The health effects of caffeine have long been debated. A naturally occurring stimulant that speeds up the body's metabolism, caffeine is an active ingredient in many diet pills.
At the same time, reports that caffeine increases blood sugar levels and insulin resistance has prompted concerns that it could actually cause weight gain or retard weight loss efforts.
Clinical research suggests that claims on both sides of the caffeine issue may be exaggerated.
Caffeine does cause the body to produce more adrenaline, which in turn increases the rate at which it releases fatty acids that muscles can convert to energy. However, the use of caffeine alone has a minor effect on weight loss.
That's because, under such conditions, the amount of fatty acids released is greater than what the body actually needs. In such cases, most of the released fatty acids travel through the bloodstream back to the fat cells and are once again stored.
In a 2004 study, researchers found that people given 10 milligrams of caffeine per kilogram of body weight nearly doubled their release of fatty acids, but their metabolic rate only increased by 13% (1).
Combining caffeine with exercise appears to yield better results...
Exercise increases the amount of fat cells burned, and caffeine ingested prior to exercise further increases that amount by about 30% during the first 15 to 30 minutes of the workout (2).
Meanwhile, the body continues to burn fat at an accelerated rate after exercise, and caffeine also appears to augment this effect (3).
Even so, most of these studies have been conducted on subjects who fasted prior to exercise. Consumption of carbohydrates appears to substantially reduce the effect of caffeine on the number of fat calories burned during exercise (4).
Just as with weight loss claims, the extension of a few simple physiological truths about caffeine can yield misleading conclusions about the connection between caffeine and weight gain.
When fat is being burned (a process that can be accelerated by caffeine) blood sugars are being reserved, so sugar levels do remain higher for longer, as do levels of the insulin that is produced to help move blood sugars through the system. High levels of insulin can reduce the body's ability to burn stored fat.
However, such effects from caffeine appear to be minor and short-term – lasting only two hours after a high-carbohydrate meal, according to one study (5).
One of the relatively few long-term studies on caffeine and weight tracked 18,417 men and 39,740 women over a 12-year period. Researchers at the University of Madrid found increased caffeine intake was associated with slightly smaller weight gains in men and women. But because it was an observational study, researchers could not say definitively that caffeine actually caused weight change (6).
A number of studies have linked caffeine to improved athletic performance and endurance, improved concentration and decreased fatigue during exercise, and decreased muscle soreness afterwards.
Laboratory studies involving running or cycling after taking a placebo or caffeine one hour before exercise have shown that caffeine improves endurance by prolonging time to exhaustion (7,8). In another study, the ingestion of caffeine reduced the time taken to swim 1,500 meters by an average of 23-seconds (9). Such studies generally involve well-trained athletes, while field studies are lacking.
As for short-term exercise, there are mixed reports. A 1998 study showed runners given caffeine supplements an hour before exercise resulted in their running 14% longer before reaching exhaustion than when they took a placebo (10). But a recent study using weightlifters found caffeine did not significantly alter muscular strength or endurance during intense bench press or leg press exercise (11).
Coffee does not provide the same level of benefit as caffeine capsules.
A study comparing the effects of coffee, caffeine supplements and a placebo among a group of runners showed no difference in run times when they drank coffee or took a placebo, but a marked improvement among most of them when they took caffeine capsules (12). This suggests that other substances in coffee can interfere with the effect of its caffeine content.
For the best results, 3-9 mg of caffeine per kilogram of body weight is recommended one hour before exercising.
However, a doctor should be consulted before caffeine is made part of a diet or fitness regime. Caffeine not only increases the body's heart rate and blood pressure, but it can interrupt sleep and cause nervousness and irritability. More than 500mg to 600mg of caffeine a day is considered heavy use and is discouraged.
When taken along with the stimulant ephedra, caffeine can increase the risk of a heart rhythm irregularity (13), while very high doses of caffeine and acetaminophen taken together can lead to liver damage (14). Increased dosages of caffeine provide very little additional benefit (15).
There is evidence to show that supplemental caffeine (not coffee) can help accelerate fat loss both during and after exercise.
However, the effects of using caffeine alone (without exercise) are likely to be small. Smaller quantities of caffeine (3-5mg per kilogram of bodyweight) seem to have the effect as larger doses and anyone using caffeine for the first time may want to build up to this level gradually.
Pregnant women and children should restrict caffeine intake (15) so supplementation is not recommended. This is also true for anyone with high blood pressure as caffeine may further exasperate this (16).