Caffeine + Calorie Burn
Coffee and tea get you charged and ready for your day. It is no wonder that they may increase your calorie burn as well.
However, the effect of caffeine on metabolism doesn't appear as clear cut as one might think. Drinking coffee and perhaps watching TV, without any other lifestyle changes, may not necessarily improve your health based on the data we've summarized.
Combining a coffee or tea habit with diet and exercise may help you improve your BMI and may protect you from diabetes.
A small double-blind controlled trial, one of the best types of clinical studies, looked at how healthy individuals respond to caffeine intake. Participants were given capsules containing either a placebo (sugar pill), 100mg of caffeine, 200mg of caffeine, or 400mg of caffeine. Their metabolic rates were then measured by collecting their blood and their breath.
They found that those who were given 100mg or 200mg of caffeine burned more calories than those given a placebo, which is a sugar pill used as a control. Of note, the participants did not know which of the pills they were given at the time.
Those given 400mg of caffeine, roughly equivalent to a venti cup of coffee from Starbucks, burned the most calories, with an approximate 10-12 calorie per hour increase in their metabolic rate.
This study demonstrates that coffee indeed increases your metabolic rate, though the effect is not huge.
Effect of Coffee & Tea on Diabetes Risk
A second study looked at the risk of diabetes among coffee and tea drinkers. Researchers followed 7006 individuals in the National Health and Nutrition Examination Survey to see whether drinking coffee and drinking tea regularly had any effect on diabetes risk.
They found that drinking 2 cups of coffee or drinking 2 cups of tea a day were both associated with a significantly lower risk of diabetes, but only when combined with weight loss.
For individuals who lost weight over the average 8-9 year course of the study, researchers estimated a 60-70% lower risk of diabetes compared to similar adults who did not regularly drink 2 cups of coffee or tea per day. This difference was found to be statistically significant, as represented by the rectangular bars in the above figure.
However, individuals who gained weight over the course of the study did not benefit from drinking coffee or tea. Tea consumption in these individuals was associated with an approximately equal risk of diabetes. Coffee was associated with a non-significantly higher risk of diabetes.
Clinical Trial Looking at Green Tea + Weight Loss
This data is supported by a small clinical trial that examined the effects of green tea and weight loss.
In a randomized controlled trial, researchers had 76 overweight individuals lose an average of 13 pounds through a low calorie diet. They then gave some of the individuals caffeinated tea and others a placebo. They found that the individuals who were given tea maintained their weight loss, while individuals in the control group quickly regained much of their lost weight.
Of note, this only applied to individuals who were not already heavy caffeine users. Subjects who had a heavy coffee or caffeine habit did not significantly benefit from the caffeinated tea.
Effect of Caffeine on Mortality
These data provide a possible explaination for the association between coffee and a longer life. Researchers in 2012 found that coffee was associated with a significant reduction in risk of death for men and women. Additionally, those who drank more appeared to have a higher risk reduction.