Title and Abstract
Greenberg JA et. al. Coffee, diabetes, and weight control. Am J Clin Nutr. (2006) 84(4):682-93.
Several prospective epidemiologic studies over the past 4 y concluded that ingestion of caffeinated and decaffeinated coffee can reduce the risk of diabetes. This finding is at odds with the results of trials in humans showing that glucose tolerance is reduced shortly after ingestion of caffeine or caffeinated coffee and suggesting that coffee consumption could increase the risk of diabetes. This review discusses epidemiologic and laboratory studies of the effects of coffee and its constituents, with a focus on diabetes risk. Weight loss may be an explanatory factor, because one prospective epidemiologic study found that consumption of coffee was followed by lower diabetes risk but only in participants who had lost weight. A second such study found that both caffeine and coffee intakes were modestly and inversely associated with weight gain. It is possible that caffeine and other constituents of coffee, such as chlorogenic acid and quinides, are involved in causing weight loss. Caffeine and caffeinated coffee have been shown to acutely increase blood pressure and thereby to pose a health threat to persons with cardiovascular disease risk. One short-term study found that ground decaffeinated coffee did not increase blood pressure. Decaffeinated coffee, therefore, may be the type of coffee that can safely help persons decrease diabetes risk. However, the ability of decaffeinated coffee to achieve these effects is based on a limited number of studies, and the underlying biological mechanisms have yet to be elucidated.
Caffeine is another one of those compounds about which there is endless argument and debate. Some feel that it is evil, too much causes all manners of problems, and should be eliminated completely. Others like me feel that the only problem with caffeine is when there isn’t enough of it.
Specific to this research review, it’s often been claimed that caffeine raises insulin, causes insulin resistance and deteriorates blood glucose control. Thus individuals suffering from the Metabolic Syndrome/Insulin Resistance/Pre-Type II diabetes (all being the same name for essentially the same thing) should avoid it.
But what does the research actually say in this regards?
First and foremost, there is actual some truth to the idea that caffeine can cause problems with blood glucose control and insulin levels, at least if you’re looking at high doses of caffeine right before a meal tested under acute (single meal) conditions.
Typically doses of 5 mg/kg are given which is 500 mg of caffeine for a 100kg (220 lb) person. Under those conditions, at least in short-term studies, problems are often seen. This is an enormous amount of caffeine.
Putting this in a real world perspective, a typical soda or cup of coffee might contain 60-100 mg of caffeine, or approximately 1/5th to 1/8th the amount used in most studies. Of course, we all know people who’s idea of caffeine intake means drinking the entire pot and, in that situation, the above might actually apply. But the person drinking a soda with a meal or a cup of coffee won’t even be close.
At the same time, epidemiological studies (which are not the strongest data set in my opinion) suggest that regular caffeine/coffee intake may actually be beneficial in terms of limiting the incidence of diabetes and may play a role in weight loss.
So clearly it’s a bit more complicated than it looks (or is claimed) and this paper set out to uncomplicate things.
The first data set the review looked at was epidemiological data. Now, I’m no fan of epidemiology in general, there can be a lot of confounding factors when you’re trying to determine what causes what. At best, that kind of data gives a starting point and some possible correlational data to do direct work; at worst it’s useless.
That said, looking at 20 studies on the topic, the researchers found that 17 of the 20 showed a beneficial effect of habitual coffee/caffeine intake on diabetes and glucose metabolism, 3 found no effect and none showed a negative effect.
Getting a bit more detailed, four of the studies suggested that non-caffeine components of coffee were involved (that is, pure caffeine and coffee per se may have different effects, a topic I’ll come back to below) and four studies found an effect of decaf coffee (suggesting that non-caffeine components are playing a role here). One study suggested that the impact of coffee was due to an effect on body weight (weight loss) which was the next topic of the paper and what I’ll discuss next.
Looking first at rat data, the paper examines data showing that caffeine can reduce bodyweight, fat pad weight and even fat cell number. However, humans aren’t rats and human data on this topic is mixed at best. One human study found no impact of caffeine on weight loss but as mentioned above it may be that coffee and other non-caffeine components explains the epidemiological data.
Next up, the paper looked at the impact of caffeine on thermogenesis (calorie burning) and lipolysis (fat mobilization). It comments that a habitual caffeine of 600 mg/day (~6 strong cups of coffee) could lead to an extra caloric expenditure of 100 cal/day (equivalent to walking about 1 mile for a 150 lb person). This effect also occurs with ground and instant coffee, but not decaf so the effect is probably mediated via the caffeine itself. Do note that the body can develop tolerance to these effects so any effect might not be very long lasting.
Related to this, caffeine has also been shown to increase lipolysis (fat mobilization) and fat oxidation and both caffeine and coffee have this effect; decaf does not so it’s clearly an effect of the caffeine per se. Interestingly, both the impact on lipolysis and fat oxidation is more pronounced in non-obese than obese individuals; leaner individuals, probably due to a greater sensitivity to lipolytic stimuli, get a larger effect.
In any case, I want to expand on that a bit, even if caffeine is having a negative impact on insulin sensitivity or insulin levels, the simple fact is that it increases fat mobilization. Combined with a caloric deficit or exercise, this means that those fatty acids can be burned off the body. The idea that caffeine is somehow bad on a diet because of an impact on insulin sensitivity is missing the forest for the trees; caffeine increases fat mobilization and burning and that’s what matters in the long run for losing body fat.
The paper also mentions that caffeine may increase energy expenditure. Doses of 3-30 mg/kg in rats increase spontaneous activity and this type of activity (called NEAT or non-exercise activity thermogenesis in humans) can amount to a fairly considerable energy expenditure. Basically, caffeine may help with weight loss by making you move around more. Again, decaf does not have this effect.
Additionally, a very well known effect of caffeine is improved exercise performance. Caffeine pre workout decreases fatigue, causes more fat to be used (sparing glycogen) and has a host of other effects. By allowing exercisers to work harder, caloric expenditure can be increased. Which can only facilitate fat loss.
The next topic discussed has to do with the direct impact of caffeine/coffee on insulin and blood glucose tolerance with a majority of short-term studies showing a negative impact of coffee/caffeine on glucose tolerance when given right before a carbohydrate containing meal.
Note that caffeine was not found to raise insulin or blood glucose when not given with a carbohydrate meal; the fear of caffeine on low-carb diets (it’s often claimed that caffeine will raise insulin and should be avoided on such diets) appears to be unfounded. However, this data is at odds with the epidemiological data suggesting that chronic caffeine/coffee intake decreases diabetes risk.
Data comparing the effects of decaf to caffeinated coffee suggests a possible explanation; decaf coffee tends to lower blood glucose, suggesting the presence of non-caffeine compounds in coffee that may beneficially impact on blood glucose levels.
Note also that animal research suggests a tolerance to any impact of caffeine on blood glucose levels although this has not been studied in humans. However, humans are known to develop a tolerance to the stimulant, thermogenic and other effects of caffeine, it may be that chronic intake of caffeine has a very different effect on blood glucose levels that studies looking at single dose intakes.
Mechanistically, caffeine probably impacts on blood glucose tolerance by raising blood fatty acids and catecholamine levels, both of which impair skeletal muscle insulin sensitivity. That is, the way that caffeine might have an impact on insulin resistance (and thus indirectly insulin levels when carbohydrates are consumed) is by its effects on lipolysis.
Additionally, the potential impact of coffee/caffeine on fullness was noted but this has not been well researched in humans, some studies indicate higher satiety in folks using coffee/caffeine habitually.
Next the paper delved into other potential health effects. Acutely, caffeine/coffee can raise blood pressure a bit but the body develops partial tolerance rapidly. High caffeine intakes have been found, in animal studies, to cause problems with pregnancy; as well, it may potentiate the negative effects of alcohol and tobacco in this regards. Intakes of >3 cups/day of coffee can decrease fetal birth weight.
Additionally, caffeine withdrawal can cause headaches, irritability, anxiety, depression, drowsiness and fatigue. Folks wanting to reduce their caffeine/coffee intake (for whatever reason) should do so gradually to avoid problems.
High doses of caffeine can also contribute to the risk of kidney stones in elderly individuals and could cause problems with osteoporosis; this is mainly seen with daily calcium intake is low to begin with.
Early research suggested a link between coffee and an elevation of blood lipids but this turns out to only hold for boiled coffee, not brewed.
Finally, the paper discussed the issue of non-caffeine compounds in coffee that might have additional effects on the body. One (I’ll spare you the name) has been shown to decrease glucose uptake from the intestine, this might offset negative potential effects of caffeine on blood glucose levels (caffeine alone accelerates glucose uptake from the gut).
Another compound (called a quinide) was shown to enhance glucose uptake and insulin sensitivity in rats, and both the high antioxidant content of coffee along with the magnesium intake may improve insulin sensitivity in the long-term; this might explain the discrepancy in the short-term and epidemiological data.
More research into the non-caffeine components of coffee still needs to be done.
So what’s the take home in this? Caffeine/coffee intake appears to have different effects when looked at in the short and long term; looking only at acute dosing studies (esp with extremely high doses of caffeine) aren’t that relevant to how people actually consume caffeine (over the long term).
In the short-term, caffeine can impact positively on a number of factors (such as delaying fatigue during exercise, increasing lipolysis, and increasing fat oxidation and caloric expenditure) but negatively on others (decreased glucose tolerance/increased insulin response, slight increase in blood pressure).
However, longer term studies suggest that habitual caffeine/coffee intake is, overall, beneficial: it decreases the risk of diabetes and may contribute to preventing weight gain Tangentially, of course, any benefit of coffee/caffeine itself is going to be more than outweighed if you fill it up with sugar, cream and other high calorie goodies.
My take on the topic: used in reasonable amounts, caffeine pretty much does nothing but help fat loss. The impact on insulin sensitivity is overstated (in my opinion) only applying to acute studies with massive doses. The known impact of caffeine on lipolysis and improving exercise performance is so well-established as to be beyond debate.
So use caffeine, just don’t go nuts with it.