Caffeine

The main functions of caffeine

• Increases energy production.
• Increases fat loss.
• Increases endurance.

Mechanism of action of caffeine

Caffeine has been used for hundreds of years. Although it is not a nutrient, it is widely used in normal diets. During metabolism, it is converted in the liver into three dimethylxanthines - paraxanthine, theophylline and theobromine.

Three main theories have been proposed for the ergogenic effect of caffeine.

• Being a CNS stimulant, it reduces the perception of fatigue.
• Enhances muscle contraction due to its beneficial effect on ion transport.
• Enhances fat utilization and thus preserves muscle glycogen.

Since caffeine enters the CNS and skeletal muscles, it is impossible to separate its effects on the CNS from its effects on the peripheral nervous system. It is possible that different mechanisms are responsible for the improvement in performance in different situations.

Research results

Interest in caffeine as an ergogenic aid was sparked by work at the Costill Laboratory over 40 years ago. In a 1978 study, nine competitive cyclists took 330 mg caffeine (5 mg kg -1 ) 1 h before a race at 80% V02max and were able to race to failure 19% longer (90 min compared to 75 min).

A 1979 study found that consuming 250 mg caffeine increased the amount of work that could be done in a 2-hour period by 20%. These two studies found that fat utilization for energy increased by approximately 30% in the caffeine trials. A 1980 study found that consuming 5 mg caffeine kg-1 reduced muscle glycogen utilization by 42% and increased muscle triglyceride utilization by 150% during a 30-min cycling session at 70% V02max.

Subsequent studies on caffeine and exercise performance have yielded conflicting results. However, in the last 10 years, it has been established that caffeine can improve endurance.

In 1991, Graham and Spriet evaluated the effects of caffeine intake on runners and cyclists. The athletes took 9 mg caffeine kg-1 1 h before cycling and running to exhaustion at an intensity of about 85% V02max. The average increase in endurance in running was 44% and in cycling 51%. However, caffeine levels in four of 12 urine samples were close to or above the IOC threshold.

Graham and Spriet conducted another study to examine the effects of different doses of caffeine in well-trained athletes. Eight subjects avoided caffeine for 48 h, then consumed 3, 6, and 9 mg caffeine/kg body weight or placebo 1 h before exercise at 85% V02max. Endurance performance increased at 3 and 6 mg kg-1, but not at 9 mg g-1. Plasma epinephrine did not increase at 3 mg, but did increase at higher doses. Only the 9 mg dose showed increases in glycerol and free fatty acid levels.

These data indicate that even the lowest dose, 3 mg kg-1, exhibits an ergogenic effect without increasing epinephrine levels.

Recommendations for caffeine consumption

Graham and Spriet found that consumption of 3-13 mg caffeine-kg-1 increases endurance by 20-50% in elite and amateur athletes during cycling or running at 80-90% V02max.

They indicate that caffeine doses of 3 to 6 mg kg-1 1 h before exercise provide an ergogenic effect without raising urinary caffeine levels above the IOC doping threshold.

Although higher doses of caffeine from 9 to 13 mg kg-1 also improve athletic performance, they are likely to cause adverse effects and raise urinary caffeine levels above the IOC (12 μg dL-1) and NCAA (15 μg dL-1) doping thresholds.

Although caffeine is relatively harmless, large doses can cause side effects including nausea, muscle tremors, increased heart rate, and headache. Athletes who are sensitive to caffeine may experience these symptoms even with small doses.

Athletes should be aware that the ergogenic effects of some proprietary supplements may be due to the caffeine they contain. Nuts, Paraguayan tea, and guarana contain caffeine.