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Does a Vegetarian Diet Lead to Better Glycemic Control?

Meta-analysis shows some reduction in HbA1c but not a significant change in fasting glucose….

Previous studies have shown a possible association between vegetarian diets and lower rates of type 2 diabetes. Studies have also shown improved glycemic control when subjects were switched from an omnivorous diet to a vegetarian diet. Many of these studies were limited by a small sample size. Trial length for several of the studies was also a limitation. With durations of less than one month, the effect of the diet may not be fully recognized.

The purpose of this study was to use a meta-analysis to assess the association between consuming a vegetarian diet and glycemic control in patients with type 2 diabetes. Studies selected to be part of this meta-analysis needed a vegetarian diet that did not include meat, poultry or fish and was at least four weeks in length. The primary outcome was HbA1c level comparing those in the vegetarian diet group to those in an omnivorous/control diet group. Six studies were found to meet inclusion criteria with a total of 255 subjects.

Pooled analysis showed that a vegetarian diet was associated with a reduction in HbA1c (-0.39%, 95% CI -0.62 to -0.15, P=0.001) when compared to consuming an omnivorous diet. A significant difference in reduction of fasting blood glucose was not observed between the two groups (-0.36mmol/L, 95% CI -1.04 to 0.32, P=0.301).

The results of this study show an association between following a vegetarian diet and a reduction in HbA1c. A reduction in fasting blood glucose was not observed when consuming a vegetarian diet. While the results of this study showed a statistically significant difference, the HbA1c reduction of 0.39% was still quite small. A previous meta-analysis found that metformin therapy had over double the effect size with an HbA1c reduction of 0.9%. Advising a patient to begin a vegetarian diet could be very difficult. Telling a patient to completely alter their eating habits will likely be met with resistance and low compliance. Such a recommendation should be reserved to patients not yet ready or not yet willing to begin using medication therapies.

Practice Pearls:

  • Previous studies have shown that those following vegetarian diets may have lower rates of diabetes and better glycemic control
  • The results of this meta-analysis showed that a vegetarian diet was associated with a 0.39% reduction in HbA1c levels
  • The sudden implementation of a vegetarian diet in type 2 diabetes would not be very practical as many medications can offer greater HbA1c reductions without an extreme change in lifestyle

Yokoyama Y, Barnard ND, Levin SM and Watanabe M. Vegetarian Diets and Glycemic Control in Diabetes: a Systematic Review and Meta-Analysis. Cardiovascular Diagnosis and Therapy. 2014.