Exercise calories: Don't top off the tank

Energy bars. Performance drinks. Gels. Gu. Sport Beans. Shot Blocks. All doused with a whey protein shake. By the time you're done with the concoctions being marketed to help you work out, there may be a half a day's worth of calories consumed and not much to show for it — aside from a bit of extra flab.

The landscape these days is thick with sometimes conflicting advice about how, what and when you should eat and drink to get the most out of your workout: Exercising on an empty stomach burns more fat ... eating fat before a workout burns more fat ... eating right after a workout burns more fat ... not eating after a workout burns more fat. Gatorade and the other sports-drink companies make it seem as if casual sweating requires an Olympic refueling.

For the majority of us, however, life can be simpler. Unless you are preparing for a competition or an endurance event or exercising hard for a long time, deciding what and when to eat is not complicated.

The "what" is plain old food: fruit, vegetables, grains and protein, in reasonable balance.

The "when" is whenever it feels right. Don't like to exercise on an empty stomach? Have a small snack an hour or so beforehand. Inclined to stomach cramps? Skip the pre-exercise fueling. Don't have time to eat afterward? No worry. The standard trip to the gym won't run you down enough to demand immediate attention.

When it comes to liquids, stick to water unless you are pushing beyond 90 minutes or so.

"An average health-club workout — 20 or 30 minutes on a bike, 20 or 30 minutes of weightlifting, the whole thing done in an hour — 60 minutes of exercise, even at a fairly high level, you are not going to deplete your system," said Jo B. Zimmerman, a trainer and doctoral student in the University of Maryland's kinesiology department.

The marketing for must-have potions plays on vanity (if we're drinking what Peyton Manning drinks, we must be doing something right) as well as fear that without these things we won't become as strong or as fast or as lean as we otherwise might.

Of course, sports drinks and energy bars and gels can be useful — if you sweat a lot or exercise hard.

Depending on the person and the level of effort, there is enough stored glycogen — the body's main fuel — for between 90 minutes and two hours of continuous exercise. If you are going to train longer, you need to consume enough simple carbohydrates along the way and with enough lead time so the sugar is available — hence those nifty little bottles tucked on the backsides of distance runners. Sweat pulls salt and other electrolytes from your body, which sports drinks will replenish during a longer or harder workout. If you lift weights like a maniac, a dose of protein afterward is generally recommended.

But how many of us — be honest — are working that hard? Probably far fewer than the number goaded into eating or drinking things we don't need. What the discussion over pre- and post-workout meals normally overlooks, Zimmerman said, is how the body processes food and uses energy. The system is a dynamic one, with several stores of fuel to draw on and several different tasks always under way — from repairing muscle and building bone to keeping the major organs functioning.

The food we eat is made up of three macronutrients: carbohydrates, fat and protein. (Vitamins and the micronutrients? We'll get back to them in the future.) Each has unique benefits — carbs being the quickest form of energy, fat helping with vitamin transport and cell construction, and protein providing the amino acids to build muscle.

But as we eat, these nutrients are broken down into a stew of constituent parts that are then reassembled based on what the system needs. As long as intake is balanced and steady, the body will sort out what goes where.

Absent an acute demand — the energy to finish a marathon, for example — exercising doesn't require you to eat a certain thing at a certain time, but rather to eat a healthy mix through the day.

There are exceptions. Those with gastric reflux should avoid eating within two hours of a workout, said Robynne Chutkan, assistant professor of gastroenterology at Georgetown University.

Diabetics and people with the broad mix of prediabetic symptoms known as metabolic syndrome need to keep their blood sugar regulated for a workout, and should consult with a doctor or dietitian, Zimmerman said.

But otherwise, keep it simple, and don't get lured into 300 to 400 extra calories unless you're sure you need them.