A controlled trial in Africa found that supplementing with both vitamin B1 (25 mg per day) and vitamin B6 (50 mg per day) led to significant improvement of symptoms of diabetic neuropathy after four weeks. However, since this was a trial conducted among people in a vitamin B1–deficient developing country, these improvements might not occur in other people with diabetes. Another trial found that combining vitamin B1 (in a special fat-soluble form) and vitamin B6 plus vitamin B12 in high but variable amounts led to improvement in some aspects of diabetic neuropathy in 12 weeks. As a result, some doctors recommend that people with diabetic neuropathy supplement with vitamin B1, though the optimal level of intake remains unknown.
The intake of large amounts of niacin (a form of vitamin B3), such as 2 to 3 grams per day, may impair glucose tolerance and should be used by people with type 2 diabetes only with medical supervision. Smaller amounts (500 to 750 mg per day for one month followed by 250 mg per day) may help some people with type 2 diabetes, though this research remains preliminary.
Many people with type 2 diabetes have low blood levels of vitamin B6. Levels are even lower in people with diabetes who also have nerve damage (neuropathy). Vitamin B6 supplementation has improved glucose tolerance in women with diabetes caused by pregnancy. Vitamin B6 supplementation is also effective for glucose intolerance induced by birth control pills. In a trial that included people with type 2 diabetes, 1,800 mg per day of a special form of vitamin B6—pyridoxine alpha-ketoglutarate—improved glucose tolerance dramatically. Standard vitamin B6 has helped in some, but not all, trials.
Vitamin B12 is needed for normal functioning of nerve cells. Vitamin B12 taken orally has reduced symptoms of nerve damage caused by diabetes in 39% of people studied; when given both intravenously and orally, two-thirds of people improved. In a preliminary trial, people with nerve damage due to kidney disease or to diabetes plus kidney disease received intravenous injections of 500 mcg of methylcobalamin (the main form of vitamin B12 found in the blood) three times a day for six months in addition to kidney dialysis. Nerve pain was significantly reduced and nerve function significantly improved in those who received the injections. Oral vitamin B12 up to 500 mcg three times per day is recommended by some practitioners.
Vitamin D is needed to maintain adequate blood levels of insulin. Vitamin D receptors have been found in the pancreas where insulin is made, and preliminary evidence suggests that supplementation can improve some measures of blood sugar control in people with type 2 diabetes. Not enough is known about optimal amounts of vitamin D for people with diabetes, and high amounts of vitamin D can be toxic; therefore, people with diabetes considering vitamin D supplementation should talk with a doctor and have their vitamin D status assessed.
People with low blood levels of vitamin E are more likely to develop type 1 and type 2 diabetes. Vitamin E supplementation has improved glucose tolerance in people with type 2 diabetes in most, but not all, double-blind trials. Vitamin E has also improved glucose tolerance in elderly people without diabetes. Three months or more of at least 900 IU of vitamin E per day may be required for benefits to become apparent.
People with type 2 diabetes tend to be zinc deficient, but some evidence indicates that zinc supplementation does not improve their ability to process sugar. Nonetheless, many doctors recommend that people with type 2 diabetes supplement with moderate amounts of zinc (15 to 25 mg per day) as a way to correct the deficit.