More green, less blood glucose
Using a research method called meta-analysis, the study authors combines 17 previously published clinical trials on green tea and blood sugar (glucose) metabolism. The studies included a mix of:
healthy, normal-weight adults,
overweight and obese adults,
people with type 2 diabetes, and
people with "borderline" type 2 diabetes (a condition sometimes called prediabetes or insulin resistance).
All of the studies met specific criteria to ensure high quality, including that:
participants drank green tea or took green tea extract for two or more weeks,
the study was a randomized, controlled human trial,
the outcomes included baseline fasting glucose or insulin levels,
other dietary supplements and black and oolong teas were not allowed,
the study used a concurrent control group--people taking no green tea (placebo) to serve as a comparison to the active-treatment group.
The 17 studies created a total sample of 1,133 adults, and from this large pool of information (data), the researchers determined that green tea significantly reduced fasting glucose and insulin levels, and lowered hemoglobin A1c values (HbA1c).
HbA1c is a measure of how much glucose is bound to your red blood cells. Compared with glucose levels, which give a snapshot in time of your glucose levels, HbA1c gives an assessment of long-term glucose control.
Putting green tea to work for you
This study has a lot of strengths, including that it only considered randomized controlled trials, and eliminated other types of tea and dietary supplements that could have contributed to effects on glucose control. There are few downsides to green tea, so if you're looking for ways to boost your diabetes-prevention plan, regularly sipping this brew is a smart step. And according to the study authors, previous research suggests green tea is most effective in those who already have risk factors for diabetes (such as metabolic syndrome), so take advantage if you are at risk.
Along with maintaining a healthy weight, exercising regularly, taking your medications as prescribed, and avoiding cigarettes:
- Catch a catechin. Green tea nutrients called catechins contribute to the brew's beneficial effects. In this study, catechin intake ranged from 208 to 1,207 mg per day. A cup of green tea has about 200 mg of catechins, so aim for at least one cup daily, and more if you can.
- Go low and watch the clock. If green tea tastes bitter to you, brew at a temperature slightly less than boiling, and steep for two to four minutes. Higher temperatures and longer brew times make for a more bitter brew.
- Squeeze it. After brewing, dip your teabag up and down in the cup, then squeeze the liquid out, to maximize the catechins in your cup.
- Add citrus, avoid milk. Sip green tea with a squeeze of lemon to help your body best use the nutrients, and don't add milk to your tea; it may block catechin absorption.
(Am J Clin Nutr 2013; doi: 10.3945/ajcn.112.052746)