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OTC dietary supplements have inundated American culture: their ads appear on television, in magazines, and on the Internet. The inherent ambiguity of dietary supplements can put the pharmacist in an uncomfortable position when he or she is asked across the counter, "Is this product safe for me to take?" Claims for supplements can be vague and their risks can be undetermined, and, when patients' preexisting disease states are considered, uncertainty is bound to arise. Most studies of vitamin, mineral, herbal, or other alternative supplements have been inconclusive. More research needs to be done before any clinician can fully advocate their use.
Although evidence of beneficial effects may be lacking, however, most supplements have been found to be safe for patients with diabetes. Four general categories with potential use for diabetic patients are vitamins, minerals, supplements, and herbs.
Supplemental folic acid is promoted to help prevent macrovascular and microvascular complications of diabetes through its role in the regulation of blood homocysteine levels. Increased levels of homocysteine are associated with a greater risk of cardiovascular disease and mortality, especially in diabetic patients.1 Whereas folic acid supplementation poses minimal risk for toxicity, its significance in cardiovascular health is undetermined.2
Niacin therapy is not a new strategy in the treatment of hypercholesterolemia. Its place in therapy for the diabetic patient, however, is growing. In a recent study, patients taking niacin 500 mg 3 times a day in combination with pravastatin showed a significant drop in low-density lipoprotein cholesterol, when compared with patients taking pravastatin alone.3
High doses of pantethine, a derivative of pantothenic acid, may aid in lowering cholesterol and triglycerides. Its mechanism is believed to involve 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibition.4 Vitamin E has not proven to be effective in producing positive outcomes in diabetic patients, particularly in preventing cardiovascular complications.
Vitamin E given with nicotinic acid, however, in patients with new-onset type 1 diabetes, may lead to prevention of beta-cell loss. Doses generally range from 400 to 800 mg per day.5,6
Vitamin C deficiency may be more prevalent in diabetic patients than in nondiabetic patients, although findings are inconclusive.7,8
Some correlation between vitamin D deficiency, insulin resistance, and beta-cell dysfunction exists, even in glucose-tolerant patients.9 Vitamin D may protect against autoimmune attack on the insulin-producing cells.
Chromium, an element required for glucose maintenance, is promoted to benefit patients with glucose intolerance and insulin insensitivity.10,11 Deficiency in chromium typically presents as increased blood glucose levels, glucosuria, weight loss, elevated plasma free fatty acid concentrations, neuropathy, and altered nitrogen metabolism. One study of patients with gestational diabetes showed improved postprandial glucose levels with supplementation of 4 to 8 mcg/kg/day of chromium. When the mineral was administered in high doses (1000 mcg/day) to type 2 diabetic patients, glycemic control improved. Interestingly, a decrease in total cholesterol was noted as well.10 Despite these results, most clinicians agree that further research is required to establish the significance of chromium supplementation in the diabetic patient.2
Copper supplementation is promoted to enhance such functions as blood cell maturation, iron transport, cholesterol metabolism, and myocardial contractility. It does not appear, however, to play a role in either type 1 or type 2 diabetes.12
Iron, which can act as a catalyst of free radical particles, has been suggested to influence the onset of diabetes. A prospective, randomized 4-year study in Finland found that men with greater iron stores were 2.5 times more likely to develop type 2 diabetes than men with lower iron stores.13
Low plasma magnesium concentrations often are found in diabetic patients. This magnesium deficiency may exacerbate insulin resistance and put diabetics at risk for cardiovascular complications.14
Conjugated linoleic acid (CLA) supplementation could be beneficial to type 2 diabetics in losing weight, and it appears safe. The average range is 1 to 5 g/day of CLA.15
Fiber is an important factor in reducing the risk of diabetes. The combination of high glycemic load and a low cereal fiber content further increased the risk of diabetes in one study of more than 65,000 US women.16 No Recommended Daily Allowance for dietary fiber exists, although between 20 and 35 g of fiber a day is suggested.17
Alpha-lipoic acid (ALA) is believed to help prevent cellular damage in neural and renal tissue in diabetic patients through its antioxidant properties. ALA has been used for decades in Germany for the treatment of diabetic neuropathy. Recent studies, however, have had mixed results: moderate improvement or no improvement at all. ALA also may play a role in the prevention of renal damage. In a prospective, nonrandomized study, patients not using ALA were shown to have a significant increase in urinary albumin concentration; however, further investigation is warranted. ALA also has been studied in glucose uptake: 600 mg twice a day was shown to increase glucose sensitivity in lean and obese patients with type 2 diabetes.18
Coenzyme Q10 (CoQ10) is an antioxidant agent promoted to improve endothelial dysfunction of the brachial artery in type 2 diabetic patients with dyslipidemia. Supplementation in patients deficient of CoQ10 also is suggested to improve glycemic control.19
American ginseng has shown some of the best evidence of benefits in patients. It appears to decrease postprandial glycemia in healthy nondiabetic patients and to lower hemoglobin A1C levels.20 Three grams per day is the standard dose of American ginseng. Patients wishing to try ginseng should be warned of the potential interactions between the herb and other drugs. Asian ginseng may decrease warfarin activity, and ginseng may increase the effects of caffeine and other stimulants. Ginseng also may cause a reduction in the dose of insulin or oral hypoglycemic agents. In addition, it can interact with digoxin and nonsteroidal anti-inflammatory drugs.
Ginseng may cause mild irritability and excitation, insomnia, depression, headache, hypertension, and possible weight loss. Ginseng should be avoided in patients with hypertension or cardiovascular disease.
L-carnitine, which influences free fatty acid and glucose oxidation, has been studied in patients with diabetes. Trials show that intravenous carnitine administration can possibly affect insulin sensitivity as well as enhance glucose uptake and storage.21
Other herbal supplements that have potential benefits in treating diabetic patients include Coccinia indica, Aloe vera, vanadium, Momordica charantia, and Nopal. These agents have shown promise, but more conclusive data are needed to show benefits.10
Recommending a dietary supplement is a challenging task for any clinician. Diabetic patients present a more complicated picture, as their health may be compromised or they may have comorbidities. Especially in light of minimal clinical testing and data, the pharmacist must weigh the potential benefits and risks of dietary supplements in the diabetic population.
Dr. Schott is a pharmacist with Stop and Shop, Wallingford, Conn. Dr. Holmberg is a pharmacist with Phoenix Indian Medical Center, Phoenix, Ariz.
For a list of references, send a stamped, self-addressed envelope to: References Department, Attn. A. Stahl, Pharmacy Times, 241 Forsgate Drive, Jamesburg, NJ 08831; or send an e-mail request to: firstname.lastname@example.org.