Mysterious protective effects of coffee against diabetes have been reported in the past. A 2010 meta-analysis analyzing data from 18 studies reported that each additional cup of coffee consumed per day was associated with a 7% reduction in risk of diabetes.1 This was surprising, especially because coffee consumption has been shown to raise glucose levels after a meal so you would expect it to worsen diabetes, not help it. However, this is true of both decaffeinated and regular coffee, although regular coffee raises blood glucose more than decaf.2
The reason for the decreased diabetes risk remains uncertain, but since coffee comes from a darkly colored bean, it is likely that antioxidants, minerals, or other phytochemicals present in coffee may be responsible for the long term benefits seen in the observational studies.3 With this in mind, we must also remember that almost all of the subjects in the observational studies were eating the standard American diet and therefore starving for antioxidants and phytochemicals.
Is the standard American diet so nutrient-poor that a significant portion of people’s phytochemical intake comes from their morning coffee?
It’s likely. Additional studies support this possibility. One observational study of 28,000 postmenopausal women actually found that decaffeinated coffee was more protective than regular coffee – which suggests that the caffeine in coffee might be increasing risk, while the phytochemicals decrease risk.4 Chlorogenic acid and trigonelline, two of the major phytochemicals in coffee, have been shown to decrease blood glucose and insulin concentrations in the blood compared to placebo after ingesting sugar, so these phytochemicals likely increase insulin sensitivity.5 It is doubtful that coffee would offer any additional protection on top of a nutrient dense diet - the responsible phytochemicals can be obtained from other plant foods and the diet would not be so lacking in antioxidants. For example, blueberries contain the antioxidant chlorogenic acid, and the phytoestrogen trigonelline is also found in peas, lentils, soybeans, and sunflower seeds. 6-8 The only reason coffee is beneficial is because of the severe deficiencies in the plant-derived phytochemicals in the diet of most Americans, and coffee at least supplies something.
Research has found something that makes the insulin desensitizing effects of caffeine even worse - ingesting caffeine with a high-fat meal.
High-fat meals are another factor known to impair glucose tolerance, and saturated fat consumption causes the body to produce inflammatory molecules that contribute to insulin resistance.9 This study demonstrated caffeine consumption and a high-fat meal had additive insulin desensitizing effects, and this did not merely raise the blood glucose - but also when the insulin doesn’t work well the body has to make more of it, and higher insulin causes weight gain and increases cancer risk.10-12 When subjects ingested a high-fat meal followed by a sugary drink, and blood glucose levels were 32% higher compared to subjects who had water in place of the high-fat meal. In the second part of the study, subjects were given two cups of caffeinated coffee in addition to the high-fat meal and sugary beverage – this time, blood glucose was even higher – 65% higher than the subjects who had only water before the sugary drink.13 Apparently, coffee can have good or bad effects on insulin depending on whether it is consumed with high fat animal products or not.
The message here is that coffee can be both good and bad, but its powerful addictive qualities, with the potential for withdrawal headaches and to increase blood pressure should make people cautious;14-16 the most likely risks are almost never mentioned in news reports. I do not think anyone should rely on coffee to protect themselves against diabetes. If you do choose to drink coffee, stick to water-processed (non-chemical) decaf, and of course skip the doughnuts!
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