Antioxidants Boost Insulin Sensitivity
SAN DIEGO—A diet rich in natural antioxidants may help improve insulin sensitivity in insulin-resistant obese adults while enhancing the effect of metformin, according to new Italian study data presented at The Endocrine Society's 92nd Annual Meeting.
“The beneficial effects of antioxidants are known, but we have revealed for the first time one of their biological bases of action—the improvement of hormonal action in obese subjects with the metabolic syndrome,” said principal investigator Antonio Mancini, MD, Professor of Endocrinology at The Catholic University of Sacred Heart in Rome.
Evidence suggests that oxidative stress may play a role in metabolic syndrome. Oxidative stress results from an imbalance between an excessive amount of oxidants and decreased antioxidant defenses. Antioxidants include vitamins E and C, selenium and carotenoids. Previously published studies have demonstrated that antioxidants can prevent oxidative damage to cells and in some cases also help repair damage.
Dr. Mancini and his colleagues studied the effects of dietary antioxidants on insulin resistance in 16 men and 13 women aged 18-66 years who were obese and insulin-resistant but were not yet diabetic. The investigators randomly assigned the subjects to one of four treatment groups. All groups ate a low-calorie, Mediterranean-type diet that averaged 1,500 calories a day. This diet contained only 25% from protein foods, with the rest made up of low-gylcemic-index carbohydrates. Group A only ate this kind of diet and group B ate the same diet but also took metformin. For group C and group D, the researchers prescribed a diet enriched with antioxidants, with a calculated intake of 800-1,000 mg a day coming from fruits and vegetables. Group D also took metformin.
Despite similar weight loss in all the groups only the two groups receiving the antioxidant diet (groups C and D) had a significant decrease in insulin resistance. Group D had the most significant improvements in insulin resistance based on an oral glucose tolerance test.
“Our suggestion is to eat five a day servings of fruits and vegetables,” Dr. Mancini said. “This way you can lower insulin levels.”
The study showed that the groups that received a diet rich in antioxidants experienced improvements in lipid profiles. The high fiber content in the enriched diets may also play a significant role, he said. Additionally, study findings suggest that combining metformin with a diet high in antioxidants may have a synergistic effect.
In a separate study presented at this meeting, British researchers reported that high intake of fructose by children may increase their risk of developing type 2 diabetes. The investigators studied biopsy specimens of subcutaneous and visceral fat from 32 normal-weight pre-pubescent children. From these samples, they obtained preadipocytes, precursors to fat cells that have the potential to differentiate or mature into fat-containing adipocytes. The researchers allowed the precursor cells to mature for 14 days in culture media containing normal or high levels of glucose or high levels of fructose.
Exposing the preadipocytes to high levels of fructose resulted in the formation of more mature fat cells in the visceral fat compared with glucose exposure. In addition, fructose was associated with a decrease in insulin sensitivity. The investigators concluded that high levels of fructose, which may result from eating a diet high in fructose throughout childhood, may lead to an increase in visceral obesity and an increased cardiometabolic risk.
“I would avoid carbonated drinks which are extremely high in fructose corn syrup,” said study investigator Jeff Holly, PhD, Professor of Clinical Science at the University of Bristol. The new findings suggest that such avoidance would reduce the body's propensity to put fat into abdominal stores and thus increase the risk of diabetes, he noted.