Many questions that I get from friends and family are related to keto and caffeine. Since I am a frequent coffee drinker and usually have 5 to 6 cups of black coffee every day I decided to check it out a little to see if there is any research or indications that caffeine will impact on ketosis.
So far in my researching in this topic I have not found any real proof that shows that it has any impact. There is some indication of people who claim they have a slower keto progress when drinking coffee or tea compared to when not. But there is not a big enough sample to draw any real conclusion on this. It might be that caffeine could interrupt the glucose metabolism, which in turn could affect ketosis but there is no real evidence to support this.
Then when considering the insulin resistance there is another thing to consider. Ketosis decreases the insulin resistance by improving the insulin sensitivity, which means the ability for the cells to absorb glucose. There is some indications that caffeine might increase the insulin resistance but this has also not been proven in any studies. There was a study made at Duke University Medical Center in August 2004 that looked at the effects of caffeine on the blood glucose levels and insulin resistance. The study result showed that 250mg of caffeine did not change the glucose levels. The interesting part of this study however was that there was actually an increase in the glucose levels if you had 75g of carbohydrates together with the caffeine.
As the result from the study shows the glucose levels might increase if having caffeine together with a meal with a high amount of carbohydrates but there is no indication what the impact is for people who eat according to a low carb diet.
This has also been mentioned by Lyle McDonald who is the author of the book “The Ketogenic Diet: A Complete Guide for the Dieter and Practitioner”. McDonald says that the idea that caffeine can interfere with ketosis is not possible.
There is also one study that has looked at the effect of coffee and tea on the glycaemic index of foods. This study is not really focusing on people following a low carbohydrate diet but the result is still interesting. The study compared people who either drank 250ml coffee or 250ml water and showed that there was in fact an increased blood glucose levels in the people who drank coffee. However the impact was not very big and the study also concludes that coffee or tea did not affect the meal GI value.
In total it is still hard to find specific studies on people following a low carbohydrate diet such as the keto diet and caffeine. I have not been able to find any study that shows this exact information so far. I have been on the keto diet now for around 5 months and I am down over 65 lbs (30+kg) and I also have 5-6 cups of coffee every day. Since I have not seen any real slowdown in my weight loss during this time I draw the conclusion that at least for me there is no negative impact between keto and caffeine.
I also think you can drink your coffee or tea without any problems and you should not worry that it will kick you out of ketosis. As the studies do not show any conclusive results that it will impact your weight loss I draw the conclusion there is no or extremely small impact by combining keto and caffeine.