Can Green Tea or Garlic Prevent Diabetes, Prediabetes?

Written by Rosemary Black
Reviewed by Angela Fitch MD, FACP

Could drinking green tea and adding garlic reduce your risk of prediabetes and diabetes? Two recent studies—one on green tea and the other on aged garlic—found that extracts from these foods may be beneficial to individuals who have diabetes or who are at risk for developing the disorder.1,2

But don’t run out and buy green tea extract or garlic extract just yet! If they offer any health benefits, the best result occurs when they are taken along with other strategies, experts say.

 “Everybody wants that one-pill fix,” says Sherri Findley, MS, RD, a dietitian at University of Florida Health Shands Hospital in Gainesville, Florida, “Of course, it’s tempting to take a product and hope it will prevent these diseases.”

Findings of AntiDiabetes, Obesity Properties  

In the recent clinical trial on aged garlic extract that focused on adults with obesity,1 researchers focused on whether daily supplements of aged garlic extract might reduce inflammation and improve immune function. The results are promising given some evidence that taking aged garlic extract (3.6 grams) might improve blood cholesterol levels and have a favorable effect on the immune system.1

In a second study, also published in the journal Clinical Nutrition ESPEN, 120 women who were overweight (body mass index >24) were assigned to consume green tea extract (1 gram), metformin, or green tea with metformin.2 The evidence suggested that green tea extract outperformed metformin in terms of improving blood sugar control in women who did not have diabetes but were overweight and therefore considered at risk for developing the condition. 

Such Small Studies Only Offer a Bit of Hope, for Now

Angela Fitch, MD, FACP, associate professor and vice president of primary care at University of Cincinnati College of Medicine/UC Health in Ohio, says that the catechins in green tea do provide health benefits. “We know that these catechins can help with blood sugar regulation,” she says.

It is still unclear whether either of these extracts could prove useful in managing glucose in people with diabetes. Nearly half of all Americans have diabetes, or prediabetes,3 which is a condition that causes blood glucose levels to rise above normal but not high enough for a diagnosis of diabetes. Individuals with prediabetes, who are at risk of developing diabetes as well as cardiovascular disease,4 are often advised to make changes to their diet and to try to lose weight as the best way to reduce the risks associated with diabetes and cardiovascular disease.

While results from the two studies offer some promise of health benefits, it is important to keep in mind that they both involved a small number of people and that if the same research was conducted with larger numbers of subjects, the findings might end up being quite different, Dr. Fitch says. “The two studies show a benefit, but if we repeat these studies with 10,000 patients, they may show no connection between the extracts and improvements in diabetes symptoms,” she says.

Ms. Findley says taking aged garlic extract would not be a part of her current recommendation for obesity-related disease states including diabetes. “There is some evidence that these foods might be useful for concerns about heart disease and cholesterol levels,” says Dr. Fitch. 

Go with Known Strategies for the Greatest Benefits

While green tea extract and aged garlic extract may be helpful in reducing the risk of developing diabetes or prediabetes, it makes more sense to embrace more proven and sure measures as well, the experts say.

Here are a few things to try.

If you decide to take a supplement of either green tea or aged garlic extract, keep in mind that these pills are not regulated by the Food and Drug Administration, says Dr. Fitch. “You can’t be sure of the quality of the supplement you are taking,” she says. At the very least, be sure to look for products that carry the seal from independent testing labs such as, the USP, ConsumerLab, or NSF International, which offer some assurance that the ingredients have been evaluated.


  1. Ferreira MA et al. Green tea extract outperforms metformin in lipid profile and glycaemic control in overweight women: A double-blind, placebo-controlled, randomized trial.” Clinical Nutrition ESPEN. Published ahead of print, August 2018. Available at: Accessed January 31, 2018.
  2. Xu C, Mathews AE, Rodrigues C, et al. Aged garlic extract supplementation modifies inflammation and immunity of adults with obesity: A randomized, double-blind, placebo-controlled clinical trial. Clinical Nutrition ESPEN. Published ahead of print, December 2017. Available at: Accessed January 31, 2018.
  3. American Diabetes Association. Economic burden of prediabetes up 74 percent over five years. Available at: Accessed February 5, 2018.
  4. National Institute of Diabetes and Digestive and Kidney Diseases. Diabetes Prevention Program. Available at: Accessed February 5, 2018.

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