A meta analysis study shows that acute caffeine ingestion reduces insulin sensitivity as measured by the methods of oral glucose tolerance testing and the insulin sensitivity index testing. This research is fairly recent (2016) and so far researchers do not know the impact of chronic caffeine ingestion on human health. Some question I would have are whether or not these changes to insulin sensitivity are temporary or permanent? Do genetics play a role? It appears some people do have a gene which causes caffeine to stay in the system for a longer period of time. Additionally some people are genetically vulnerable to diabetes but insulin resistance alone doesn't always lead to type 2 diabetes but does lead to what is known as "pre-diabetes" or glucose intolerance.
Does caffeine cause glucose intolerance?
References
Liu, K., Zhou, R., Wang, B., Chen, K., Shi, L. Y., Zhu, J. D., & Mi, M. T. (2013). Effect of green tea on glucose control and insulin sensitivity: a meta-analysis of 17 randomized controlled trials. The American journal of clinical nutrition, 98(2), 340-348.
Robertson, T. M., Clifford, M. N., Penson, S., Chope, G., & Robertson, M. D. (2015). A single serving of caffeinated coffee impairs postprandial glucose metabolism in overweight men. British Journal of Nutrition, 114(8), 1218-1225.--
Shi, X., Xue, W., Liang, S., Zhao, J., & Zhang, X. (2016). Acute caffeine ingestion reduces insulin sensitivity in healthy subjects: a systematic review and meta-analysis. Nutrition journal, 15(1), 103.