Antioxidants have not been shown to prevent metabolic syndrome

Even though antioxidant supplements might not cut the risk of metabolic syndrome in healthy people with no major risk of micronutrient deficiencies, low concentrations of beta-carotene and vitamin C were associated with an increased risk of the disorder, according to a new study.

The study included 5,220 adults who were randomly assigned to take either a mix of vitamin C, vitamin E, beta-carotene, selenium and zinc in capsule form or inactive placebo capsules (1). After an average of 7.5 years, 263 study participants had been diagnosed with metabolic syndrome. There was no significant difference in risk between the supplement and placebo groups.

There were differences, though, when the researchers looked at participants' antioxidant blood levels at the study's outset. The one-third with the highest vitamin C levels had about half the risk of metabolic syndrome as those with the lowest levels. Similarly, the third with the highest beta-carotene levels had only one-third of the risk of metabolic syndrome as those with the lowest beta-carotene concentrations.

The decreased risk of metabolic syndrome in people with higher blood antioxidant concentrations at the beginning of the study probably reflects their overall healthy dietary patterns, consuming antioxidant-rich foods such as fruit and vegetables.

In contrast, higher zinc levels in the blood were linked to an increased risk of metabolic syndrome. It's not clear why this is, but the researchers speculate that high zinc levels might, in some people, reflect heavy consumption of red meat ? one of the prime food sources of the mineral.

The researchers conceded that the participants in this study may have had healthier diets and lifestyles than the population at large, which could affect the generalizability of the study findings. The subjects with metabolic syndrome at the end of follow-up had more unhealthy behaviors than did subjects without the disorder; as expected, they were more often active smokers, were less physically active, had a higher BMI, and were more likely to be taking medications, particularly antihypertensive medications.

Metabolic syndrome refers to a collection of risk factors for type 2 diabetes, heart disease and stroke ? including high blood pressure, abdominal obesity, low levels of "good" HDL cholesterol, elevated triglycerides and high blood sugar. The condition is diagnosed when a person has at least three of those risk factors (see also Expert Opinion).

1. Czernichow S. et al. Effects of long-term antioxidant supplementation and association of serum antioxidant concentrations with risk of metabolic syndrome in adults. Am J Clin Nutr, 2009; 90:329–35.