March 18, 2009

Caffeine has so many metabolic effects that it is difficult to sort out which are responsible for the increase in physical and mental energy that most users experience, says Susan Roberts. Photo: iStockphoto

Ask The Professor

Why does caffeine give you energy?

This month’s faculty expert, Susan Roberts, professor of nutrition at the Friedman School and director of the Energy Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging, fills us in:

Caffeine itself wasn’t discovered until 1821, but caffeine-containing foods have been known and cultivated for centuries, since at least the fourth century A.D., when historical records show coffee cultivation began in Ethiopia. According to legend, Ethiopians became enamored with coffee after observing that grazing animals were more energetic after munching on particular shrubs which we now know as coffee.

Today, these same effects of caffeine are widely recognized, and research studies broadly document favorable changes such as increases in metabolic rate, increased fat oxidation and a perception of increased physical and mental energy, even while recognizing that excess caffeine leads to jitteriness and sleeplessness.

There are so many metabolic effects of caffeine that it has been hard to sort out which, exactly, are responsible for the increase in physical and mental energy that most users experience.

Caffeine is known to boost the effects of the neurotransmitters serotonin, dopamine and acetylcholine. Dopamine, for example, is known to affect levels of concentration. It blocks adenosine receptors in the basal forebrain, which when not impeded, are what typically signal the brain when it’s time to go to bed. Caffeine also increases the release of catecholamines (such as adrenaline) via the sympathetic nervous system, which among other things can make your heart beat faster, send more blood to your muscles and tell your liver to release sugar into the bloodstream for energy.

Caffeine can help muscles to contract by encouraging the sarcoplasmic reticulum in muscle fibers to release calcium ions, and it reduces the percentage of maximum exertion that a given exercise requires, to name just a few effects. Increasing circulating and intracellular substrate availability, or fuel for the muscles, occurs in response to changes brought on by caffeine, and may help to explain the perception of reduced exertion during exercise.

Research also shows that the amount of caffeine we consume matters. Consumption of 3 milligrams of caffeine per kilogram of body weight (about one to one and a half cups of brewed coffee) appears not to produce some of the energizing effects, and as much as 6 mg/kg may be needed. That’s a lot of coffee, especially for women, considering that high caffeine consumption is linked to greater bone loss. On the other hand, depending on your family history, with caffeine consumption linked to a reduced risk of Alzheimer’s disease, Type 2 diabetes and Parkinson’s, it may feel like a bargain that works in your favor.

How much caffeine you consume is a personal choice. As with so many other small decisions we make about our health every day, research doesn’t have all the answers yet, and being self-aware about the benefits we get from caffeine and understanding the potential downsides gives us our own our best solution.

Susan B. Roberts, professor of nutrition and professor of psychiatry at Tufts University, is also author of The Instinct Diet (Workman Publishing, 2009), a book on weight control that has both behavioral advice and an 8-week program with menus and recipes. See our story on the book at

Article Tools

emailE-mail printPrint