The caffeine in coffee is a methylxanthine -- the most commonly consumed group of psychoactive substances in the world. As a stimulant, caffeine increases your metabolism, which is why it's a common ingredient in a number of "fat-burning" supplements. Just as you become tolerant to the stimulating effects of caffeine, your metabolism can also adjust to the slight boost that caffeine in coffee gives you.
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Coffee's Long-Proven Metabolic Benefits
A hallmark study published in 1980 in the American Journal of Clinical Nutrition showed that coffee has immediate effects on your metabolism after eating a meal -- particularly on improved fat oxidation. A study published nine years later, also in the American Journal of Clinical Nutrition, determined that 100 milligrams of caffeine increased the metabolic rate by 3 to 4 percent for about 150 minutes after consumption. One cup of coffee has roughly 100 milligrams of caffeine, but this varies, depending on how the coffee is brewed and on the coffee beans themselves.
In 2004, another study in the same journal showed that metabolism increased by 13 percent after a 4.5-miligram-per-pound dose of caffeine; the study also showed that lipid turnover doubled -- meaning, the participants processed fat much more readily. This study involved just eight participants. Caffeine works to break down fat by stimulating the nervous system, which in turn, signals fat cells to break down fat, according to a 2010 study in Food Science and Biotechnology.
Coffee Is Less Effective for Obese Individuals
The metabolic boosting effects of coffee seem less effective for overweight and obese people. In the 1995 American Journal of Clinical Nutrition study, the results showed that, on average, lean participants experienced an extra burn of about 150 calories while obese participants burned only 79 additional calories. Obese women also showed less fat burning as a result of caffeine intake compared to lean women, according to a 1995 study published in the American Journal of Physiology, Endocrinology and Metabolism. In this study, caffeine increased lipid oxidation by 29 percent in lean women but only 10 percent in obese women.
Tolerance to Caffeine
When your nervous system gets a boost from a cup of coffee, your heart rate increases slightly, as does the amount of blood pumped through your veins. This speeds up your metabolism, but it also makes some people jittery. With habitual use, your body becomes accustomed to the effects of caffeine. This includes metabolic boosting effects as well as enhanced alertness and energy. Also, some people are far more affected by caffeine's stimulation of the nervous system. If you don't typically get a "caffeine buzz," you might not get much metabolic boosting benefit, either.
Exercise Boost and Coffee
Coffee, or any other source of caffeine, has benefits for exercise performance, especially endurance sports. If coffee gives you the energy for you to do a particularly intense workout, you'll burn more calories. Caffeine actually helps you use fat for fuel, as well as stored glycogen from carbohydrates. As a result, you have glycogen stores available for longer periods, prolonging your stamina.
Caffeine can also make exercise feel easier. You may rate a workout as feeling less difficult when you've had a cup of coffee than you would if you had missed your morning Joe.