed, nicked and create infarcts in the retina. Severe hypertension additionally affects the retina's veins and increases fatty deposits, as well as occludes quadrants of the retina's arteries and veins. Individuals suffering cardiovascular disease experience vision loss of varying severity, from hemorrhages to occlusions in the eye's blood flow.20
From another perspective, observing retinal arteriolar vessel narrowing in women can identify their risk of coronary heart disease. These warning signs were not found in men in the NIH's National Heart Lung Blood Institute's study of 9,648 people.21 Studies conclude that arteriolar narrowing is related to coronary heart disease in women but not men, theoretically because of a more prominent microvascular role in the development of coronary heart disease in women than men. With women, according to this study, the corollary of being at greater risk for arteriolar narrowing is an increased risk for secondary ocular vascular complications and subsequent vision loss. Those with the narrowest retinal vasculature have a double risk.22
The ophthalmic arteries are carotid branches that can cause central retinal artery occlusions as a secondary complication of this systemic disease. High blood pressure can also cause these occlusions. A diet rich in fruits, vegetables, soluble fiber and soy provides phytosterols, phytochemicals and isoflavones that can help lower cholesterol and maintain good blood flow through the eye vessels.23-25
Optimal functioning of the microvasculature in the eye depends on adequate nutrients from the heart, which is their primary source. The retina is highly vascularised as well as extremely fragile. When its vessels become clogged with plaques, for example, and they break loose and travel to the brain, a stroke occurs. This causes an automatic visual field loss. It can cause central or branch retinal artery or vein occlusions with haemorrhages, atrophy and vision loss.
Numerous nutrients are necessary to allow proper metabolism of the cardia vasculature. This macro-vasculature plays an even more critical role in the eyes' micro-vasculature to prevent secondary ocular complications.
The mechanism of the ocular system to allow clear acuity at all distances and the ability to focus and to move in visual space all rely on the heart's ability to function. The average 24mm eyeball functions with its connection to the brain, and any interruption, such as oxidation or cell or vascular damage, causes secondary visual losses.
Vitamin E, in addition to neutralising free radicals, prevents oxidative damage, thus helping to prevent platelet clumping.26 This vitamin plays a role in haemoglobin production, collagen formation and the metabolism of polyunsaturated fatty acids, thereby reducing cholesterol.27
NCCAM in Washington, DC, is currently in Phase II of a study to evaluate the effect of d-alpha-tocopherol supplementation on the progression of carotid atherosclerosis in persons with coronary artery disease. It is hoped vitamin E will reduce oxidation and adhesion to endothelial blood cell linings. Results of this study may demonstrate that vitamin E can be an adjunctive therapy in the management of cardiovascular disease (CVD). Because these conditions can cause high intraocular pressure and glaucoma-related vision loss, vitamin E could prevent ischemia of the ophthalmic vasculature, which causes vision loss and is also related to intraocular pressure.
E's antioxidant effects are synergised by vitamins C and beta-carotene. Researchers have found that the interaction of water-soluble vitamin C with fat-soluble vitamin E broadens the range of protection from free radical damage.28 In another study, the combination of vitamins C and E reduced by 53 per cent the risk of death from heart disease in more than 11,000 individuals.29
Beta-carotene nourishes the eye's rod and cone photoreceptors and is related to retina and macular functioning because it provides day, night and sharp central vision. Its highest concentration is found in the yellow/orange and dark-green phytochemicals. These functional food sources are more beneficial, compared with nutraceutical forms, against cardiovascular disease.30 Examples of functional foods that are beneficial are sweet potatoes, carrots, mangoes, spinach, squashes and papaya.
Red yeast rice (Monascus purpureus), a Chinese food and medicine, has the same reductase inhibitors found in the statin drugs to lower cholesterol. It also contains flavonoids and helped lower cholesterol production levels by 18 per cent in one study.31 Lower cholesterol production is associated with a decreased likelihood of Hollenhorst plaques, which are seen only in the eye as precursors, most often, to stroke and its accompanying visual field losses.32
Omega-3 fatty acids such as docosahexaenoic acid (DHA) have a vital role in replenishing the retinal photoreceptor cells that provide colour, black-and-white, and detailed vision.33 Intake of DHA reduced triglyceride levels by 25 per cent in one study, while another found post-prandial triglyceride levels were reduced by nearly 50 per cent. Traditionally, the more fatty the meal, the higher the triglyceride level.34 The higher the fat from triglycerides, the greater chance for plaque formation, which impedes the blood flow through the vessels to photoreceptors for proper regeneration and vision. DHA sources show the same benefit against cardiovascular disease of reducing triglycerides, increasing HDL cholesterol, increasing blood DHA levels and reducing the risk of this disease.35
Garlic (Allium sativum) lowers blood cholesterol and triglycerides as it reduces the risk of developing heart disease.34 Allicin in garlic rapidly reacts with free radicals, has antioxidant abilities and rapidly crosses membranes, including those in red blood cells. It also combines with the antioxidant glutathione36 and as a thiosulfinate contributes to reducing blood pressure.37
Garlic extract strongly inhibits calcium 2+ by binding to proteoheparan sulfate from arteriolar endothelial cells. This newly formed complex blunts plaque formation.38 Anti-hypertensive effects of garlic increase nitric oxide production, inhibit angiotensin-I converting enzyme and prevent ischemia.39 By preventing plaque formation, blood flow is unimpeded and haemorrhages are prevented, as are occlusions of vessels that cause visual field losses, elevated intraocular pressure and glaucoma.
In a recent WHO longevity report, epidemiologists and biostatisticians worldwide suggest targeting disease prevention by intervening amongst entire at-risk people by supplementing with vitamin A, reducing processed food intake, and increasing fruit and vegetable intake. Enhancing beta-carotene in rice could prevent an estimated 350,000 pre-school children from becoming partially or totally blind, and 60 per cent of those from dying within a few months after going blind. It is estimated that 14 million pre-schoolers, mostly in underdeveloped nations, have some eye damage from vitamin A deficiency.40
With modifications in diet and dietary supplementation, blindness caused by diabetes and cardiovascular disease can be prevented, as can secondary ocular complications such as dry eyes, nyctalopia and age-related macular degeneration.
Barbara Anan Kogan, PhD, OD, was the peer-review editor for the National Eye Research Foundation's journal. She provided conventional and preventive vision care in her Washington, DC, practice for 13 years.
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