HOW COME? Why we get a caffeine jolt

How does caffeine make us feel more wide-awake and energetic? asks a readerWhen we think of caffeine, we usually imagine coffee, from hot and black to iced and topped with whipped cream and chocolate shavings. But caffeine is also found in tea leaves and cocoa beans. In fact, more than 100 plants have caffeine molecules hidden in their leaves, seeds or bark.

Extracted, caffeine is a white, crystalline, bitter-tasting powder, added to everything from cola and energy drinks to some pain relievers. Caffeine is one of many natural alkaloids, like the nicotine in tobacco leaves. Botanists say alkaloids act as a kind of plant self-defense, their taste or side effects warding off hungry animals.

Alkaloids easily penetrate human cells, and when enough caffeine enters the bloodstream, heart rate and blood pressure temporarily increase. We may have more energy and focus, and memory and athletic skills may improve.

Why do we feel more wide-awake? Without caffeine, a chemical called adenosine accumulates in our cells following a burst of energy, the molecules binding to brain receptors like keys into locks. The receptors then release chemicals that lead to that sleepy feeling.

But a caffeine molecule is shaped enough like an adenosine molecule that it fits neatly into its receptors, locking the real adenosine out -- and allowing us to feel alert longer. Caffeine also resembles a molecule called cAMP, which keeps fight-or-flight adrenaline flowing.

Caffeine also increases the brain's production of dopamine, a neurotransmitter that makes us feel excited and confident. But too much caffeine can actually make it hard to concentrate, and leave us irritable, shaky, and unable to fall asleep.

However, for most adults, having a daily cup (or three) of a caffeinated beverage is probably not harmful, and may even have health benefits. In addition to the many antioxidants in tea leaves and coffee and cocoa beans, caffeine itself has antioxidant activity, fighting "free radical" molecules that can damage cells.

One study showed that those drinking about three cups of coffee a day had a nearly 20 percent lower risk of basal cell skin cancer, the most common kind. Other studies found that people drinking several cups of coffee a day had a 25 percent reduced risk of developing Parkinson's disease.

A study published in June had more good caffeine news. Researchers studied people age 65 to 88 with mild memory impairment. Over a period of two to four years, no one who consumed the caffeine equivalent of three cups of coffee a day progressed to Alzheimer's disease.