Researchers in New Zealand found that high-dose vitamin E appeared to temporarily improve insulin resistance - a precursor to type 2 diabetes - among adults, who were overweight.
The improvement was short-lived, but another diabetes risk factor, namely elevations in a liver enzyme called alanine transferase, changed for the better throughout the six-month study.
"These results suggest that vitamin E could have a role to play in delaying the onset of diabetes in at-risk individuals," said Dr. Patrick Manning and colleagues from the University of Otago in Dunedin in the journal Diabetes Care.
This research supported the conclusions of recent studies, which found that people whose diets had a healthy dose of antioxidants, including vitamin E, had a lower diabetes risk than those with lower antioxidant intakes. Vitamin E has also been shown to help some diabetics gain better control over their blood sugar.
The new study included 80 overweight adults ages 31 to 65. Overweight and obese individuals have an increased risk of developing insulin resistance, in which the body loses sensitivity to the hormone insulin, causing blood sugar levels to soar.
According to Manning's team, excess fat may speed up the production of oxygen free radicals, the potentially cell-damaging byproducts of normal metabolism. Moreover, overweight people tend to have low levels of antioxidants, which counter the effects of free radicals. It is thought that the resulting oxidative stress may contribute to insulin resistance.
To see whether vitamin E can alter oxidative stress and insulin resistance, Manning and his colleagues assigned participants to take either vitamin E or a placebo pill every day for six months. For the first three months, the treatment group took 800 IU of vitamin E each day, followed by 1,200 IU per day for the next three months - a much higher dosage than the recommended dietary allowance of 22 IU.
The researchers found that after both three- and six-months, plasma peroxides, markers of oxidative stress, had fallen in the vitamin E group. After three months, blood sugar levels and insulin sensitivity had also improved, though the gains did not continue.
However, there was a lasting decline in blood levels of alanine transferase liver enzymes, elevations of which have been linked to a heightened diabetes risk. The researchers noted that the liver plays a key role in sugar and insulin metabolism, and is the main site of insulin clearance from the blood.
According to Manning's team, vitamin E may boost insulin sensitivity and decrease diabetes risk in a number of ways, including by reducing oxidative stress to cells and by improving liver function.
However, they emphasized that it is unclear why blood sugar levels and insulin resistance improved only temporarily, when markers of oxidative stress and liver function continued to look better. They concluded that a larger study is needed to clarify the picture.