Antioxidants Increase Insulin Sensitivity

Antonio Mancini, MD, of Catholic University of the Sacred Heart, Rome, reported that, among a group of 29 obese patients, those who ate an antioxidant-rich diet, homeostasis model assessment-insulin resistance (HOMA IR) fell from 4.20 to 3.13 (P<0.05) after three months, while those eating the same diet who also took metformin saw their HOMA-IR scores drop from 5.71 to 3.06 (P<0.05). 

"Our suggestion is that five servings a day of fruits and vegetables can lower insulin levels," Mancini said at a press briefing at the annual meeting of the Endocrine Society.

"It is well known that antioxidants exert beneficial effects, but, in our opinion, there is not sufficient scientific support for this theory," he continued. "There is a need for a well-controlled study to give it statistical significance."

Oxidative stress is an imbalance between damaging free radicals and antioxidant defenses in the body, Mancini explained. "Free radicals are unstable molecules ... they can transfer instability to the molecules they meet, so there is a propagation of the chain of oxidative damage."

But antioxidant defenses can repair or prevent the damage and can break that damage chain, he said. "Oxidative stress is not an innocent bystander or consequence of metabolic syndrome -- in our opinion it's a contributing factor to insulin resistance. Therefore we try to influence the antioxidant defense of the body and to observe what happens in insulin resistance."

To see what happens when antioxidant defenses are shored up, Mancini and colleagues enrolled 29 obese patients in their study. Of those, 16 were male; the patients ranged in age from 18 to 66, and their average body mass index was 36 kg/m².

Patients were divided into four groups: one group ate a typical Mediterranean diet with 400 to 500 mg of antioxidants per day, and the second group ate the same diet but also received metformin. A third group ate an antioxidant-enhanced diet with 900 to 1000 mg of antioxidants per day, and a fourth group ate the enhanced diet and also received metformin.

The enhanced diet included antioxidant-rich foods such as wild berries, orange juice, tea, chicory, carrots, capers, and tomatoes.

Parameters measured before and after the study included plasma glucose and insulin levels after an oral glucose tolerance test; total cholesterol, HDL cholesterol, LDL cholesterol, and triglycerides; uric acid, albumin, and total antioxidant capacity; and FT3, FT4, TSH, cortisol, ACTH, IGF-1, and testosterone (in males).

In addition to the decrease in HOMA-IR, the researchers also found that the insulin peak declined from 181.61 to 157.62 in the group that was fed the Mediterranean diet plus metformin (P<0.05) and from 246.44 to 141.73 in the group that was given the antioxidant-rich diet plus metformin (P<0.05).

No differences were observed in glucose area under the curve or in lipid values and weight loss did not differ among the study groups.

Mancini noted that the improved results the researchers found when metformin was added to the diets indicate that the antioxidants increase the response to the drug, "but they also had an effect when administered alone." He said his group intends to expand the study, enrolling a total of 30 patients in each group.

• Note that this small study (29 patients) suggests that an antioxidant-rich diet helped to increase insulin sensitivity and enhance the effects of metformin.
• The study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

A Mancini, et al "Oxidative stress and metabolic syndrome: Effects of natural dietary antioxidants in obese patients with insulin resistance" ENDO 2010; Abstract P3-428.