Caffeine taken 45-60 minutes before exercise can facilitate a shifting to burn more fat and less glycogen. It increases the breakdown of fat, freeing fatty acids, which are used for producing energy during exercise.12
The caffeine-enhanced fat breakdown can only occur during exercise. While the fat is being burned, glycogen and glucose are being reserved, allowing blood sugar levels to remain higher for longer. Higher glucose levels ward off hunger. The brain functions exclusively on glucose, and higher blood sugar levels facilitate thinking.
Caffeine Effects on Weight Loss & Performance
You may experience an enhancement in both performance (ease at which you complete a session), especially in endurance sports, and the rate in which your body releases and uses fat during your cardio session by consuming a small to moderate amount of caffeine.34 You just need cycle the use of caffeine on and off, approximately every 4 weeks, otherwise your body will become accustomed to it, thus losing the benefits.
However, there are some definite downsides. Caffeine can increase urination by up to 30% (leaving you dehydrated), increase anxiety and restlessness, cause upset stomachs and increase heart rate. In rare cases, high caffeine intake in combination with excessive exercise may lead to rhabdomyolysis (a life-threatening condition caused by muscle breakdown which may cause kidney failure).5
Some people are more sensitive to caffeine and may which to avoid. Furthermore, people with severe anxiety, cardiovascular disease, reflux disease, liver or kidney impairment, seizures, and pregnant women should be more cautious.
While caffeine tolerance is highly individual, daily doses of up to 400 mg of caffeine are considered safe for most adults.6 According to the American College of Obstetricians and Gynecologists (ACOG) up to 200 mg daily is safe during pregnancy and should seek advice from a qualified medical professional before using coffee or other caffeine sources for sports performance.7
An average cup of coffee (8 fl oz.) contains nearly 100 mg of caffeine.8 Therefore, consuming 1–2 cups of coffee 60 minutes before your workout should easily provide sufficient caffeine to improve performance.9 However, you may need less caffeine or want to avoid entirely if you are more sensitive to caffeine.
Expert advice is that for the average gym user a coffee before going to the gym is fine to perk you up, but stay away from caffeine pills and supplements.