Advances in Diabetes for the Millennium: Vitamins and Oxidant Stress in Diabetes and Its Complications

Abstract

Hyperinduced oxidant stress may have a role in the pathogenesis of diabetes and its micro- and macrovascular complications. Attaining euglycemia and the use of antioxidant vitamins could reduce oxidant stress and complications. In general, evidence does not support the use of supplements, and supplements are not recommended unless patients are deficient. Use of vitamins in excess may have adverse effects. Vitamin supplements are indicated in patients deficient in vitamins due to inadequate dietary intake or intestinal disease. Treatment with proper amounts of vitamins and antioxidants is best accomplished with a balanced diet including 3 servings of vegetables and 2 servings of fruits. Regarding supplementation of specific vitamins: carotene cannot be recommended in view of the possible harm and lack of benefit in clinical studies. Vitamin A (retinol) and Vitamin D should be repleted if deficient by laboratory assay. Excesses should be avoided. Vitamin A supplements, particularly in pregnancy, should not exceed 10,000 IU daily or a supplement should not exceed 25,000 units weekly. Vitamin E (alpha-tocopherol) alone in doses of 400 units is of questionable value, and larger doses may cause intracranial hemorrhage or interact negatively with lipid-lowering drugs. Vitamin E should not be used in patients who have bleeding disorders or patients on anticoagulants or acetylsalicylic acid (ASA). Vitamin C (ascorbic acid) losses in urine may be excessive in diabetic patients and may require repletion to 200 mg in nonsmokers and 250 mg in smokers. Further studies are needed testing: (1) vitamin supplementation in subgroups of patients at high risk for specific complications using tissue-specific indicators of oxidative stress; (2) the role of oxidative stress in nephropathy, diabetic myocardiopathy, dermopathy, joint limitation syndromes, peripheral edema, metabolic bone disease, and pregnancy; (3) the impact of renal failure on oxidative stress; and (4) the effects of diabetes and dietary vitamins on the relative amounts of retinoids, carotenoids, and vitamin E in the chylomicron and lipoproteins, and how this affects assimilation, oxidation of lipids, and atherosclerotic plaque formation.

Introduction

The role of oxidant stress and vitamins in the pathogenesis of diabetes and its complications is reviewed here. Also considered is the possible role of antioxidants and vitamins as treatment to prevent complications.

Oxidative stress and free radicals result from either an increase in production or decrease in clearance. An excess of free radicals is detrimental to cell function including beta cells,[1-3] endothelial cells,[4] fat,[5] muscle,[6] and nerve cells.[7] Decreasing production or increasing clearance should reduce the net amount of free radicals and cell damage.

Different patients[8] or the organs, tissues, or cells of an individual patient may be more or less sensitive to free radicals and have different susceptibility to oxidants or greater antioxidant defenses.[9-12] The same level of oxidative stress may be more or less deleterious depending upon the protective antioxidant enzyme defense system and reparative process. These differences may alter the requirements or effective doses of antioxidants for different target tissues.

Although oxidative stress can adversely affect cell function, not all free radical production is harmful. For example, oxidative processes may be advantageous by inhibiting proliferation of cells[13] or initiating programmed cell death of cells whose DNA is damaged or mutated beyond repair.[14,15]

In states of oxidative stress, vitamins may have an antioxidant role and have more or less antioxidant activity (retinol > beta-carotene > alpha-tocopheroxyl > ascorbate).[16] One vitamin may alter the requirement for another.[17] Aside from their role as antioxidants, vitamins have specific nongenomic[18] or genomic[19] functions or both acting at the membrane, cytoplasmic, or nuclear level. Vitamins are important in regulation of specific metabolic pathways. Vitamins A and D are important in the regulation of gene expression, cell growth, and differentiation.[19,20] Dual functions as an antioxidant and regulator of growth and differentiation would be of obvious importance in the repair and regeneration of cells that have died from oxidative damage.

Oxidative Stress and the Chronic Complications of Diabetes

Oxidative stress could worsen the complications, and complications could alter requirements for antioxidant. Hyperglycemia causes oxidative stress, which increases glycosylation and oxidation of proteins involved in the pathogenesis of the complications of diabetes.[17,21] Oxidative stress contributes to impairment of islet function,[1,3,22,23] insulin resistance, and microvascular and macrovascular disease.[24-26] Diabetic patients with uncontrolled hyperglycemia are at risk for oxidative stress and complications, and oxidative stress may increase their requirement for vitamins with antioxidant effects.[8,17,27-31] Damaged tissues may have altered responses to vitamins and differing requirements. Reduction of hyperglycemia and improvement of blood sugar control reduces oxidative stress, and reduction of free radical levels should improve metabolic function of beta cells, vascular endothelial cells, fat and muscle cells, and platelets.[17,21,23,29,32] Decreased glycosylation and oxidation of proteins should reduce atherosclerosis, retinopathy, nephropathy, and neuropathy attributable to these processes.[21]

Microvascular Complications

The pathogenesis of retinopathy involves endothelial dysfunction and the proliferation of new vessels. Nephropathy involves endothelial cell dysfunction and proliferation of glomerular capillary and mesangial cells. Diabetic neuropathy is associated with nerve cell damage. Impaired myocardial function is often due to coronary artery disease (CAD). However, myocardiopathy may exist without significant coronary occlusion, suggesting microvascular disease. Intracellularly, oxidant stress is thought to play a key role in the pathogenesis of these complications. Hyperglycemia with overproduction of superoxide radical is a first and common step in diverse pathways leading to an increase in polyols, hexosamines, advanced glycosylation products, NF-kappaB (a proinflammatory factor), and protein kinase C (PKC), which, in turn, contribute to the vascular complications and neuropathy.[21]

Tissue-specific alterations in vitamin and micronutrient metabolism are associated with these complications. Each tissue's requirements for and metabolism of vitamins is different. Retina,[33] kidney,[34,35] and nerve tissues[36-40] have tissue-specific antioxidant defense mechanisms that are needed to prevent or minimize oxidative stress. Lack of vitamins and oxidant damage may contribute to endothelial dysfunction and cell proliferation observed with these complications.[26] Excess of specific vitamins acting as an oxidant instead of an antioxidant could worsen microvascular complications.

Macrovascular Disease, Hyperlipoproteinemia, and Oxidants

Oxidized low density lipoprotein-cholesterol (LDL-C) is thought to play a role in the development of arterial plaque.[41,42] Vitamins in the diet, free levels in the blood and tissue, and amounts in lipoprotein particles may alter oxidative stress and LDL-C particle size and metabolism.[43-46] Alterations of vitamin amounts and ratios in chylomicrons or lipoproteins may lead to oxidation of lipoproteins. LDL-C may be oxidized and penetrate the microvasculature. For example, if retinyl esters are high, vitamin E is deficient, and enough antioxidants are not present, lipoproteins may be subjected to auto-oxidation in the lipoprotein particle.[17] Deficiency of vitamin E in the lipoprotein particle may result in auto-oxidation of LDL-C or defective endothelial-dependent vasodilatation.[47] Alternatively, lack of vitamin C at the cellular level or in macrophages may result in oxidation of LDL-C. Oxidized LDL-C then penetrates the vessel and is taken up by macrophages that become foam cells.[26] This leads to plaque formation.

Vitamin A may regulate gene expression of apolipoprotein A1 and C3, and vitamin A blood concentrations are associated with plasma levels of these lipoproteins in dyslipemic patients and patients with familial combined hyperlipidemia.[43,48,49] Deficiencies of vitamin A may lead to a higher atherogenic index in type 1 diabetes,[44] and deficiencies of vitamin A and D may cause endothelial cell or vascular smooth muscle cell proliferation.

Other Complications

In diabetic patients, metabolic defects in an organ system may contribute to organ-specific disease.

Diabetic Dermopathy

Necrotic skin lesions may be associated with depleted levels of vitamins in skin,[50] and tretinoin has been used to treat necrobiosis diabeticorum lipoidica.[51]

Diabetic Nephropathy

Chronic renal failure is associated with high plasma levels of retinoids[52] and diminished 1-25 dihydroxy vitamin D3 levels. Proteinuria is associated with protein deficiency and alterations of vitamin binding and action, impaired renal production of 1,25 dihydroxy vitamin D3, and loss of vitamins in the urine. Excess levels of retinol could lead to further renal damage.

Diabetic Myocardiopathy

Cardiac disease with diastolic dysfunction may be associated with cardiac tissue-specific changes in vitamin metabolism and vitamin turnover. After a myocardial infarction (MI), the levels of production and clearance may be altered.[53,54]

Autonomic Neuropathy

Severe gastroparesis and diarrhea are associated with malnutrition and vitamin deficiency.

Joint Limitation Syndromes

Adhesive capsulitis, Dupuytren's contractures, trigger finger, and limited mobility of the metacarpophalangeal joints giving a positive prayer sign are not uncommon complications of type 1 diabetes mellitus.[55] This could be related to oxidative damage to tendons and synovial tissues.[56]

Metabolic Bone Disease

Diabetic patients with type 1 diabetes are at risk for osteopenia.[57] Calcium losses are excessive, and vitamin D production is reduced with impaired action. Bone turnover is slowed. Supplementation for the type 1 diabetic patient to replace losses of calcium and altered vitamin D status is appropriate. 25-hydroxyvitamin D3 levels and urinary calcium measurements should prove useful in deciding on the need for calcium and vitamin D supplementation. Excessive calcium replacement, over 1.5-2 g, should be avoided in individuals who form kidney stones. An additional reason to avoid excess calcium is the theoretical risk of suppressing parathyroid hormone secretion. Because low levels of parathyroid hormone are required for bone formation, excessive suppression may reduce osteoblastic bone formation and cause low bone turnover osteoporosis.

Peripheral Edema

Diabetic patients may develop peripheral edema in the presence or absence of heart failure and hypoproteinemia. This reflects endothelial dysfunction.[58] Diuretic use or abuse to treat edema may lead to vitamin loss in the urine, mineral depletion, and sudden death.[59] Treatment of diabetic patients with thiazolidinediones may worsen edema.

Gestational and Pregestational Diabetes

Oxidative stress is also increased in the diabetic pregnancy[60] and could contribute to congenital defects and abnormal fetal growth. (See the review by Jovanovic in this symposium.) The fetus requires macronutrients and vitamins for growth and development. Caloric or protein restriction in the mother can lead to a small-for-gestational-age fetus and impaired fetal insulin secretion with subsequent development of diabetes.[61] Likewise, fetal malnutrition associated with vitamin deficiency could lead to abnormalities in body growth, development and differentiation of pancreatic islet cells, peripheral insulin insensitivity, and subsequent development of diabetes.[62]

Vitamin Replacement and Treatment

General Concepts

Several concepts must be considered when considering treatment with vitamins and antioxidants:

Dietary Sources

Treatment with proper amounts of vitamins and antioxidants is best accomplished with a balanced diet including 3 servings of vegetables and 2 servings of fruits. Most individuals do not meet these requirements for fruits and vegetables. Pockets of our population in poor or rural areas of the United States do not have adequate intake of vitamins. Acquired vitamin deficiencies occur with malabsorption secondary to chronic liver, pancreatic, or inflammatory bowel disease, or gastric or small bowel surgery.

Dietary Sources vs Supplemental Source

The effect of a vitamin may depend upon the intake and presence of other vitamins and their relative concentrations in the diet.[16] As components of food, vitamins may act differently from when they are given alone. The mixture of foods becomes important and the dose of a vitamin may be different in combination with other vitamins or nutrients. A mixture of vitamins and minerals to serve both specific metabolic functions and as antioxidants allows for normal growth, differentiation, and function. This proper mixture should reduce oxidative stress while allowing for specific biological actions.

Oxidant vs Antioxidant Effects

Under some conditions, a vitamin may have oxidant effects. Whether a vitamin behaves as an oxidant or antioxidant depends upon the mixture of vitamins and the redox state in the blood or intracellularly. Giving too much of one vitamin in an altered redox environment may change its effect from an antioxidant to a pro-oxidant.[16] After acting as an antioxidant (donating an electron), a vitamin may become an oxidant if the net redox state is not corrected.[63] Administration of antioxidants could be detrimental under these conditions. Sometimes a vitamin could have dual functions as an oxidant and antioxidant. For example, retinoids bind to cysteine-rich domains of PKC alpha and serve as a redox activator of PKC alpha or reversible redox switch in the activation of PKC alpha.[64]

Antioxidant vs Specific Functions

The antioxidant properties of a vitamin should be considered separately from other more specific functions. The blood level and dose required for an antioxidant effect will be different from the dose required for a specific effect. Dose response curves will be different for an antioxidant effect and a specific effect. The proper dosing (amount and frequency) of vitamins must be function and site specific. Doses will be limited by toxicity. Upper tolerable dietary levels and doses are usually based on a side effect of a large dose or blood levels considered higher than normal. Sometimes a specific function can result in changes in the redox state or production of free radicals. Retinoids and 1,25 dihydroxyvitamin D₃ also inhibit nitric oxide synthase, which, in turn, reduces nitric oxide and free radical production in monocytes and vascular smooth muscle cells.[65,66]

Vitamin Transport and Delivery

The effects of vitamins depend upon the availability of transport proteins and delivery of the vitamin to its target tissue and its free level at the site of action.[67] Insufficient levels of transport or binding proteins in blood or intracellularly could lead to increased free concentrations and actions at target sites. Chronic diseases are frequently associated with reduced amounts of protein or their binding affinity and binding capacity. Further, mobilization of liver stores of vitamins may be impaired in chronic disease. As a result, chronic diseases may be associated with low or high levels of free vitamin at their site of action. In chronic disease states, vitamin replacement is likely needed when circulating and tissue free vitamin levels are low, but on the other hand, the reduced release and blood levels may represent an adaptive response to avoid excess oxidant effects. Vitamins should be avoided when free levels are high.

Vitamin Therapy: Vitamin A, Beta-Carotene, Vitamin E, and Vitamin C -- Dietary Requirements and Vitamins as Therapeutic Agents

It is possible that treatment with a single vitamin could act sufficiently as an antioxidant and prevent one or more complications of diabetes. However, the mixture of vitamins in food (see above) more likely determines the total oxidant status of vitamins. Therefore, treatment needs to be considered in the context of providing a diet sufficient and balanced in all vitamins and minerals. The most benefit would be obtained from vitamin repletion in a patient deficient in vitamins from inadequate intake or gastrointestinal or urine losses and under high oxidant stress.

Specific antioxidant treatment should be considered with other medications used to control hyperglycemia, hyperlipemia, and hypertension for high-risk diabetic patients to protect their islets, reduce insulin resistance, and reduce the microvascular and macrovascular complications of diabetes.[26,68-70] Vitamin status should be assessed and levels should be measured and, if low, replaced with vegetables and fruit if possible or, if not feasible, with dietary supplements. Increasing the level of dietary fat-soluble antioxidant vitamins in patients with hyperlipoproteinemia could conceivably reduce auto-oxidation in the chylomicrons or lipoprotein particle.[26,68,69]

Vitamin Therapy: General Use

Vitamin A

The daily estimated average requirement of vitamin A for adults is 500 mcg for women and 625 mcg for men[71] (.3 mcg retinol =1 IU; 3.6 mcg all-trans-beta-carotene = 1 IU). Recommended dietary allowance for vitamin A that meets the requirements for > 97% of people is 900 mcg for men and 700 mcg for women. The upper intake level for adults is set at 3000 mcg/day (10,000 IU). In patients with hyperlipoproteinemias or dyslipemia, the maximal daily dose should probably be restricted to less than 5000 units.

Vitamin A in excessive amounts is teratogenic. Therefore, during pregnancy, a daily supplement should not exceed 10,000 IU or a weekly supplement should not exceed 25,000 units.[20]

The Beta-Carotene and Retinol Efficacy Trial (30 mg of beta-carotene and 25,000 IU of retinol (vitamin A) in the form of retinyl palmitate) showed an increase in lung cancer and cardiovascular events in smokers exposed to asbestos. An excess of retinoid could have acted as an oxidant instead of an antioxidant.[72]

Studies need to be done to determine the effects of absolute and relative amounts of retinoids and carotene along with other carotenoids in chylomicron and lipoprotein particles.

Carotene

The Alpha Tocopherol Beta-Carotene Cancer Prevention Study (ATBC) also raised the possibility that carotene supplementation increased the incidence of lung cancer as well as hemorrhagic stroke in male smokers, although it may have decreased ischemic infarct modestly in a hypertensive subgroup. In view of the possible harm and lack of clear benefit in clinical studies to date, supplements cannot be recommended.[73-76]

Vitamin E (Alpha-Tocopherol)

The recommended dietary requirement of alpha-tocopherol is 15 mg or about 20 units (35 mcmol)/day.[77] The ATBC study showed alpha-tocopherol increased the incidence of subarachnoid hemorrhage but reduced the risk of ischemic stoke in a group of hypertensive men with diabetes without increasing the risk of hemorrhage.[26,71,73,75] In general, supplements would not be recommended, but supplements of 400 units are probably safe (see above) in the absence of bleeding and in patients not on anticoagulants. It would be prudent to test for bleeding tendency or low platelets before supplementing with alpha-tocopherol. In view of the general use of ASA, studies are needed testing the combination of alpha-tocopherol and ASA to assure the lack of a detrimental additive effect on bleeding.

Vitamin C (Ascorbic Acid)

Ascorbic acid is needed for proper leukocyte function and wound healing. Hyperglycemia and oxidative stress adversely affect leukocyte function. Leukocyte ascorbate levels may be reduced by oxidative stress. Ascorbic acid improves endothelial-dependent vasodilatation[5] and is important as an electron donor cofactor in hydroxylation of collagen and connective tissue synthesis.[56] Hyperglycemia increases urinary losses of ascorbate. This may explain why diabetic patients have low levels of ascorbic acid.[78,79] Low levels of ascorbic acid may make the diabetic patient more susceptible to wound infection, delayed healing, endothelial dysfunction, and tenosynovial disease. Therefore, diabetic patients who are not controlled require greater amounts of ascorbic acid in their diet. The normal amount to provide antioxidant protection (recommended dietary allowance) is 90 mg and 75 mg for men and women, respectively, and is based on the minimal urinary excretion of ascorbate. Smoking increased the requirement by 35 mg/day.[26,56] Hyperglycemia should increase the requirement by as much as smoking, if not more. Since diabetic patients are deficient in ascorbic acid and have high levels of retinoid esters in their lipoprotein particles,[25] the oxidation of their LDL-C may be increased and lead to an increase in foam cells. Providing the diabetic patient with enough ascorbic acid to cover losses in the urine and avoiding excessive retinoids in the diet may increase cellular antioxidant capacity and prevent oxidation and glycosylation exposure at the cellular level.

The upper tolerable limit is 2 g. Studies need to be done to determine dietary intake resulting in maximal retention of ascorbate. Until such studies are completed, the daily requirement of ascorbic acid should be increased to 200 mg daily. Smokers should have an additional 50 mg of ascorbic acid in their diet.

Vitamin Therapy: Use in Selected Populations at Risk for Oxidative Stress

High Risk for Macro- and Microvascular Disease

Further studies are needed testing dose responses of alpha-tocopherol, retinol, and ascorbic acid alone or in combination in diabetic patients at high risk for CAD and stroke and at high risk for nephropathy, retinopathy, and neuropathy.

Studies in animals support the potential role for antioxidants in reducing oxidative stress and complications. Epidemiologic observational studies suggested that low levels of vitamins A, E, and C may be associated with an increased oxidative stress and CAD.[80,81] Studies in diabetic patients have shown that type 1 patients may have low levels of retinol, and they have high levels of retinyl esters associated with insulin resistance[25,82] and low levels of ascorbic acid.[78] Some prospective clinical trials testing vitamin E for cardiovascular protection have been encouraging: Secondary Prevention with Antioxidants in End Stage Renal Disease (SPACE, vitamin E, 800 IU) showed a reduction in acute MI[83]; the Cambridge Heart Antioxidant Study (CHAOS, alpha-tocopherol, 400-800 units)[84] showed a reduction in nonfatal MI in patients with coronary heart disease; the Antioxidant Supplementation in Atherosclerosis Prevention Study (ASAP, twice daily vitamin E, 136 IU, + slow release vitamin C, 250 mg) showed slowed progression of carotid artery intima media thickness in men;[85] and the Transplant Associated Arteriosclerosis Study[86] (twice daily vitamin E, 400 IU +ascorbic acid, 500 mg in post cardiac transplants) showed decreased progression of coronary lesions.

However, several other large, prospective, controlled clinical trials testing daily antioxidant vitamins alone or in combination in high risk individuals have not been convincing or supportive of a benefit: the Physicians Health Study[87-89] (PHS, beta-carotene, 50 mg on alternating days); studies by Brown and colleagues[90] (HATS, vitamin E, 800 units + vitamin C, 1000 mg + beta-carotene 25 mg + and selenium, 100 mcg in patients with CAD with low high-density lipoprotein cholesterol [HDL-C]); The Heart Protection Study[91,92] (MRC/BHF HPS, vitamin E, 600 mg + vitamin C , 250 mg + beta-carotene, 20 mg in nondiabetic and diabetic patients at high risk for cardiovascular events); the HOPE Study[93] (vitamin E, 400 units in diabetic patients at high risk for cardiovascular events); the MICRO-HOPE Substudy[93] (vitamin E, 400 units in diabetic patients with microalbuminuria); the Primary Prevention Project[94] (PPP, vitamin E, 300 mg in type 2 diabetic patients); and the Women's Angiographic Vitamin and Estrogen Trial[95] (WAVE, vitamin C, 1 g + vitamin E, 800 units). Furthermore, in studies by Brown and colleagues,[90] antioxidants appeared to attenuate the effects of lipid-lowering agents; and in other studies, vitamin treatment led to excessive deaths from cancer (retinol) or hemorrhagic stroke (alpha-tocopherol).[73,75,76,96]

A strong linear relationship exists between serum creatinine and retinol levels.[97] Although cause and effect are not proven, retinol excess in chronic renal insufficiency could potentially worsen renal function. In dialysis patients, supplementation can lead to significant elevation in plasma alpha-tocopherol without reducing indicators of oxidant stress.[98]

In view of the above negative findings and potential adverse effects, it would seem prudent not to supplement with vitamin A or carotene unless levels are low. Vitamin E at 400 units is safe in the absence of a significant bleeding history or anticoagulation. Vitamin C at doses of 1 g or less appears to be safe and can be used with vitamin E if vitamin E is supplemented. In view of the negative effect of antioxidants on HDL2-C in HATS,[90] antioxidants might not be a wise choice in patients with low HDL-C levels. Further long-term studies are needed testing the effects of combinations of vitamins E and C on cardiovascular events in subgroups with high oxidant stress. Very little is known about preventive effects of 1,25-dihydroxyvitamin D₃ on cardiovascular events. In view of the high mortality from cardiovascular disease in end-stage renal patients, this is an area that needs study.

Dysmetabolic Syndrome (Insulin Resistance Syndrome) and Impaired Insulin Secretion

Adequate amounts of vitamins A, C, D, and E are required for normal insulin secretion[99-102] and action, lipoprotein gene expression,[48] and blood pressure control. Deficiencies or excesses of these vitamins have been implicated in impaired glucose disposition and insulin resistance.[103,104] Vitamins A, D, and E may alter vascular endothelial cell function.[22] Retinoids and vitamin D may affect vascular remodeling, tone, and blood pressure.[26,105-107] The roles of vitamins A, C, D, and E in insulin secretion and action and their contribution to insulin resistance syndrome need to be further defined.

In diabetic patients with dyslipemia, ie, low HDL-C and high triglycerides, trials are needed testing whether dietary restriction of retinoids or increasing dietary alpha-tocopherol reduces complications. Studies need to be done measuring the amount of fat-soluble vitamins in lipoproteins and optimal ratio of vitamins to lipoproteins[108,109] to prevent auto-oxidation and secondary foam cell formation, steaks, and proliferation of vascular endothelial and smooth muscle cells.[43,44] On the other hand, avoiding excesses of vitamins, which act as oxidants in the lipoprotein particle, may also reduce auto-oxidation of LDL-C.[17] The same concern about the negative effects of antioxidants on HDL2-C apply in this subgroup of patients.

The Elderly, Growing Fetus, and Child

Treatment with vitamins and antioxidants should be age specific because the requirements of vitamins and antioxidants for specific effects may vary with age. Free radical production is increased in the elderly.[46] Compared with healthy adults, requirements for vitamins may be increased in the pregnant mother, growing fetus, diabetic children, and the elderly.[17] Fetuses, low-birth-weight newborns, growing children, and the elderly may require more vitamins than healthy adults, but excesses need to be avoided. Also, the requirement of antioxidant vitamins may be increased in low-birth-weight or premature infants.

Vitamin D and retinoids are important in cell development, differentiation, and secretion. The current dose recommendations for vitamin D and A during pregnancy may be insufficient for fetal and childhood growth in cases of fetal malnutrition. However, teratogenicity limits their use.

Poorly controlled pregnant diabetic patients might benefit from antioxidants. Excessive oxidative stress in pregnancy can be reduced by alpha-tocopherol. Studies need to be performed to determine the variation in oxidative stress and its significance in pregnancy and antioxidant requirements and doses of vitamins in high-risk pregnancies.

The Regenerating Islet or Beta Cell

The architecture of the islet of patients with type 2 diabetes is disrupted, regenerative repairs are inadequate, and function is impaired. This may be due to alterations of retinoid receptors, peroxisome proliferator-activated receptors (PPARs), vitamin D receptors, and other transcription factors important in growth and differentiation. Targeting the islet with the right combination of ligands, eg, retinoids, vitamin D metabolites, or PPAR agonists, may correct or facilitate repair.

Transplanted islets may be at risk for oxidative stress. In the future, with development of artificial beta cells and pancreatic duct stem cells for transplantation, the external milieu, including oxidative stress and vitamin environment, may be important for beta cell maintenance and growth in vitro and in vivo.

Conclusion

Oxidant stress plays a role in the pathological processes ongoing in the diabetic patient. Excessive oxidant stress has adverse effects on islet cell survival and function and accelerates complications in target organs and tissues. Potentially, antioxidant therapy may play a critical role in reducing morbidity and mortality in diabetes. However, at this time, studies do not allow us to determine quantitatively to what extent oxidative stress plays a role in deterioration of islet function and worsening of the complications.

Further studies need to be performed on the evolution of oxidative stress over time, which complications are affected, when and to what extent, whether intervention with antioxidants in the diet or by supplementation slows or reverses the process, and, if so, at what stage of the disease. Studies will have to be placebo-controlled, prospective, and empowered with a large number of patients to detect significant differences in antioxidant effects. Patients need to be selected for test groups and subgroups with early disease without complications (primary prevention) or with mild to severe disease (second intervention). Clinical and biochemical end points for different targets will need to be defined to assess oxidative stress and effects of antioxidants. Specific methods will have to be used to measure oxidative stress. These methods may differ depending upon the target organs and tissues under study. Methods will have to be standardized and reproducible between laboratories to assure valid results and consistency of observations between investigators. These studies will be difficult to perform in humans because oxidative stress is only one component of the dysmetabolic processes that are ongoing in the diabetic patient. Hyperglycemia, hypertension, and lipid disorders cannot go untreated, so it will be hard to separate out benefits of antioxidants from benefits of glucose or lipid-lowering agents and antihypertensive medication. Studies in animal models may help in this regard, allowing specific treatment of oxidative stress and assessment of effects on specific organs or tissues. Understanding the molecular mechanisms of oxidative stress and damage will provide insight into the role of specific antioxidants in different disease processes. Progress may come as specific therapies are developed to correct components of oxidant stress. In the interim, providing sufficient amounts of vitamins in the form of food or physiological repletion with dietary supplements if intake is inadequate would serve our patients best.